From ca945a254a7687226ebabd662a50ce7f56d0805e Mon Sep 17 00:00:00 2001
From: Nebojsa Cvetkovic <nebkat@gmail.com>
Date: Tue, 13 Dec 2022 03:58:35 +0000
Subject: [PATCH] Consider all union types matching discriminator for excess
 property checks

Fixes #51873
---
 src/compiler/checker.ts                       |  10 +-
 ...yCheckWithMultipleDiscriminants.errors.txt |  54 +++-
 ...sPropertyCheckWithMultipleDiscriminants.js |  53 +++-
 ...ertyCheckWithMultipleDiscriminants.symbols |  82 +++++-
 ...opertyCheckWithMultipleDiscriminants.types |  76 ++++-
 .../excessPropertyCheckWithUnions.errors.txt  |  55 ++--
 .../excessPropertyCheckWithUnions.js          |  10 +-
 .../excessPropertyCheckWithUnions.symbols     | 260 +++++++++---------
 .../excessPropertyCheckWithUnions.types       |  12 +-
 .../objectLiteralNormalization.errors.txt     |   6 +-
 ...sPropertyCheckWithMultipleDiscriminants.ts |  37 ++-
 .../compiler/excessPropertyCheckWithUnions.ts |   6 +-
 12 files changed, 463 insertions(+), 198 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7a9572c993c51..8ed96e399cd28 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -22061,15 +22061,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         if (match === -1) {
             return defaultValue;
         }
-        // make sure exactly 1 matches before returning it
-        let nextMatch = discriminable.indexOf(/*searchElement*/ true, match + 1);
-        while (nextMatch !== -1) {
-            if (!isTypeIdenticalTo(target.types[match], target.types[nextMatch])) {
-                return defaultValue;
-            }
-            nextMatch = discriminable.indexOf(/*searchElement*/ true, nextMatch + 1);
-        }
-        return target.types[match];
+        return getUnionType(target.types.filter((_, index) => discriminable[index]));
     }
 
     /**
diff --git a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.errors.txt b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.errors.txt
index 29575580e8087..3b5ac675bab34 100644
--- a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.errors.txt
+++ b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.errors.txt
@@ -2,11 +2,17 @@ tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(30,5): erro
   Object literal may only specify known properties, and 'multipleOf' does not exist in type 'Float'.
 tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(41,5): error TS2322: Type '{ p1: "left"; p2: false; p3: number; p4: string; }' is not assignable to type 'DisjointDiscriminants'.
   Object literal may only specify known properties, and 'p3' does not exist in type '{ p1: "left"; p2: boolean; }'.
+tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(50,5): error TS2322: Type '{ p1: "left"; p2: true; p3: number; p4: string; }' is not assignable to type 'DisjointDiscriminants'.
+  Object literal may only specify known properties, and 'p4' does not exist in type '{ p1: "left"; p2: true; p3: number; } | { p1: "left"; p2: boolean; }'.
 tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(57,5): error TS2322: Type '{ p1: "right"; p2: false; p3: number; p4: string; }' is not assignable to type 'DisjointDiscriminants'.
   Object literal may only specify known properties, and 'p3' does not exist in type '{ p1: "right"; p2: false; p4: string; }'.
+tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(83,5): error TS2322: Type '{ type: "A"; n: number; a: number; b: number; }' is not assignable to type 'CommonWithOverlappingOptionals'.
+  Object literal may only specify known properties, and 'b' does not exist in type 'Common | (Common & A)'.
+tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(93,5): error TS2322: Type '{ type: "A"; n: number; a: number; b: number; }' is not assignable to type 'CommonWithDisjointOverlappingOptionals'.
+  Object literal may only specify known properties, and 'b' does not exist in type 'Common | A'.
 
 
-==== tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts (3 errors) ====
+==== tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts (6 errors) ====
     // Repro from #32657
     
     interface Base<T> {
@@ -57,12 +63,15 @@ tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(57,5): erro
         p4: "hello"
     };
     
-    // This has no excess error because variant one and three are both applicable.
+    // This has excess error because variant two is not applicable.
     const b: DisjointDiscriminants = {
         p1: 'left',
         p2: true,
         p3: 42,
         p4: "hello"
+        ~~~~~~~~~~~
+!!! error TS2322: Type '{ p1: "left"; p2: true; p3: number; p4: string; }' is not assignable to type 'DisjointDiscriminants'.
+!!! error TS2322:   Object literal may only specify known properties, and 'p4' does not exist in type '{ p1: "left"; p2: true; p3: number; } | { p1: "left"; p2: boolean; }'.
     };
     
     // This has excess error because variant two is the only applicable case
@@ -75,4 +84,45 @@ tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts(57,5): erro
 !!! error TS2322:   Object literal may only specify known properties, and 'p3' does not exist in type '{ p1: "right"; p2: false; p4: string; }'.
         p4: "hello"
     };
+    
+    // Repro from #51873
+    
+    interface Common {
+        type: "A" | "B" | "C" | "D";
+        n: number;
+    }
+    interface A {
+        type: "A";
+        a?: number;
+    }
+    interface B {
+        type: "B";
+        b?: number;
+    }
+    
+    type CommonWithOverlappingOptionals = Common | (Common & A) | (Common & B);
+    
+    // Should reject { b } because reduced to Common | (Common & A)
+    const c1: CommonWithOverlappingOptionals = {
+        type: "A",
+        n: 1,
+        a: 1,
+        b: 1  // excess property
+        ~~~~
+!!! error TS2322: Type '{ type: "A"; n: number; a: number; b: number; }' is not assignable to type 'CommonWithOverlappingOptionals'.
+!!! error TS2322:   Object literal may only specify known properties, and 'b' does not exist in type 'Common | (Common & A)'.
+    }
+    
+    type CommonWithDisjointOverlappingOptionals = Common | A | B;
+    
+    // Should still reject { b } because reduced to Common | A, even though these are now disjoint
+    const c2: CommonWithDisjointOverlappingOptionals = {
+        type: "A",
+        n: 1,
+        a: 1,
+        b: 1  // excess property
+        ~~~~
+!!! error TS2322: Type '{ type: "A"; n: number; a: number; b: number; }' is not assignable to type 'CommonWithDisjointOverlappingOptionals'.
+!!! error TS2322:   Object literal may only specify known properties, and 'b' does not exist in type 'Common | A'.
