[ms] Don't try to delay lookup for lookup or overload resolution failures in SFINAE context (PR23823)

[hans]

Summary

The underlying problem in PR23823 already existed before my recent change in r239558, but that change made it worse (failing not only for undeclared symbols, but also failed overload resolution). This makes us not try to delay the lookup in SFINAE context. I assume no current code is relying on SFINAE working with lookups that need to be delayed, because that never seems to have worked :-)

Please take a look!

[rsmith]

LGTM

lib/Sema/SemaOverload.cpp

if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts)) {
     *Result = ExprError();
     return true;
   }
 
   // Add the functions denoted by the callee to the set of candidate
   // functions, including those from argument-dependent lookup.
   AddOverloadedCallCandidates(ULE, Args, *CandidateSet);
 
-  if (getLangOpts().MSVCCompat && CurContext->isDependentContext() &&
+  if (getLangOpts().MSVCCompat &&
+      CurContext->isDependentContext() && !isSFINAEContext() &&
       (isa<FunctionDecl>(CurContext) || isa<CXXRecordDecl>(CurContext))) {
 
     OverloadCandidateSet::iterator Best;
     if (CandidateSet->empty() ||
         CandidateSet->BestViableFunction(*this, Fn->getLocStart(), Best) ==
             OR_No_Viable_Function) {
       // In Microsoft mode, if we are inside a template class member function then
       // create a type dependent CallExpr. The goal is to postpone name lookup

test/SemaTemplate/ms-lookup-template-base-classes.cpp

void member() {
     f(); // expected-warning {{found via unqualified lookup into dependent bases}}
   }
 };
 void test() {
   Template<Base> x;
   x.member(); // expected-note{{requested here}}
 };
 }
+
+namespace PR23823 {
+// Don't delay lookup in SFINAE context.
+template <typename T> decltype(g(T())) check(); // expected-note{{candidate template ignored: substitution failure [with T = int]: use of undeclared identifier 'g'}}
+decltype(check<int>()) x; // expected-error{{no matching function for call to 'check'}}
+
+void h();
+template <typename T> decltype(h(T())) check2(); // expected-note{{candidate template ignored: substitution failure [with T = int]: no matching function for call to 'h'}}
+decltype(check2<int>()) y; // expected-error{{no matching function for call to 'check2'}}
+}