early return if pattern type is never

src/compiler/checker.ts

@@ -25160,6 +25160,9 @@ namespace ts {
                             if (parentType && parentType.flags & TypeFlags.Union && !(parent.kind === SyntaxKind.Parameter && isSymbolAssigned(symbol))) {
                                 const pattern = declaration.parent;
                                 const narrowedType = getFlowTypeOfReference(pattern, parentType, parentType, /*flowContainer*/ undefined, location.flowNode);
+                                if (narrowedType.flags & TypeFlags.Never) {
+                                    return neverType;
+                                }
                                 return getBindingElementTypeFromParentType(declaration, narrowedType);
                             }
                         }

tests/baselines/reference/arrayDestructuringInSwitch1.js

@@ -0,0 +1,47 @@
+//// [arrayDestructuringInSwitch1.ts]
+export type Expression = BooleanLogicExpression | 'true' | 'false';
+export type BooleanLogicExpression = ['and', ...Expression[]] | ['not', Expression];
+
+export function evaluate(expression: Expression): boolean {
+  if (Array.isArray(expression)) {
+    const [operator, ...operands] = expression;
+    switch (operator) {
+      case 'and': {
+        return operands.every((child) => evaluate(child));
+      }
+      case 'not': {
+        return !evaluate(operands[0]);
+      }
+      default: {
+        throw new Error(`${operator} is not a supported operator`);
+      }
+    }
+  } else {
+    return expression === 'true';
+  }
+}
+
+//// [arrayDestructuringInSwitch1.js]
+"use strict";
+exports.__esModule = true;
+exports.evaluate = void 0;
+function evaluate(expression) {
+    if (Array.isArray(expression)) {
+        var operator = expression[0], operands = expression.slice(1);
+        switch (operator) {
+            case 'and': {
+                return operands.every(function (child) { return evaluate(child); });
+            }
+            case 'not': {
+                return !evaluate(operands[0]);
+            }
+            default: {
+                throw new Error("".concat(operator, " is not a supported operator"));
+            }
+        }
+    }
+    else {
+        return expression === 'true';
+    }
+}
+exports.evaluate = evaluate;

tests/baselines/reference/arrayDestructuringInSwitch1.symbols

@@ -0,0 +1,55 @@
+=== tests/cases/compiler/arrayDestructuringInSwitch1.ts ===
+export type Expression = BooleanLogicExpression | 'true' | 'false';
+>Expression : Symbol(Expression, Decl(arrayDestructuringInSwitch1.ts, 0, 0))
+>BooleanLogicExpression : Symbol(BooleanLogicExpression, Decl(arrayDestructuringInSwitch1.ts, 0, 67))
+
+export type BooleanLogicExpression = ['and', ...Expression[]] | ['not', Expression];
+>BooleanLogicExpression : Symbol(BooleanLogicExpression, Decl(arrayDestructuringInSwitch1.ts, 0, 67))
+>Expression : Symbol(Expression, Decl(arrayDestructuringInSwitch1.ts, 0, 0))
+>Expression : Symbol(Expression, Decl(arrayDestructuringInSwitch1.ts, 0, 0))
+
+export function evaluate(expression: Expression): boolean {
+>evaluate : Symbol(evaluate, Decl(arrayDestructuringInSwitch1.ts, 1, 84))
+>expression : Symbol(expression, Decl(arrayDestructuringInSwitch1.ts, 3, 25))
+>Expression : Symbol(Expression, Decl(arrayDestructuringInSwitch1.ts, 0, 0))
+
+  if (Array.isArray(expression)) {
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.es5.d.ts, --, --))
+>expression : Symbol(expression, Decl(arrayDestructuringInSwitch1.ts, 3, 25))
+
+    const [operator, ...operands] = expression;
+>operator : Symbol(operator, Decl(arrayDestructuringInSwitch1.ts, 5, 11))
+>operands : Symbol(operands, Decl(arrayDestructuringInSwitch1.ts, 5, 20))
+>expression : Symbol(expression, Decl(arrayDestructuringInSwitch1.ts, 3, 25))
+
+    switch (operator) {
+>operator : Symbol(operator, Decl(arrayDestructuringInSwitch1.ts, 5, 11))
+
+      case 'and': {
+        return operands.every((child) => evaluate(child));
+>operands.every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>operands : Symbol(operands, Decl(arrayDestructuringInSwitch1.ts, 5, 20))
+>every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>child : Symbol(child, Decl(arrayDestructuringInSwitch1.ts, 8, 31))
+>evaluate : Symbol(evaluate, Decl(arrayDestructuringInSwitch1.ts, 1, 84))
+>child : Symbol(child, Decl(arrayDestructuringInSwitch1.ts, 8, 31))
+      }
+      case 'not': {
+        return !evaluate(operands[0]);
+>evaluate : Symbol(evaluate, Decl(arrayDestructuringInSwitch1.ts, 1, 84))
+>operands : Symbol(operands, Decl(arrayDestructuringInSwitch1.ts, 5, 20))
+>0 : Symbol(0)
+      }
+      default: {
+        throw new Error(`${operator} is not a supported operator`);
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>operator : Symbol(operator, Decl(arrayDestructuringInSwitch1.ts, 5, 11))
+      }
+    }
+  } else {
+    return expression === 'true';
+>expression : Symbol(expression, Decl(arrayDestructuringInSwitch1.ts, 3, 25))
+  }
+}

