Defer reduction of identical function types in unions and intersections

Author: ahejlsberg

src/compiler/checker.ts

@@ -8691,10 +8691,7 @@ namespace ts {
                     const len = typeSet.length;
                     const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearch(typeSet, type, getTypeId, compareValues);
                     if (index < 0) {
-                        if (!(flags & TypeFlags.Object && (<ObjectType>type).objectFlags & ObjectFlags.Anonymous &&
-                            type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method) && containsIdenticalType(typeSet, type))) {
-                            typeSet.splice(~index, 0, type);
-                        }
+                        typeSet.splice(~index, 0, type);
                     }
                 }
             }
@@ -8710,15 +8707,6 @@ namespace ts {
             return includes;
         }
 
-        function containsIdenticalType(types: ReadonlyArray<Type>, type: Type) {
-            for (const t of types) {
-                if (isTypeIdenticalTo(t, type)) {
-                    return true;
-                }
-            }
-            return false;
-        }
-
         function isSubtypeOfAny(source: Type, targets: ReadonlyArray<Type>): boolean {
             for (const target of targets) {
                 if (source !== target && isTypeSubtypeOf(source, target) && (
@@ -8899,10 +8887,7 @@ namespace ts {
                 if (flags & TypeFlags.AnyOrUnknown) {
                     if (type === wildcardType) includes |= TypeFlags.Wildcard;
                 }
-                else if ((strictNullChecks || !(flags & TypeFlags.Nullable)) && !contains(typeSet, type) &&
-                    !(flags & TypeFlags.Object && (<ObjectType>type).objectFlags & ObjectFlags.Anonymous &&
-                    type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method) &&
-                    containsIdenticalType(typeSet, type))) {
+                else if ((strictNullChecks || !(flags & TypeFlags.Nullable)) && !contains(typeSet, type)) {
                     typeSet.push(type);
                 }
             }