[MSVC Compat] Don't evaluate member base expressions w/o side effects

[majnemer]

Summary

A member expression's base doesn't always have an impact on what the
member decl would evaluate to. In such a case, the base is used as a
poor man's scope qualifier.

This fixes PR26738.

[majnemer]

• Add support for static data members too.

[majnemer]

• Refactor this change.

[rsmith]

Looks terrible, please commit :)

[andreybokhanko]

@majnemer, this error in the header is now gone. Thank you! On to more errors! ;-)

Yours,
Andrey

lib/AST/ExprConstant.cpp

public:
   bool CheckReferencedDecl(const Expr *E, const Decl *D);
   bool VisitDeclRefExpr(const DeclRefExpr *E) {
     if (CheckReferencedDecl(E, E->getDecl()))
       return true;
 
     return ExprEvaluatorBaseTy::VisitDeclRefExpr(E);
   }
   bool VisitMemberExpr(const MemberExpr *E) {
-    if (CheckReferencedDecl(E, E->getMemberDecl())) {
+    ValueDecl *MemberDecl = E->getMemberDecl();
+    ASTContext &Ctx = MemberDecl->getASTContext();
+    if (CheckReferencedDecl(E, MemberDecl)) {
+      // While MSVC doesn't evaluate the base expression, it does diagnose the
+      // presence of side-effecting behavior.
+      if (!Info.getLangOpts().MSVCCompat || E->getBase()->HasSideEffects(Ctx)) {
         VisitIgnoredValue(E->getBase());
+      }
       return true;
     }
 
     return ExprEvaluatorBaseTy::VisitMemberExpr(E);
   }
 
   bool VisitCallExpr(const CallExpr *E);
   bool VisitBinaryOperator(const BinaryOperator *E);

test/SemaCXX/ms-const-member-expr.cpp

+// RUN: %clang_cc1 %s -std=c++11 -fms-compatibility -fsyntax-only -verify
+
+struct S {
+  enum { E = 1 };
+};
+
+void f(S *s) {
+  char array[s->E] = { 0 };
+}
+
+extern S *s;
+constexpr int e1 = s->E;
+
+S *side_effect();  // expected-note{{declared here}}
+constexpr int e2 = // expected-error{{must be initialized by a constant expression}}
+    side_effect()->E; // expected-note{{cannot be used in a constant expression}}