experiment

src/compiler/checker.ts

@@ -19311,6 +19311,7 @@ namespace ts {
 
             function getTypeAtFlowBranchLabel(flow: FlowLabel): FlowType {
                 const antecedentTypes: Type[] = [];
+                const containedUnions: Type[] = [];
                 let subtypeReduction = false;
                 let seenIncomplete = false;
                 for (const antecedent of flow.antecedents!) {
@@ -19330,6 +19331,7 @@ namespace ts {
                         return type;
                     }
                     pushIfUnique(antecedentTypes, type);
+                    pushIfUnique(containedUnions, containingUnion || type);
                     // If an antecedent type is not a subset of the declared type, we need to perform
                     // subtype reduction. This happens when a "foreign" type is injected into the control
                     // flow using the instanceof operator or a user defined type predicate.
@@ -19340,6 +19342,8 @@ namespace ts {
                         seenIncomplete = true;
                     }
                 }
+                const containingUnionType = createFlowType(getUnionOrEvolvingArrayType(containedUnions, subtypeReduction ? UnionReduction.Subtype : UnionReduction.Literal), seenIncomplete);
+                captureContainingUnion(containingUnionType);
                 return createFlowType(getUnionOrEvolvingArrayType(antecedentTypes, subtypeReduction ? UnionReduction.Subtype : UnionReduction.Literal), seenIncomplete);
             }