[MSVC] PR27337: allow static_cast from private base to derived for WTL

[DmitryPolukhin]

Summary

MSVC doesn't report even warning for cast from private base class to derived.

[rnk]

lgtm

include/clang/Basic/DiagnosticSemaKinds.td

5768

Maybe:

casting from %select{private|protected}2 base class %1 to derived class %0 is a Microsoft extension

[DmitryPolukhin]

Thank you for the review!

include/clang/Basic/DiagnosticSemaKinds.td

 // These messages don't adhere to the pattern.
 // FIXME: Display the path somehow better.
 def err_ambiguous_base_to_derived_cast : Error<
   "ambiguous cast from base %0 to derived %1:%2">;
 def err_static_downcast_via_virtual : Error<
   "cannot cast %0 to %1 via virtual base %2">;
 def err_downcast_from_inaccessible_base : Error<
   "cannot cast %select{private|protected}2 base class %1 to %0">;
+def ext_ms_downcast_from_inaccessible_base : ExtWarn<
+  "cast %select{private|protected}2 base class %1 to %0 is a Microsoft extension">,
+  InGroup<MicrosoftCast>;
 def err_upcast_to_inaccessible_base : Error<
   "cannot cast %0 to its %select{private|protected}2 base class %1">;
 def err_bad_dynamic_cast_not_ref_or_ptr : Error<
   "%0 is not a reference or pointer">;
 def err_bad_dynamic_cast_not_class : Error<"%0 is not a class">;
 def err_bad_dynamic_cast_incomplete : Error<"%0 is an incomplete type">;
 def err_bad_dynamic_cast_not_ptr : Error<"%0 is not a pointer">;
 def err_bad_dynamic_cast_not_polymorphic : Error<"%0 is not polymorphic">;

lib/Sema/SemaCast.cpp

if (Paths.getDetectedVirtual() != nullptr) {
     QualType VirtualBase(Paths.getDetectedVirtual(), 0);
     Self.Diag(OpRange.getBegin(), diag::err_static_downcast_via_virtual)
       << OrigSrcType << OrigDestType << VirtualBase << OpRange;
     msg = 0;
     return TC_Failed;
   }
 
   if (!CStyle) {
-    switch (Self.CheckBaseClassAccess(OpRange.getBegin(),
-                                      SrcType, DestType,
-                                      Paths.front(),
-                                diag::err_downcast_from_inaccessible_base)) {
+    unsigned Diag = Self.getLangOpts().MSVCCompat
+                        ? diag::ext_ms_downcast_from_inaccessible_base
+                        : diag::err_downcast_from_inaccessible_base;
+    switch (Self.CheckBaseClassAccess(OpRange.getBegin(), SrcType, DestType,
+                                      Paths.front(), Diag)) {
     case Sema::AR_accessible:
     case Sema::AR_delayed:     // be optimistic
     case Sema::AR_dependent:   // be optimistic
       break;
 
     case Sema::AR_inaccessible:
       msg = 0;
       return TC_Failed;

test/SemaCXX/ext_ms_downcast.cpp

+// RUN: %clang_cc1 -fsyntax-only -fms-compatibility -verify %s
+// RUN: %clang_cc1 -fsyntax-only -DNO_MS_COMPATIBILITY -verify %s
+
+// Minimal reproducer.
+class A {};
+class B : A {}; // expected-note 2 {{implicitly declared private here}}
+
+B* foo(A* p) {
+  return static_cast<B*>(p);
+#ifdef NO_MS_COMPATIBILITY
+  // expected-error@-2 {{cannot cast private base class 'A' to 'B'}}
+#else
+  // expected-warning@-4 {{cast private base class 'A' to 'B' is a Microsoft extension}}
+#endif
+}
+
+A* bar(B* p) {
+  return static_cast<A*>(p); // expected-error {{cannot cast 'B' to its private base class 'A'}}
+}
+
+// from atlframe.h
+template <class T>
+class CUpdateUI {
+public:
+  CUpdateUI() {
+    T* pT = static_cast<T*>(this);
+#ifdef NO_MS_COMPATIBILITY
+    // expected-error@-2 {{cannot cast private base class}}
+#else
+    // expected-warning@-4 {{cast private base class 'CUpdateUI<CMDIFrame>' to 'CMDIFrame' is a Microsoft extension}}
+#endif
+  }
+};
+
+// from sample WTL/MDIDocVw (mainfrm.h
+class CMDIFrame : CUpdateUI<CMDIFrame> {};
+// expected-note@-1 {{implicitly declared private here}}
+// expected-note@-2 {{in instantiation of member function}}
+
+CMDIFrame wndMain;