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.js b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.js
index a847e63aa5a1f..6f930bbc9ef29 100644
--- a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.js
+++ b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.js
@@ -43,7 +43,7 @@ const a: DisjointDiscriminants = {
     p4: "hello"
 };
 
-// This has no excess error because variant one and three are both applicable.
+// This has excess error because variant two is not applicable.
 const b: DisjointDiscriminants = {
     p1: 'left',
     p2: true,
@@ -58,6 +58,41 @@ const c: DisjointDiscriminants = {
     p3: 42,
     p4: "hello"
 };
+
+// Repro from #51873
+
+interface Common {
+    type: "A" | "B" | "C" | "D";
+    n: number;
+}
+interface A {
+    type: "A";
+    a?: number;
+}
+interface B {
+    type: "B";
+    b?: number;
+}
+
+type CommonWithOverlappingOptionals = Common | (Common & A) | (Common & B);
+
+// Should reject { b } because reduced to Common | (Common & A)
+const c1: CommonWithOverlappingOptionals = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1  // excess property
+}
+
+type CommonWithDisjointOverlappingOptionals = Common | A | B;
+
+// Should still reject { b } because reduced to Common | A, even though these are now disjoint
+const c2: CommonWithDisjointOverlappingOptionals = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1  // excess property
+}
 
 
 //// [excessPropertyCheckWithMultipleDiscriminants.js]
@@ -75,7 +110,7 @@ var a = {
     p3: 42,
     p4: "hello"
 };
-// This has no excess error because variant one and three are both applicable.
+// This has excess error because variant two is not applicable.
 var b = {
     p1: 'left',
     p2: true,
@@ -89,3 +124,17 @@ var c = {
     p3: 42,
     p4: "hello"
 };
+// Should reject { b } because reduced to Common | (Common & A)
+var c1 = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1 // excess property
+};
+// Should still reject { b } because reduced to Common | A, even though these are now disjoint
+var c2 = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1 // excess property
+};
diff --git a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.symbols b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.symbols
index 7a1b06b045a74..090932206afca 100644
--- a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.symbols
+++ b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.symbols
@@ -103,7 +103,7 @@ const a: DisjointDiscriminants = {
 
 };
 
-// This has no excess error because variant one and three are both applicable.
+// This has excess error because variant two is not applicable.
 const b: DisjointDiscriminants = {
 >b : Symbol(b, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 45, 5))
 >DisjointDiscriminants : Symbol(DisjointDiscriminants, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 31, 1))
@@ -141,3 +141,83 @@ const c: DisjointDiscriminants = {
 
 };
 
+// Repro from #51873
+
+interface Common {
+>Common : Symbol(Common, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 58, 2))
+
+    type: "A" | "B" | "C" | "D";
+>type : Symbol(Common.type, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 62, 18))
+
+    n: number;
+>n : Symbol(Common.n, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 63, 32))
+}
+interface A {
+>A : Symbol(A, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 65, 1))
+
+    type: "A";
+>type : Symbol(A.type, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 66, 13))
+
+    a?: number;
+>a : Symbol(A.a, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 67, 14))
+}
+interface B {
+>B : Symbol(B, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 69, 1))
+
+    type: "B";
+>type : Symbol(B.type, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 70, 13))
+
+    b?: number;
+>b : Symbol(B.b, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 71, 14))
+}
+
+type CommonWithOverlappingOptionals = Common | (Common & A) | (Common & B);
+>CommonWithOverlappingOptionals : Symbol(CommonWithOverlappingOptionals, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 73, 1))
+>Common : Symbol(Common, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 58, 2))
+>Common : Symbol(Common, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 58, 2))
+>A : Symbol(A, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 65, 1))
+>Common : Symbol(Common, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 58, 2))
+>B : Symbol(B, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 69, 1))
+
+// Should reject { b } because reduced to Common | (Common & A)
+const c1: CommonWithOverlappingOptionals = {
+>c1 : Symbol(c1, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 78, 5))
+>CommonWithOverlappingOptionals : Symbol(CommonWithOverlappingOptionals, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 73, 1))
+
+    type: "A",
+>type : Symbol(type, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 78, 44))
+
+    n: 1,
+>n : Symbol(n, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 79, 14))
+
+    a: 1,
+>a : Symbol(a, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 80, 9))
+
+    b: 1  // excess property
+>b : Symbol(b, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 81, 9))
+}
+
+type CommonWithDisjointOverlappingOptionals = Common | A | B;
+>CommonWithDisjointOverlappingOptionals : Symbol(CommonWithDisjointOverlappingOptionals, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 83, 1))
+>Common : Symbol(Common, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 58, 2))
+>A : Symbol(A, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 65, 1))
+>B : Symbol(B, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 69, 1))
+
+// Should still reject { b } because reduced to Common | A, even though these are now disjoint
+const c2: CommonWithDisjointOverlappingOptionals = {
+>c2 : Symbol(c2, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 88, 5))
+>CommonWithDisjointOverlappingOptionals : Symbol(CommonWithDisjointOverlappingOptionals, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 83, 1))
+
+    type: "A",
+>type : Symbol(type, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 88, 52))
+
+    n: 1,
+>n : Symbol(n, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 89, 14))
+
+    a: 1,
+>a : Symbol(a, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 90, 9))
+
+    b: 1  // excess property
+>b : Symbol(b, Decl(excessPropertyCheckWithMultipleDiscriminants.ts, 91, 9))
+}
+
diff --git a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.types b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.types
index 2c5b07e591444..79dd2fce363a6 100644
--- a/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.types
+++ b/tests/baselines/reference/excessPropertyCheckWithMultipleDiscriminants.types
@@ -93,7 +93,7 @@ const a: DisjointDiscriminants = {
 
 };
 
-// This has no excess error because variant one and three are both applicable.
