[ms] Prevent explicit constructor name lookup if scope is missing

[lantictac]

Summary

MicrosoftExt allows explicit constructor calls. Prevent lookup of constructor name unless the name has explicit scope.
This avoids a compile-time crash due to confusing a member access for a constructor name.

Test case included. All tests pass.

[rnk]

lgtm, thanks!

lib/Parse/ParseExpr.cpp

case tok::period: {
         // Objective-C method names, one must use the message send syntax.
         IdentifierInfo *Id = Tok.getIdentifierInfo();
         SourceLocation Loc = ConsumeToken();
         Name.setIdentifier(Id, Loc);
       } else if (ParseUnqualifiedId(SS,
                                     /*EnteringContext=*/false,
                                     /*AllowDestructorName=*/true,
                                     /*AllowConstructorName=*/
-                                      getLangOpts().MicrosoftExt,
+                                    getLangOpts().MicrosoftExt &&
+                                        SS.isNotEmpty(),
                                     /*AllowDeductionGuide=*/false,
                                     ObjectType, &TemplateKWLoc, Name)) {
         (void)Actions.CorrectDelayedTyposInExpr(LHS);
         LHS = ExprError();
       }
 
       if (!LHS.isInvalid())
         LHS = Actions.ActOnMemberAccessExpr(getCurScope(), LHS.get(), OpLoc,

test/SemaCXX/MicrosoftCompatibility.cpp

 long function_prototype(int a);
 long (*function_ptr)(int a);
 
 void function_to_voidptr_conv() {
   void *a1 = function_prototype;  // expected-warning {{implicit conversion between pointer-to-function and pointer-to-object is a Microsoft extension}}
   void *a2 = &function_prototype; // expected-warning {{implicit conversion between pointer-to-function and pointer-to-object is a Microsoft extension}}
   void *a3 = function_ptr;        // expected-warning {{implicit conversion between pointer-to-function and pointer-to-object is a Microsoft extension}}
 }
+
+namespace member_lookup {
+
+template<typename T>
+struct ConfuseLookup {
+  T* m_val;
+  struct m_val {
+    static size_t ms_test;
+  };
+};
+
+// Microsoft mode allows explicit constructor calls
+// This could confuse name lookup in cases such as this
+template<typename T>
+size_t ConfuseLookup<T>::m_val::ms_test
+  = size_t(&(char&)(reinterpret_cast<ConfuseLookup<T>*>(0)->m_val));
+
+void instantiate() { ConfuseLookup<int>::m_val::ms_test = 1; }
+}
+