[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

bool VisitStmtExpr(const StmtExpr *E) {
 
     llvm_unreachable("Return from function from the loop above.");
   }
 
   /// Visit a value which is evaluated, but whose value is ignored.
   void VisitIgnoredValue(const Expr *E) {
     EvaluateIgnoredValue(Info, E);
   }
+
+  /// Potentially visit a MemberExpr's base expression.
+  void VisitIgnoredBaseExpression(const Expr *E) {
+    // While MSVC doesn't evaluate the base expression, it does diagnose the
+    // presence of side-effecting behavior.
+    if (Info.getLangOpts().MSVCCompat && !E->HasSideEffects(Info.Ctx))
+      return;
+    VisitIgnoredValue(E);
+  }
 };
 
 }
 
 //===----------------------------------------------------------------------===//
 // Common base class for lvalue and temporary evaluation.
 //===----------------------------------------------------------------------===//
 namespace {

 
 bool LValueExprEvaluator::VisitCXXUuidofExpr(const CXXUuidofExpr *E) {
   return Success(E);
 }
 
 bool LValueExprEvaluator::VisitMemberExpr(const MemberExpr *E) {
   // Handle static data members.
   if (const VarDecl *VD = dyn_cast<VarDecl>(E->getMemberDecl())) {
-    VisitIgnoredValue(E->getBase());
+    VisitIgnoredBaseExpression(E->getBase());
     return VisitVarDecl(E, VD);
   }
 
   // Handle static member functions.
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(E->getMemberDecl())) {
     if (MD->isStatic()) {
-      VisitIgnoredValue(E->getBase());
+      VisitIgnoredBaseExpression(E->getBase());
       return Success(MD);
     }
   }
 
   // Handle non-static data members.
   return LValueExprEvaluatorBaseTy::VisitMemberExpr(E);
 }
 
public:
   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())) {
-      VisitIgnoredValue(E->getBase());
+      VisitIgnoredBaseExpression(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 };
+  static const int sdm = 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}}
+
+constexpr int e4 = s->sdm;