+// This has excess error because variant two is not applicable.
 const b: DisjointDiscriminants = {
 >b : DisjointDiscriminants
 >{    p1: 'left',    p2: true,    p3: 42,    p4: "hello"} : { p1: "left"; p2: true; p3: number; p4: string; }
@@ -139,3 +139,77 @@ const c: DisjointDiscriminants = {
 
 };
 
+// Repro from #51873
+
+interface Common {
+    type: "A" | "B" | "C" | "D";
+>type : "A" | "B" | "C" | "D"
+
+    n: number;
+>n : number
+}
+interface A {
+    type: "A";
+>type : "A"
+
+    a?: number;
+>a : number
+}
+interface B {
+    type: "B";
+>type : "B"
+
+    b?: number;
+>b : number
+}
+
+type CommonWithOverlappingOptionals = Common | (Common & A) | (Common & B);
+>CommonWithOverlappingOptionals : Common | (Common & A) | (Common & B)
+
+// Should reject { b } because reduced to Common | (Common & A)
+const c1: CommonWithOverlappingOptionals = {
+>c1 : CommonWithOverlappingOptionals
+>{    type: "A",    n: 1,    a: 1,    b: 1  // excess property} : { type: "A"; n: number; a: number; b: number; }
+
+    type: "A",
+>type : "A"
+>"A" : "A"
+
+    n: 1,
+>n : number
+>1 : 1
+
+    a: 1,
+>a : number
+>1 : 1
+
+    b: 1  // excess property
+>b : number
+>1 : 1
+}
+
+type CommonWithDisjointOverlappingOptionals = Common | A | B;
+>CommonWithDisjointOverlappingOptionals : Common | A | B
+
+// Should still reject { b } because reduced to Common | A, even though these are now disjoint
+const c2: CommonWithDisjointOverlappingOptionals = {
+>c2 : CommonWithDisjointOverlappingOptionals
+>{    type: "A",    n: 1,    a: 1,    b: 1  // excess property} : { type: "A"; n: number; a: number; b: number; }
+
+    type: "A",
+>type : "A"
+>"A" : "A"
+
+    n: 1,
+>n : number
+>1 : 1
+
+    a: 1,
+>a : number
+>1 : 1
+
+    b: 1  // excess property
+>b : number
+>1 : 1
+}
+
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt b/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
index 8c34a0f5d69cf..fb33e81d25664 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.errors.txt
@@ -5,30 +5,27 @@ tests/cases/compiler/excessPropertyCheckWithUnions.ts(11,21): error TS2322: Type
 tests/cases/compiler/excessPropertyCheckWithUnions.ts(12,1): error TS2322: Type '{ tag: "D"; }' is not assignable to type 'ADT'.
   Property 'd20' is missing in type '{ tag: "D"; }' but required in type '{ tag: "D"; d20: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20; }'.
 tests/cases/compiler/excessPropertyCheckWithUnions.ts(33,28): error TS2322: Type '{ tag: "A"; x: string; extra: number; }' is not assignable to type 'Ambiguous'.
-  Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+  Object literal may only specify known properties, and 'extra' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
 tests/cases/compiler/excessPropertyCheckWithUnions.ts(34,26): error TS2322: Type '{ tag: "A"; y: number; extra: number; }' is not assignable to type 'Ambiguous'.
-  Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(39,1): error TS2322: Type '{ tag: "A"; }' is not assignable to type 'Ambiguous'.
-  Type '{ tag: "A"; }' is not assignable to type '{ tag: "C"; }'.
-    Types of property 'tag' are incompatible.
-      Type '"A"' is not assignable to type '"C"'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(40,1): error TS2322: Type '{ tag: "A"; z: true; }' is not assignable to type 'Ambiguous'.
-  Type '{ tag: "A"; z: true; }' is not assignable to type '{ tag: "B"; z: boolean; }'.
-    Types of property 'tag' are incompatible.
-      Type '"A"' is not assignable to type '"B"'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(49,35): error TS2322: Type '{ a: 1; b: 1; first: string; second: string; }' is not assignable to type 'Overlapping'.
+  Object literal may only specify known properties, and 'extra' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(37,1): error TS2322: Type '{ tag: "A"; }' is not assignable to type 'Ambiguous'.
+  Type '{ tag: "A"; }' is not assignable to type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
+    Property 'y' is missing in type '{ tag: "A"; }' but required in type '{ tag: "A"; y: number; }'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(38,19): error TS2322: Type '{ tag: "A"; z: boolean; }' is not assignable to type 'Ambiguous'.
+  Object literal may only specify known properties, and 'z' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(47,35): error TS2322: Type '{ a: 1; b: 1; first: string; second: string; }' is not assignable to type 'Overlapping'.
   Object literal may only specify known properties, and 'second' does not exist in type '{ a: 1; b: 1; first: string; }'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(50,35): error TS2322: Type '{ a: 1; b: 1; first: string; third: string; }' is not assignable to type 'Overlapping'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(48,35): error TS2322: Type '{ a: 1; b: 1; first: string; third: string; }' is not assignable to type 'Overlapping'.
   Object literal may only specify known properties, and 'third' does not exist in type '{ a: 1; b: 1; first: string; }'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(66,9): error TS2322: Type '{ kind: "A"; n: { a: string; b: string; }; }' is not assignable to type 'AB'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(64,9): error TS2322: Type '{ kind: "A"; n: { a: string; b: string; }; }' is not assignable to type 'AB'.
   Types of property 'n' are incompatible.
     Type '{ a: string; b: string; }' is not assignable to type 'AN'.
       Object literal may only specify known properties, and 'b' does not exist in type 'AN'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(87,5): error TS2322: Type '{ tag: "button"; type: "submit"; href: string; }' is not assignable to type 'Union'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(85,5): error TS2322: Type '{ tag: "button"; type: "submit"; href: string; }' is not assignable to type 'Union'.
   Object literal may only specify known properties, and 'href' does not exist in type 'Button'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(108,5): error TS2322: Type 'string' is not assignable to type 'IValue'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(113,67): error TS2322: Type 'string' is not assignable to type 'number'.
-tests/cases/compiler/excessPropertyCheckWithUnions.ts(114,63): error TS2322: Type 'string' is not assignable to type 'number'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(106,5): error TS2322: Type 'string' is not assignable to type 'IValue'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(111,67): error TS2322: Type 'string' is not assignable to type 'number'.
+tests/cases/compiler/excessPropertyCheckWithUnions.ts(112,63): error TS2322: Type 'string' is not assignable to type 'number'.
 
 
 ==== tests/cases/compiler/excessPropertyCheckWithUnions.ts (14 errors) ====
@@ -77,27 +74,23 @@ tests/cases/compiler/excessPropertyCheckWithUnions.ts(114,63): error TS2322: Typ
     amb = { tag: "A", x: "hi", extra: 12 }
                                ~~~~~~~~~
 !!! error TS2322: Type '{ tag: "A"; x: string; extra: number; }' is not assignable to type 'Ambiguous'.
