Merge pull request #5920 from Microsoft/fixUnionTypeInference

Fix slowdown in type inference involving union types

Author: ahejlsberg

src/compiler/checker.ts

@@ -6178,9 +6178,12 @@ namespace ts {
                 }
                 else {
                     source = getApparentType(source);
-                    if (source.flags & TypeFlags.ObjectType && (target.flags & (TypeFlags.Reference | TypeFlags.Tuple) ||
-                        (target.flags & TypeFlags.Anonymous) && target.symbol && target.symbol.flags & (SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class))) {
-                        // If source is an object type, and target is a type reference, a tuple type, the type of a method, or a type literal, infer from members
+                    if (source.flags & TypeFlags.ObjectType && (
+                        target.flags & TypeFlags.Reference && (<TypeReference>target).typeArguments ||
+                        target.flags & TypeFlags.Tuple ||
+                        target.flags & TypeFlags.Anonymous && target.symbol && target.symbol.flags & (SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class))) {
+                        // If source is an object type, and target is a type reference with type arguments, a tuple type,
+                        // the type of a method, or a type literal, infer from members
                         if (isInProcess(source, target)) {
                             return;
                         }