Fix undeduced type when instanciating template member

[serge-sans-paille]

Summary

Basically, if the instantiation of a member variable makes it possible to compute a previously undeduced type, we should use that piece of information.

Should fix https://bugs.llvm.org/show_bug.cgi?id=50590

[aaron.ballman]

I'm not 100% certain these changes are correct because template instantiation isn't always the easiest thing to reason about, so adding @rsmith for his expertise in the area.

That said, if this change is along the lines of correct, I think a similar change is needed in MemberExpr::setMemberDecl() as well.

There are some SemaCXX tests that we can add that this fixes as well. e.g.,

template <typename>
struct pack {
  template <typename T>
  constexpr static auto some_boolean_cx_value = true;
};

bool usage() { // Changing the return type here from `auto` to `bool` gives nonsense diagnostics in both the DeclRefExpr and MemberExpr cases
  return pack<char>{}.some_boolean_cx_value<int>; // This is the MemberExpr version of your test, which fails in Sema but not CodeGen
}

https://godbolt.org/z/KMeE9ssns

[aaron.ballman]

Adding the mailing lists to the review.

[serge-sans-paille]

Added test + fix suggested by @aaron.ballman

[aaron.ballman]

I think this is reasonable, but I'd like to hear from @rsmith before landing this.

[serge-sans-paille]

@rsmith : any opinion on that one?

[aaron.ballman]

This LGTM as far as the fix seems like an improvement, though I'm not 100% certain that the fix is in the right place. Please give a few more days for @rsmith to consider the changes before landing, but if we don't hear from him by Fri (6/25), go ahead and land.

[serge-sans-paille]

@rsmith : gentle ping ;-)

clang/lib/AST/Expr.cpp

std::size_t Size =
           HasQualifier ? 1 : 0, HasFoundDecl ? 1 : 0, HasTemplateKWAndArgsInfo,
           NumTemplateArgs);
   void *Mem = Context.Allocate(Size, alignof(DeclRefExpr));
   return new (Mem) DeclRefExpr(EmptyShell());
 }
 
 void DeclRefExpr::setDecl(ValueDecl *NewD) {
   D = NewD;
+  if (getType()->isUndeducedType())
+    setType(NewD->getType());
   setDependence(computeDependence(this, NewD->getASTContext()));
 }
 
 SourceLocation DeclRefExpr::getBeginLoc() const {
   if (hasQualifier())
     return getQualifierLoc().getBeginLoc();
   return getNameInfo().getBeginLoc();
 }
std::size_t Size =
       totalSizeToAlloc<MemberExprNameQualifier, ASTTemplateKWAndArgsInfo,
                        TemplateArgumentLoc>(HasQualOrFound ? 1 : 0,
                                             HasTemplateKWAndArgsInfo ? 1 : 0,
                                             NumTemplateArgs);
   void *Mem = Context.Allocate(Size, alignof(MemberExpr));
   return new (Mem) MemberExpr(EmptyShell());
 }
 
-void MemberExpr::setMemberDecl(ValueDecl *D) {
-  MemberDecl = D;
+void MemberExpr::setMemberDecl(ValueDecl *NewD) {
+  MemberDecl = NewD;
+  if (getType()->isUndeducedType())
+    setType(NewD->getType());
   setDependence(computeDependence(this));
 }
 
 SourceLocation MemberExpr::getBeginLoc() const {
   if (isImplicitAccess()) {
     if (hasQualifier())
       return getQualifierLoc().getBeginLoc();
     return MemberLoc;

clang/test/CodeGenCXX/auto-variable-template.cpp

 // RUN: %clang_cc1 -std=c++14 %s -triple=x86_64-linux -emit-llvm -o - | FileCheck %s
 
 struct f {
   void operator()() const {}
 };
 
 template <typename T> auto vtemplate = f{};
 
 int main() { vtemplate<int>(); }
 
 // CHECK: @_Z9vtemplateIiE = linkonce_odr global %struct.f undef, comdat
 
 // CHECK: define{{.*}} i32 @main()
 // CHECK: call void @_ZNK1fclEv(%struct.f* {{[^,]*}} @_Z9vtemplateIiE)
+
+template <typename>
+struct pack {
+  template <typename T>
+  constexpr static auto some_boolean_cx_value = true;
+};
+
+auto usage() {
+  return pack<char>::some_boolean_cx_value<int>;
+}
+
+// CHECK: define{{.*}} i1 @_Z5usagev()
+
+auto otherusage() {
+  return pack<char>{}.some_boolean_cx_value<int>;
+}
+
+// CHECK: define{{.*}} i1 @_Z10otherusagev()