-!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
     amb = { tag: "A", y: 12, extra: 12 }
                              ~~~~~~~~~
 !!! error TS2322: Type '{ tag: "A"; y: number; extra: number; }' is not assignable to type 'Ambiguous'.
-!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type 'Ambiguous'.
+!!! error TS2322:   Object literal may only specify known properties, and 'extra' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
     
-    // assignability errors still work.
-    // But note that the error for `z: true` is the fallback one of reporting on
-    // the last constituent since assignability error reporting can't find a single best discriminant either.
+    // assignability errors still work
     amb = { tag: "A" }
     ~~~
 !!! error TS2322: Type '{ tag: "A"; }' is not assignable to type 'Ambiguous'.
-!!! error TS2322:   Type '{ tag: "A"; }' is not assignable to type '{ tag: "C"; }'.
-!!! error TS2322:     Types of property 'tag' are incompatible.
-!!! error TS2322:       Type '"A"' is not assignable to type '"C"'.
+!!! error TS2322:   Type '{ tag: "A"; }' is not assignable to type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
+!!! error TS2322:     Property 'y' is missing in type '{ tag: "A"; }' but required in type '{ tag: "A"; y: number; }'.
+!!! related TS2728 tests/cases/compiler/excessPropertyCheckWithUnions.ts:19:5: 'y' is declared here.
     amb = { tag: "A", z: true }
-    ~~~
-!!! error TS2322: Type '{ tag: "A"; z: true; }' is not assignable to type 'Ambiguous'.
-!!! error TS2322:   Type '{ tag: "A"; z: true; }' is not assignable to type '{ tag: "B"; z: boolean; }'.
-!!! error TS2322:     Types of property 'tag' are incompatible.
-!!! error TS2322:       Type '"A"' is not assignable to type '"B"'.
+                      ~~~~~~~
+!!! error TS2322: Type '{ tag: "A"; z: boolean; }' is not assignable to type 'Ambiguous'.
+!!! error TS2322:   Object literal may only specify known properties, and 'z' does not exist in type '{ tag: "A"; x: string; } | { tag: "A"; y: number; }'.
     
     type Overlapping =
         | { a: 1, b: 1, first: string }
@@ -167,7 +160,7 @@ tests/cases/compiler/excessPropertyCheckWithUnions.ts(114,63): error TS2322: Typ
       value: string
     }
     
-    interface StringKeys { 
+    interface StringKeys {
         [propertyName: string]: IValue;
     };
     
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.js b/tests/baselines/reference/excessPropertyCheckWithUnions.js
index dabcd824f3693..40be4c89c175e 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.js
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.js
@@ -34,9 +34,7 @@ amb = { tag: "A", x: "hi", y: 12 }
 amb = { tag: "A", x: "hi", extra: 12 }
 amb = { tag: "A", y: 12, extra: 12 }
 
-// assignability errors still work.
-// But note that the error for `z: true` is the fallback one of reporting on
-// the last constituent since assignability error reporting can't find a single best discriminant either.
+// assignability errors still work
 amb = { tag: "A" }
 amb = { tag: "A", z: true }
 
@@ -94,7 +92,7 @@ interface IValue {
   value: string
 }
 
-interface StringKeys { 
+interface StringKeys {
     [propertyName: string]: IValue;
 };
 
@@ -178,9 +176,7 @@ amb = { tag: "A", x: "hi", y: 12 };
 // correctly error on excess property 'extra', even when ambiguous
 amb = { tag: "A", x: "hi", extra: 12 };
 amb = { tag: "A", y: 12, extra: 12 };
-// assignability errors still work.
-// But note that the error for `z: true` is the fallback one of reporting on
-// the last constituent since assignability error reporting can't find a single best discriminant either.
+// assignability errors still work
 amb = { tag: "A" };
 amb = { tag: "A", z: true };
 var over;
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.symbols b/tests/baselines/reference/excessPropertyCheckWithUnions.symbols
index 80921fedad26c..65cec78a47b5a 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.symbols
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.symbols
@@ -95,301 +95,299 @@ amb = { tag: "A", y: 12, extra: 12 }
 >y : Symbol(y, Decl(excessPropertyCheckWithUnions.ts, 33, 17))
 >extra : Symbol(extra, Decl(excessPropertyCheckWithUnions.ts, 33, 24))
 
-// assignability errors still work.
-// But note that the error for `z: true` is the fallback one of reporting on
-// the last constituent since assignability error reporting can't find a single best discriminant either.
+// assignability errors still work
 amb = { tag: "A" }
 >amb : Symbol(amb, Decl(excessPropertyCheckWithUnions.ts, 25, 3))
->tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 38, 7))
+>tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 36, 7))
 
 amb = { tag: "A", z: true }
 >amb : Symbol(amb, Decl(excessPropertyCheckWithUnions.ts, 25, 3))
->tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 39, 7))
->z : Symbol(z, Decl(excessPropertyCheckWithUnions.ts, 39, 17))
+>tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 37, 7))
+>z : Symbol(z, Decl(excessPropertyCheckWithUnions.ts, 37, 17))
 
 type Overlapping =
->Overlapping : Symbol(Overlapping, Decl(excessPropertyCheckWithUnions.ts, 39, 27))
+>Overlapping : Symbol(Overlapping, Decl(excessPropertyCheckWithUnions.ts, 37, 27))
 
     | { a: 1, b: 1, first: string }
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 42, 7))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 42, 13))
->first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 42, 19))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 40, 7))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 40, 13))
+>first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 40, 19))
 
     | { a: 2, second: string }
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 43, 7))
->second : Symbol(second, Decl(excessPropertyCheckWithUnions.ts, 43, 13))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 41, 7))
+>second : Symbol(second, Decl(excessPropertyCheckWithUnions.ts, 41, 13))
 
     | { b: 3, third: string }
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 44, 7))
->third : Symbol(third, Decl(excessPropertyCheckWithUnions.ts, 44, 13))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 42, 7))
+>third : Symbol(third, Decl(excessPropertyCheckWithUnions.ts, 42, 13))
 