tests/baselines/reference/arrayDestructuringInSwitch1.types

@@ -0,0 +1,66 @@
+=== tests/cases/compiler/arrayDestructuringInSwitch1.ts ===
+export type Expression = BooleanLogicExpression | 'true' | 'false';
+>Expression : Expression
+
+export type BooleanLogicExpression = ['and', ...Expression[]] | ['not', Expression];
+>BooleanLogicExpression : BooleanLogicExpression
+
+export function evaluate(expression: Expression): boolean {
+>evaluate : (expression: Expression) => boolean
+>expression : Expression
+
+  if (Array.isArray(expression)) {
+>Array.isArray(expression) : boolean
+>Array.isArray : (arg: any) => arg is any[]
+>Array : ArrayConstructor
+>isArray : (arg: any) => arg is any[]
+>expression : Expression
+
+    const [operator, ...operands] = expression;
+>operator : "and" | "not"
+>operands : Expression[] | [Expression]
+>expression : BooleanLogicExpression
+
+    switch (operator) {
+>operator : "and" | "not"
+
+      case 'and': {
+>'and' : "and"
+
+        return operands.every((child) => evaluate(child));
+>operands.every((child) => evaluate(child)) : boolean
+>operands.every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; } | { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
+>operands : Expression[] | [Expression]
+>every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; } | { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
+>(child) => evaluate(child) : (child: Expression) => boolean
+>child : Expression
+>evaluate(child) : boolean
+>evaluate : (expression: Expression) => boolean
+>child : Expression
+      }
+      case 'not': {
+>'not' : "not"
+
+        return !evaluate(operands[0]);
+>!evaluate(operands[0]) : boolean
+>evaluate(operands[0]) : boolean
+>evaluate : (expression: Expression) => boolean
+>operands[0] : Expression
+>operands : Expression[] | [Expression]
+>0 : 0
+      }
+      default: {
+        throw new Error(`${operator} is not a supported operator`);
+>new Error(`${operator} is not a supported operator`) : Error
+>Error : ErrorConstructor
+>`${operator} is not a supported operator` : string
+>operator : never
+      }
+    }
+  } else {
+    return expression === 'true';
+>expression === 'true' : boolean
+>expression : "true" | "false"
+>'true' : "true"
+  }
+}

