Check assignability recursively when source is type parameter with union constraint

Author: JsonFreeman

src/compiler/checker.ts

@@ -4067,10 +4067,17 @@ module ts {
                     let reportStructuralErrors = reportErrors && errorInfo === saveErrorInfo;
                     // identity relation does not use apparent type
                     let sourceOrApparentType = relation === identityRelation ? source : getApparentType(source);
-                    if (sourceOrApparentType.flags & TypeFlags.ObjectType && target.flags & TypeFlags.ObjectType &&
-                        (result = objectTypeRelatedTo(sourceOrApparentType, <ObjectType>target, reportStructuralErrors))) {
-                        errorInfo = saveErrorInfo;
-                        return result;
+                    if (sourceOrApparentType.flags & TypeFlags.ObjectType && target.flags & TypeFlags.ObjectType) {
+                        if (result = objectTypeRelatedTo(sourceOrApparentType, <ObjectType>target, reportStructuralErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
+                    }
+                    else if (source.flags & TypeFlags.TypeParameter && sourceOrApparentType.flags & TypeFlags.Union) {
+                        if (result = isRelatedTo(sourceOrApparentType, <ObjectType>target, reportStructuralErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
                     }
                 }
                 if (reportErrors) {