 let over: Overlapping
->over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 45, 3))
->Overlapping : Symbol(Overlapping, Decl(excessPropertyCheckWithUnions.ts, 39, 27))
+>over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 43, 3))
+>Overlapping : Symbol(Overlapping, Decl(excessPropertyCheckWithUnions.ts, 37, 27))
 
 // these two are still errors despite their doubled up discriminants
 over = { a: 1, b: 1, first: "ok", second: "error" }
->over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 45, 3))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 48, 8))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 48, 14))
->first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 48, 20))
->second : Symbol(second, Decl(excessPropertyCheckWithUnions.ts, 48, 33))
+>over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 43, 3))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 46, 8))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 46, 14))
+>first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 46, 20))
+>second : Symbol(second, Decl(excessPropertyCheckWithUnions.ts, 46, 33))
 
 over = { a: 1, b: 1, first: "ok", third: "error" }
->over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 45, 3))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 49, 8))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 49, 14))
->first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 49, 20))
->third : Symbol(third, Decl(excessPropertyCheckWithUnions.ts, 49, 33))
+>over : Symbol(over, Decl(excessPropertyCheckWithUnions.ts, 43, 3))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 47, 8))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 47, 14))
+>first : Symbol(first, Decl(excessPropertyCheckWithUnions.ts, 47, 20))
+>third : Symbol(third, Decl(excessPropertyCheckWithUnions.ts, 47, 33))
 
 // Freshness disappears after spreading a union
 declare let t0: { a: any, b: any } | { d: any, e: any }
->t0 : Symbol(t0, Decl(excessPropertyCheckWithUnions.ts, 52, 11))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 52, 17))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 52, 25))
->d : Symbol(d, Decl(excessPropertyCheckWithUnions.ts, 52, 38))
->e : Symbol(e, Decl(excessPropertyCheckWithUnions.ts, 52, 46))
+>t0 : Symbol(t0, Decl(excessPropertyCheckWithUnions.ts, 50, 11))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 50, 17))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 50, 25))
+>d : Symbol(d, Decl(excessPropertyCheckWithUnions.ts, 50, 38))
+>e : Symbol(e, Decl(excessPropertyCheckWithUnions.ts, 50, 46))
 
 declare let t1: { a: any, b: any, c: any } | { c: any, d: any, e: any }
->t1 : Symbol(t1, Decl(excessPropertyCheckWithUnions.ts, 53, 11))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 53, 17))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 53, 25))
->c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 53, 33))
->c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 53, 46))
->d : Symbol(d, Decl(excessPropertyCheckWithUnions.ts, 53, 54))
->e : Symbol(e, Decl(excessPropertyCheckWithUnions.ts, 53, 62))
+>t1 : Symbol(t1, Decl(excessPropertyCheckWithUnions.ts, 51, 11))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 51, 17))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 51, 25))
+>c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 51, 33))
+>c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 51, 46))
+>d : Symbol(d, Decl(excessPropertyCheckWithUnions.ts, 51, 54))
+>e : Symbol(e, Decl(excessPropertyCheckWithUnions.ts, 51, 62))
 
 let t2 = { ...t1 }
->t2 : Symbol(t2, Decl(excessPropertyCheckWithUnions.ts, 54, 3))
->t1 : Symbol(t1, Decl(excessPropertyCheckWithUnions.ts, 53, 11))
+>t2 : Symbol(t2, Decl(excessPropertyCheckWithUnions.ts, 52, 3))
+>t1 : Symbol(t1, Decl(excessPropertyCheckWithUnions.ts, 51, 11))
 
 t0 = t2
->t0 : Symbol(t0, Decl(excessPropertyCheckWithUnions.ts, 52, 11))
->t2 : Symbol(t2, Decl(excessPropertyCheckWithUnions.ts, 54, 3))
+>t0 : Symbol(t0, Decl(excessPropertyCheckWithUnions.ts, 50, 11))
+>t2 : Symbol(t2, Decl(excessPropertyCheckWithUnions.ts, 52, 3))
 
 // Nested excess property checks work with discriminated unions
 type AN = { a: string } | { c: string }
->AN : Symbol(AN, Decl(excessPropertyCheckWithUnions.ts, 55, 7))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 58, 11))
->c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 58, 27))
+>AN : Symbol(AN, Decl(excessPropertyCheckWithUnions.ts, 53, 7))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 56, 11))
+>c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 56, 27))
 
 type BN = { b: string }
->BN : Symbol(BN, Decl(excessPropertyCheckWithUnions.ts, 58, 39))
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 59, 11))
+>BN : Symbol(BN, Decl(excessPropertyCheckWithUnions.ts, 56, 39))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 57, 11))
 
 type AB = { kind: "A", n: AN } | { kind: "B", n: BN }
->AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 59, 23))
->kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 60, 11))
->n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 60, 22))
->AN : Symbol(AN, Decl(excessPropertyCheckWithUnions.ts, 55, 7))
->kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 60, 34))
->n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 60, 45))
->BN : Symbol(BN, Decl(excessPropertyCheckWithUnions.ts, 58, 39))
+>AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 57, 23))
+>kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 58, 11))
+>n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 58, 22))
+>AN : Symbol(AN, Decl(excessPropertyCheckWithUnions.ts, 53, 7))
+>kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 58, 34))
+>n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 58, 45))
+>BN : Symbol(BN, Decl(excessPropertyCheckWithUnions.ts, 56, 39))
 
 const abab: AB = {
->abab : Symbol(abab, Decl(excessPropertyCheckWithUnions.ts, 61, 5))
->AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 59, 23))
+>abab : Symbol(abab, Decl(excessPropertyCheckWithUnions.ts, 59, 5))
+>AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 57, 23))
 
     kind: "A",
->kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 61, 18))
+>kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 59, 18))
 
