Automerge: Create a Template member to be the MemberSpec of a failed TemplVarDecl (#209604)

Patch #200092 changed to no longer check the previous var template when
setting whether the current one is a member specialization. However, if
the previous one was actually an error case (see the example here and in
the report), we ended up trying to do that anyway, which caused an
assertion.

This patch puts in a 'fake' declaration for the not-found declaration after we
emit the 'not found' error for the purposes of allowing our diagnostics to
continue evaluating these without causing problems.

Fixes: #209432
(cherry picked from commit 2c2e436)

Author: erichkeane

clang/lib/Sema/SemaDecl.cpp

@@ -8374,6 +8374,21 @@ NamedDecl *Sema::ActOnVariableDeclarator(
         << Name << computeDeclContext(D.getCXXScopeSpec(), true)
         << D.getCXXScopeSpec().getRange();
       NewVD->setInvalidDecl();
+
+      // if this is a member specialization, we don't have any primary template
+      // to be instantiated from. We set ourselves to a 'fake' clone of this so
+      // that anything that attempts to refer to this invalid declaration can
+      // act as if there IS a primary instantiation.
+      if (NewTemplate && IsMemberSpecialization) {
+        VarDecl *FakeVD =
+            VarDecl::Create(Context, DC, D.getBeginLoc(), D.getIdentifierLoc(),
+                            II, R, TInfo, SC);
+        FakeVD->setInvalidDecl();
+        VarTemplateDecl *FakeInstantiatedFrom = VarTemplateDecl::Create(
+            Context, DC, D.getIdentifierLoc(), Name, TemplateParams, FakeVD);
+        FakeInstantiatedFrom->setInvalidDecl();
+        NewTemplate->setInstantiatedFromMemberTemplate(FakeInstantiatedFrom);
+      }
     }
 
     if (!IsPlaceholderVariable)

clang/test/SemaTemplate/class-template-spec.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -triple x86_64-linux-gnu -verify=expected,cxx14 -std=c++14 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 %s
 
 template<typename T, typename U = int> struct A; // expected-note {{template is declared here}} \
@@ -251,4 +252,24 @@ namespace VarTemplateMismatch {
   template<> template<int> const int A<short>::x = 0;
   // expected-error@-1 {{template non-type parameter has a different type 'int' in template redeclaration}}
 } // namespace VarTemplateMismatch
+
+namespace VarTemplateNoMember {
+  template<typename T> struct S {};
+  // expected-error@+1{{no member named 'foo' in 'VarTemplateNoMember::S<long>'}}
+  template<> template<typename U> constexpr int S<long>::foo;
+  // In C++14, these are definitions, not declarations, so they get a
+  // redefinition error.
+  // cxx14-error@+2{{redefinition of 'foo'}}
+  // cxx14-note@-4{{previous definition is here}}
+  template<> template<typename U> constexpr int S<long>::foo;
+  // cxx14-error@+2{{redefinition of 'foo'}}
+  // cxx14-note@-2{{previous definition is here}}
+  template<> template<typename U> constexpr int S<long>::foo;
+  // cxx14-error@+2{{redefinition of 'foo'}}
+  // cxx14-note@-2{{previous definition is here}}
+  template<> template<typename U> constexpr int S<long>::foo;
+  // cxx14-error@+2{{redefinition of 'foo'}}
+  // cxx14-note@-2{{previous definition is here}}
+  template<> template<typename U> constexpr int S<long>::foo;
+} // namespace VarTemplateNoMember
 #endif