tests/baselines/reference/arrayDestructuringInSwitch2.errors.txt

@@ -0,0 +1,20 @@
+tests/cases/compiler/arrayDestructuringInSwitch2.ts(11,13): error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+
+
+==== tests/cases/compiler/arrayDestructuringInSwitch2.ts (1 errors) ====
+    type X = { kind: "a", a: [1] } | { kind: "b", a: [] }
+
+    function foo(x: X): 1 {
+      const { kind, a } = x;
+      switch (kind) {
+        case "a":
+          return a[0];
+        case "b":
+          return 1;
+        default:
+          const [n] = a;
+                ~~~
+!!! error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+          return a;
+      }
+    }

tests/baselines/reference/arrayDestructuringInSwitch2.js

@@ -0,0 +1,29 @@
+//// [arrayDestructuringInSwitch2.ts]
+type X = { kind: "a", a: [1] } | { kind: "b", a: [] }
+
+function foo(x: X): 1 {
+  const { kind, a } = x;
+  switch (kind) {
+    case "a":
+      return a[0];
+    case "b":
+      return 1;
+    default:
+      const [n] = a;
+      return a;
+  }
+}
+
+//// [arrayDestructuringInSwitch2.js]
+function foo(x) {
+    var kind = x.kind, a = x.a;
+    switch (kind) {
+        case "a":
+            return a[0];
+        case "b":
+            return 1;
+        default:
+            var n = a[0];
+            return a;
+    }
+}

tests/baselines/reference/arrayDestructuringInSwitch2.symbols

@@ -0,0 +1,37 @@
+=== tests/cases/compiler/arrayDestructuringInSwitch2.ts ===
+type X = { kind: "a", a: [1] } | { kind: "b", a: [] }
+>X : Symbol(X, Decl(arrayDestructuringInSwitch2.ts, 0, 0))
+>kind : Symbol(kind, Decl(arrayDestructuringInSwitch2.ts, 0, 10))
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 0, 21))
+>kind : Symbol(kind, Decl(arrayDestructuringInSwitch2.ts, 0, 34))
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 0, 45))
+
+function foo(x: X): 1 {
+>foo : Symbol(foo, Decl(arrayDestructuringInSwitch2.ts, 0, 53))
+>x : Symbol(x, Decl(arrayDestructuringInSwitch2.ts, 2, 13))
+>X : Symbol(X, Decl(arrayDestructuringInSwitch2.ts, 0, 0))
+
+  const { kind, a } = x;
+>kind : Symbol(kind, Decl(arrayDestructuringInSwitch2.ts, 3, 9))
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 3, 15))
+>x : Symbol(x, Decl(arrayDestructuringInSwitch2.ts, 2, 13))
+
+  switch (kind) {
+>kind : Symbol(kind, Decl(arrayDestructuringInSwitch2.ts, 3, 9))
+
+    case "a":
+      return a[0];
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 3, 15))
+>0 : Symbol(0)
+
+    case "b":
+      return 1;
+    default:
+      const [n] = a;
+>n : Symbol(n, Decl(arrayDestructuringInSwitch2.ts, 10, 13))
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 3, 15))
+
+      return a;
+>a : Symbol(a, Decl(arrayDestructuringInSwitch2.ts, 3, 15))
+  }
+}

tests/baselines/reference/arrayDestructuringInSwitch2.types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/arrayDestructuringInSwitch2.ts ===
+type X = { kind: "a", a: [1] } | { kind: "b", a: [] }
+>X : X
+>kind : "a"
+>a : [1]
+>kind : "b"
+>a : []
+
+function foo(x: X): 1 {
+>foo : (x: X) => 1
+>x : X
+
+  const { kind, a } = x;
+>kind : "a" | "b"
+>a : [1] | []
+>x : X
+
+  switch (kind) {
+>kind : "a" | "b"
+
+    case "a":
+>"a" : "a"
+
+      return a[0];
+>a[0] : 1
+>a : [1]
+>0 : 0
+
+    case "b":
+>"b" : "b"
+
+      return 1;
+>1 : 1
+
+    default:
+      const [n] = a;
+>n : never
+>a : never
+
+      return a;
+>a : never
+  }
+}

tests/cases/compiler/arrayDestructuringInSwitch1.ts

@@ -0,0 +1,21 @@
+export type Expression = BooleanLogicExpression | 'true' | 'false';
+export type BooleanLogicExpression = ['and', ...Expression[]] | ['not', Expression];
+
+export function evaluate(expression: Expression): boolean {
+  if (Array.isArray(expression)) {
+    const [operator, ...operands] = expression;
+    switch (operator) {
+      case 'and': {
+        return operands.every((child) => evaluate(child));
+      }
+      case 'not': {
+        return !evaluate(operands[0]);
+      }
+      default: {
+        throw new Error(`${operator} is not a supported operator`);
+      }
+    }
+  } else {
+    return expression === 'true';
+  }
+}

tests/cases/compiler/arrayDestructuringInSwitch2.ts

@@ -0,0 +1,14 @@
+type X = { kind: "a", a: [1] } | { kind: "b", a: [] }
+
+function foo(x: X): 1 {
+  const { kind, a } = x;
+  switch (kind) {
+    case "a":
+      return a[0];
+    case "b":
+      return 1;
+    default:
+      const [n] = a;
+      return a;
+  }
+}