     n: {
->n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 62, 14))
+>n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 60, 14))
 
         a: "a",
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 63, 8))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 61, 8))
 
         b: "b", // excess -- kind: "A"
->b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 64, 15))
+>b : Symbol(b, Decl(excessPropertyCheckWithUnions.ts, 62, 15))
     }
 }
 const abac: AB = {
->abac : Symbol(abac, Decl(excessPropertyCheckWithUnions.ts, 68, 5))
->AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 59, 23))
+>abac : Symbol(abac, Decl(excessPropertyCheckWithUnions.ts, 66, 5))
+>AB : Symbol(AB, Decl(excessPropertyCheckWithUnions.ts, 57, 23))
 
     kind: "A",
->kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 68, 18))
+>kind : Symbol(kind, Decl(excessPropertyCheckWithUnions.ts, 66, 18))
 
     n: {
->n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 69, 14))
+>n : Symbol(n, Decl(excessPropertyCheckWithUnions.ts, 67, 14))
 
         a: "a",
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 70, 8))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 68, 8))
 
         c: "c", // ok -- kind: "A", an: { a: string } | { c: string }
->c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 71, 15))
+>c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 69, 15))
     }
 }
 
 // Excess property checks must match all discriminable properties
 type Button = { tag: 'button'; type?: 'submit'; };
->Button : Symbol(Button, Decl(excessPropertyCheckWithUnions.ts, 74, 1))
->tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 77, 15))
->type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 77, 30))
+>Button : Symbol(Button, Decl(excessPropertyCheckWithUnions.ts, 72, 1))
+>tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 75, 15))
+>type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 75, 30))
 
 type Anchor = { tag: 'a'; type?: string; href: string };
->Anchor : Symbol(Anchor, Decl(excessPropertyCheckWithUnions.ts, 77, 50))
->tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 78, 15))
->type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 78, 25))
->href : Symbol(href, Decl(excessPropertyCheckWithUnions.ts, 78, 40))
+>Anchor : Symbol(Anchor, Decl(excessPropertyCheckWithUnions.ts, 75, 50))
+>tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 76, 15))
+>type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 76, 25))
+>href : Symbol(href, Decl(excessPropertyCheckWithUnions.ts, 76, 40))
 
 type Union = Button | Anchor;
->Union : Symbol(Union, Decl(excessPropertyCheckWithUnions.ts, 78, 56))
->Button : Symbol(Button, Decl(excessPropertyCheckWithUnions.ts, 74, 1))
->Anchor : Symbol(Anchor, Decl(excessPropertyCheckWithUnions.ts, 77, 50))
+>Union : Symbol(Union, Decl(excessPropertyCheckWithUnions.ts, 76, 56))
+>Button : Symbol(Button, Decl(excessPropertyCheckWithUnions.ts, 72, 1))
+>Anchor : Symbol(Anchor, Decl(excessPropertyCheckWithUnions.ts, 75, 50))
 
 const obj: Union = {
->obj : Symbol(obj, Decl(excessPropertyCheckWithUnions.ts, 81, 5))
->Union : Symbol(Union, Decl(excessPropertyCheckWithUnions.ts, 78, 56))
+>obj : Symbol(obj, Decl(excessPropertyCheckWithUnions.ts, 79, 5))
+>Union : Symbol(Union, Decl(excessPropertyCheckWithUnions.ts, 76, 56))
 
     tag: 'button',
->tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 81, 20))
+>tag : Symbol(tag, Decl(excessPropertyCheckWithUnions.ts, 79, 20))
 
     type: 'submit',
->type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 82, 18))
+>type : Symbol(type, Decl(excessPropertyCheckWithUnions.ts, 80, 18))
 
     // should have error here
     href: 'foo',
->href : Symbol(href, Decl(excessPropertyCheckWithUnions.ts, 83, 19))
+>href : Symbol(href, Decl(excessPropertyCheckWithUnions.ts, 81, 19))
 
 };
 
 // Repro from #34611
 
 interface IValue {
->IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 87, 2))
+>IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 85, 2))
 
   value: string
->value : Symbol(IValue.value, Decl(excessPropertyCheckWithUnions.ts, 91, 18))
+>value : Symbol(IValue.value, Decl(excessPropertyCheckWithUnions.ts, 89, 18))
 }
 
-interface StringKeys { 
->StringKeys : Symbol(StringKeys, Decl(excessPropertyCheckWithUnions.ts, 93, 1))
+interface StringKeys {
+>StringKeys : Symbol(StringKeys, Decl(excessPropertyCheckWithUnions.ts, 91, 1))
 
     [propertyName: string]: IValue;
->propertyName : Symbol(propertyName, Decl(excessPropertyCheckWithUnions.ts, 96, 5))
->IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 87, 2))
+>propertyName : Symbol(propertyName, Decl(excessPropertyCheckWithUnions.ts, 94, 5))
+>IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 85, 2))
 
 };
 
 interface NumberKeys {
->NumberKeys : Symbol(NumberKeys, Decl(excessPropertyCheckWithUnions.ts, 97, 2))
+>NumberKeys : Symbol(NumberKeys, Decl(excessPropertyCheckWithUnions.ts, 95, 2))
 
     [propertyName: number]: IValue;
->propertyName : Symbol(propertyName, Decl(excessPropertyCheckWithUnions.ts, 100, 5))
->IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 87, 2))
+>propertyName : Symbol(propertyName, Decl(excessPropertyCheckWithUnions.ts, 98, 5))
+>IValue : Symbol(IValue, Decl(excessPropertyCheckWithUnions.ts, 85, 2))
 }
 
 type ObjectDataSpecification = StringKeys | NumberKeys;
->ObjectDataSpecification : Symbol(ObjectDataSpecification, Decl(excessPropertyCheckWithUnions.ts, 101, 1))
->StringKeys : Symbol(StringKeys, Decl(excessPropertyCheckWithUnions.ts, 93, 1))
->NumberKeys : Symbol(NumberKeys, Decl(excessPropertyCheckWithUnions.ts, 97, 2))
+>ObjectDataSpecification : Symbol(ObjectDataSpecification, Decl(excessPropertyCheckWithUnions.ts, 99, 1))
+>StringKeys : Symbol(StringKeys, Decl(excessPropertyCheckWithUnions.ts, 91, 1))
+>NumberKeys : Symbol(NumberKeys, Decl(excessPropertyCheckWithUnions.ts, 95, 2))
 
 
 const dataSpecification: ObjectDataSpecification = {  // Error
->dataSpecification : Symbol(dataSpecification, Decl(excessPropertyCheckWithUnions.ts, 106, 5))
->ObjectDataSpecification : Symbol(ObjectDataSpecification, Decl(excessPropertyCheckWithUnions.ts, 101, 1))
+>dataSpecification : Symbol(dataSpecification, Decl(excessPropertyCheckWithUnions.ts, 104, 5))
