Check constraints deeply on singleton types.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -15764,23 +15764,13 @@ namespace ts {
                     }
                 }
 
-                if (isLiteralType(source) && !typeCouldHaveNoTopLevelSingletonTypes(target)) {
+                if (isLiteralType(source) && !typeCouldHaveTopLevelSingletonTypes(target)) {
                     generalizedSourceType = getTypeNameForErrorDisplay(getBaseTypeOfLiteralType(source));
                 }
 
                 reportError(message, generalizedSourceType, targetType);
             }
 
-            function typeCouldHaveNoTopLevelSingletonTypes(type: Type) {
-                return forEachType(type, typeCouldHaveNoTopLevelSingletonTypesWorker);
-            }
-
-            function typeCouldHaveNoTopLevelSingletonTypesWorker(type: Type): boolean {
-                return (type.flags & TypeFlags.Intersection)
-                    ? !!forEach((type as IntersectionType).types, typeCouldHaveNoTopLevelSingletonTypesWorker)
-                    : isUnitType(type) || !!(type.flags & TypeFlags.Instantiable);
-            }
-
             function tryElaborateErrorsForPrimitivesAndObjects(source: Type, target: Type) {
                 const sourceType = symbolValueDeclarationIsContextSensitive(source.symbol) ? typeToString(source, source.symbol.valueDeclaration) : typeToString(source);
                 const targetType = symbolValueDeclarationIsContextSensitive(target.symbol) ? typeToString(target, target.symbol.valueDeclaration) : typeToString(target);
@@ -17375,6 +17365,21 @@ namespace ts {
             }
         }
 
+        function typeCouldHaveTopLevelSingletonTypes(type: Type): boolean {
+            if (type.flags & TypeFlags.UnionOrIntersection) {
+                return !!forEach((type as IntersectionType).types, typeCouldHaveTopLevelSingletonTypes);
+            }
+
+            if (type.flags & TypeFlags.Instantiable) {
+                const constraint = getConstraintOfType(type);
+                if (constraint) {
+                    return typeCouldHaveTopLevelSingletonTypes(constraint);
+                }
+            }
+
+            return isUnitType(type);
+        }
+
         function getBestMatchingType(source: Type, target: UnionOrIntersectionType, isRelatedTo = compareTypesAssignable) {
             return findMatchingDiscriminantType(source, target, isRelatedTo, /*skipPartial*/ true) ||
                 findMatchingTypeReferenceOrTypeAliasReference(source, target) ||