+>ObjectDataSpecification : Symbol(ObjectDataSpecification, Decl(excessPropertyCheckWithUnions.ts, 99, 1))
 
     foo: "asdfsadffsd"
->foo : Symbol(foo, Decl(excessPropertyCheckWithUnions.ts, 106, 52))
+>foo : Symbol(foo, Decl(excessPropertyCheckWithUnions.ts, 104, 52))
 
 };
 
 // Repro from #34611
 
 const obj1: { [x: string]: number } | { [x: number]: number } = { a: 'abc' };  // Error
->obj1 : Symbol(obj1, Decl(excessPropertyCheckWithUnions.ts, 112, 5))
->x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 112, 15))
->x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 112, 41))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 112, 65))
+>obj1 : Symbol(obj1, Decl(excessPropertyCheckWithUnions.ts, 110, 5))
+>x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 110, 15))
+>x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 110, 41))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 110, 65))
 
 const obj2: { [x: string]: number } | { a: number } = { a: 5, c: 'abc' };  // Error
->obj2 : Symbol(obj2, Decl(excessPropertyCheckWithUnions.ts, 113, 5))
->x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 113, 15))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 113, 39))
->a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 113, 55))
->c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 113, 61))
+>obj2 : Symbol(obj2, Decl(excessPropertyCheckWithUnions.ts, 111, 5))
+>x : Symbol(x, Decl(excessPropertyCheckWithUnions.ts, 111, 15))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 111, 39))
+>a : Symbol(a, Decl(excessPropertyCheckWithUnions.ts, 111, 55))
+>c : Symbol(c, Decl(excessPropertyCheckWithUnions.ts, 111, 61))
 
 // Repro from #33732
 
 interface I1 {
->I1 : Symbol(I1, Decl(excessPropertyCheckWithUnions.ts, 113, 73))
+>I1 : Symbol(I1, Decl(excessPropertyCheckWithUnions.ts, 111, 73))
 
     prop1: string;
->prop1 : Symbol(I1.prop1, Decl(excessPropertyCheckWithUnions.ts, 117, 14))
+>prop1 : Symbol(I1.prop1, Decl(excessPropertyCheckWithUnions.ts, 115, 14))
 }
 
 interface I2 {
->I2 : Symbol(I2, Decl(excessPropertyCheckWithUnions.ts, 119, 1))
+>I2 : Symbol(I2, Decl(excessPropertyCheckWithUnions.ts, 117, 1))
 
     prop2: string;
->prop2 : Symbol(I2.prop2, Decl(excessPropertyCheckWithUnions.ts, 121, 14))
+>prop2 : Symbol(I2.prop2, Decl(excessPropertyCheckWithUnions.ts, 119, 14))
 }
 
 interface I3 extends Record<string, string> {
->I3 : Symbol(I3, Decl(excessPropertyCheckWithUnions.ts, 123, 1))
+>I3 : Symbol(I3, Decl(excessPropertyCheckWithUnions.ts, 121, 1))
 >Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
 
 }
 
 type Properties =
->Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 127, 1))
+>Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 125, 1))
 
     | { [key: string]: never }
->key : Symbol(key, Decl(excessPropertyCheckWithUnions.ts, 130, 9))
+>key : Symbol(key, Decl(excessPropertyCheckWithUnions.ts, 128, 9))
 
     | I1
->I1 : Symbol(I1, Decl(excessPropertyCheckWithUnions.ts, 113, 73))
+>I1 : Symbol(I1, Decl(excessPropertyCheckWithUnions.ts, 111, 73))
 
     | I2
->I2 : Symbol(I2, Decl(excessPropertyCheckWithUnions.ts, 119, 1))
+>I2 : Symbol(I2, Decl(excessPropertyCheckWithUnions.ts, 117, 1))
 
     | I3
->I3 : Symbol(I3, Decl(excessPropertyCheckWithUnions.ts, 123, 1))
+>I3 : Symbol(I3, Decl(excessPropertyCheckWithUnions.ts, 121, 1))
 
     ;
 
 
 declare const prop1: string;
->prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 137, 13))
+>prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 135, 13))
 
 declare const prop2: string | undefined;
->prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 138, 13))
+>prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 136, 13))
 
 function F1(_arg: { props: Properties }) { }
->F1 : Symbol(F1, Decl(excessPropertyCheckWithUnions.ts, 138, 40))
->_arg : Symbol(_arg, Decl(excessPropertyCheckWithUnions.ts, 140, 12))
->props : Symbol(props, Decl(excessPropertyCheckWithUnions.ts, 140, 19))
->Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 127, 1))
+>F1 : Symbol(F1, Decl(excessPropertyCheckWithUnions.ts, 136, 40))
+>_arg : Symbol(_arg, Decl(excessPropertyCheckWithUnions.ts, 138, 12))
+>props : Symbol(props, Decl(excessPropertyCheckWithUnions.ts, 138, 19))
+>Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 125, 1))
 
 F1({
->F1 : Symbol(F1, Decl(excessPropertyCheckWithUnions.ts, 138, 40))
+>F1 : Symbol(F1, Decl(excessPropertyCheckWithUnions.ts, 136, 40))
 
     props: {
->props : Symbol(props, Decl(excessPropertyCheckWithUnions.ts, 141, 4))
+>props : Symbol(props, Decl(excessPropertyCheckWithUnions.ts, 139, 4))
 
         prop1,
->prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 142, 12))
+>prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 140, 12))
 
         prop2,
->prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 143, 14))
+>prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 141, 14))
 
     },
 });
 
 function F2(_props: Properties) { }
->F2 : Symbol(F2, Decl(excessPropertyCheckWithUnions.ts, 146, 3))
->_props : Symbol(_props, Decl(excessPropertyCheckWithUnions.ts, 148, 12))
->Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 127, 1))
+>F2 : Symbol(F2, Decl(excessPropertyCheckWithUnions.ts, 144, 3))
+>_props : Symbol(_props, Decl(excessPropertyCheckWithUnions.ts, 146, 12))
+>Properties : Symbol(Properties, Decl(excessPropertyCheckWithUnions.ts, 125, 1))
 
 F2({
->F2 : Symbol(F2, Decl(excessPropertyCheckWithUnions.ts, 146, 3))
+>F2 : Symbol(F2, Decl(excessPropertyCheckWithUnions.ts, 144, 3))
 
     prop1,
->prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 149, 4))
+>prop1 : Symbol(prop1, Decl(excessPropertyCheckWithUnions.ts, 147, 4))
 
     prop2,
->prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 150, 10))
+>prop2 : Symbol(prop2, Decl(excessPropertyCheckWithUnions.ts, 148, 10))
 
 });
 
diff --git a/tests/baselines/reference/excessPropertyCheckWithUnions.types b/tests/baselines/reference/excessPropertyCheckWithUnions.types
index 2b012e13eb14c..699a13fb48640 100644
--- a/tests/baselines/reference/excessPropertyCheckWithUnions.types
+++ b/tests/baselines/reference/excessPropertyCheckWithUnions.types
@@ -126,9 +126,7 @@ amb = { tag: "A", y: 12, extra: 12 }
 >extra : number
 >12 : 12
 
-// assignability errors still work.
-// But note that the error for `z: true` is the fallback one of reporting on
-// the last constituent since assignability error reporting can't find a single best discriminant either.
+// assignability errors still work
 amb = { tag: "A" }
 >amb = { tag: "A" } : { tag: "A"; }
 >amb : Ambiguous
@@ -137,12 +135,12 @@ amb = { tag: "A" }
 >"A" : "A"
 
 amb = { tag: "A", z: true }
->amb = { tag: "A", z: true } : { tag: "A"; z: true; }
+>amb = { tag: "A", z: true } : { tag: "A"; z: boolean; }
 >amb : Ambiguous
->{ tag: "A", z: true } : { tag: "A"; z: true; }
+>{ tag: "A", z: true } : { tag: "A"; z: boolean; }
 >tag : "A"
 >"A" : "A"
->z : true
+>z : boolean
 >true : true
 
 type Overlapping =
@@ -319,7 +317,7 @@ interface IValue {
 >value : string
 }
 
-interface StringKeys { 
+interface StringKeys {
     [propertyName: string]: IValue;
 >propertyName : string
 
diff --git a/tests/baselines/reference/objectLiteralNormalization.errors.txt b/tests/baselines/reference/objectLiteralNormalization.errors.txt
index e057800725e21..a6ffc718f913c 100644
--- a/tests/baselines/reference/objectLiteralNormalization.errors.txt
+++ b/tests/baselines/reference/objectLiteralNormalization.errors.txt
@@ -1,6 +1,7 @@
 tests/cases/conformance/expressions/objectLiterals/objectLiteralNormalization.ts(7,14): error TS2322: Type 'number' is not assignable to type 'string'.
 tests/cases/conformance/expressions/objectLiterals/objectLiteralNormalization.ts(8,1): error TS2322: Type '{ b: string; }' is not assignable to type '{ a: number; b?: undefined; c?: undefined; } | { a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
-  Type '{ b: string; }' is missing the following properties from type '{ a: number; b: string; c: boolean; }': a, c
+  Type '{ b: string; }' is not assignable to type '{ a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
+    Type '{ b: string; }' is missing the following properties from type '{ a: number; b: string; c: boolean; }': a, c
 tests/cases/conformance/expressions/objectLiterals/objectLiteralNormalization.ts(9,1): error TS2322: Type '{ c: true; }' is not assignable to type '{ a: number; b?: undefined; c?: undefined; } | { a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
   Type '{ c: true; }' is missing the following properties from type '{ a: number; b: string; c: boolean; }': a, b
 tests/cases/conformance/expressions/objectLiterals/objectLiteralNormalization.ts(17,1): error TS2322: Type '{ a: string; b: number; }' is not assignable to type '{ a: number; b: number; } | { a: string; b?: undefined; } | { a?: undefined; b?: undefined; }'.
@@ -27,7 +28,8 @@ tests/cases/conformance/expressions/objectLiterals/objectLiteralNormalization.ts
     a1 = { b: "y" };  // Error
     ~~
 !!! error TS2322: Type '{ b: string; }' is not assignable to type '{ a: number; b?: undefined; c?: undefined; } | { a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
-!!! error TS2322:   Type '{ b: string; }' is missing the following properties from type '{ a: number; b: string; c: boolean; }': a, c
+!!! error TS2322:   Type '{ b: string; }' is not assignable to type '{ a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
+!!! error TS2322:     Type '{ b: string; }' is missing the following properties from type '{ a: number; b: string; c: boolean; }': a, c
     a1 = { c: true };  // Error
     ~~
 !!! error TS2322: Type '{ c: true; }' is not assignable to type '{ a: number; b?: undefined; c?: undefined; } | { a: number; b: string; c?: undefined; } | { a: number; b: string; c: boolean; }'.
diff --git a/tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts b/tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts
index 5f7abedc1f42a..518a7a930aafd 100644
--- a/tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts
+++ b/tests/cases/compiler/excessPropertyCheckWithMultipleDiscriminants.ts
@@ -42,7 +42,7 @@ const a: DisjointDiscriminants = {
     p4: "hello"
 };
 
-// This has no excess error because variant one and three are both applicable.
+// This has excess error because variant two is not applicable.
 const b: DisjointDiscriminants = {
     p1: 'left',
     p2: true,
@@ -57,3 +57,38 @@ const c: DisjointDiscriminants = {
     p3: 42,
     p4: "hello"
 };
+
+// Repro from #51873
+
+interface Common {
+    type: "A" | "B" | "C" | "D";
+    n: number;
+}
+interface A {
+    type: "A";
+    a?: number;
+}
+interface B {
+    type: "B";
+    b?: number;
+}
+
+type CommonWithOverlappingOptionals = Common | (Common & A) | (Common & B);
+
+// Should reject { b } because reduced to Common | (Common & A)
+const c1: CommonWithOverlappingOptionals = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1  // excess property
+}
+
+type CommonWithDisjointOverlappingOptionals = Common | A | B;
+
+// Should still reject { b } because reduced to Common | A, even though these are now disjoint
+const c2: CommonWithDisjointOverlappingOptionals = {
+    type: "A",
+    n: 1,
+    a: 1,
+    b: 1  // excess property
+}
diff --git a/tests/cases/compiler/excessPropertyCheckWithUnions.ts b/tests/cases/compiler/excessPropertyCheckWithUnions.ts
index 4f8ea87a3ceb2..6f26597ae912c 100644
--- a/tests/cases/compiler/excessPropertyCheckWithUnions.ts
+++ b/tests/cases/compiler/excessPropertyCheckWithUnions.ts
@@ -34,9 +34,7 @@ amb = { tag: "A", x: "hi", y: 12 }
 amb = { tag: "A", x: "hi", extra: 12 }
 amb = { tag: "A", y: 12, extra: 12 }
 
-// assignability errors still work.
-// But note that the error for `z: true` is the fallback one of reporting on
-// the last constituent since assignability error reporting can't find a single best discriminant either.
+// assignability errors still work
 amb = { tag: "A" }
 amb = { tag: "A", z: true }
 
@@ -94,7 +92,7 @@ interface IValue {
   value: string
 }
 
-interface StringKeys { 
+interface StringKeys {
     [propertyName: string]: IValue;
 };