[clang] Store the template param list of an explicit variable template specialization

[nridge]

Summary

VarTemplateSpecializationDecl does not store a template param list,
so the "template<>" needs to be stored in the ExtInfo.

[nridge]

So far the only test I added is in clangd semantic highlighting because that's how I noticed the template<> missing from the AST, but I'm open to suggestions on where to add a test at the clang layer.

[shafik]

Adding clang-language-wg for more visibility.

[erichkeane]

We have a good amount of AST-dump tests around that are likely the right place to put them perhaps? What is the reproducer that caused you to discover this issue?

[nridge]

In D142692#4090867, @erichkeane wrote:

We have a good amount of AST-dump tests around that are likely the right place to put them perhaps? What is the reproducer that caused you to discover this issue?

The following is sufficient to reproduce:

template <typename T>
T x = {};

template <>
int x<int> = 42;

However, the patch does not alter the ast-dump (it looks like the template param lists stored in DeclaratorDecl's getExtInfo()->TemplParamLists are not printed).

[nridge]

Review ping :)

[erichkeane]

sorry for the delay, just started unburying myself from the 16.0 release process. Its unfortunate we don't have a great way to test this, but I can see the value of having it anyway in clangd.

clang-tools-extra/clangd/unittests/SemanticHighlightingTests.cpp

)cpp",
         struct $Class_def[[Base]]{};
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]]
         struct $Class_def[[S]] : public $Class[[Base]] {};
         template $Bracket[[<]]$Bracket[[>]]
         struct $Class_def[[S]]$Bracket[[<]]void$Bracket[[>]] : public $Class[[Base]] {};
 
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]]
         $TemplateParameter[[T]] $Variable_def[[x]] = {};
-        template <>
+        template $Bracket[[<]]$Bracket[[>]]
         int $Variable_def[[x]]$Bracket[[<]]int$Bracket[[>]] = (int)sizeof($Class[[Base]]);
       )cpp",
       // operator calls in template
       R"cpp(
         template$Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]] class $Class_def[[C]] {
             bool $Method_def[[compare]]($TemplateParameter[[T]] $Parameter_def[[t1]], $TemplateParameter[[T]] $Parameter_def[[t2]]) { return $Parameter[[t1]] $Operator_userDefined[[==]] $Parameter[[t2]]; }
             $TemplateParameter[[T]] $Method_def[[deref]]($TemplateParameter[[T]] *$Parameter_def[[t]]) { return $Operator_userDefined[[*]]$Parameter[[t]]; }
         };
$Bracket[[>]] $LocalVariable_def[[s7]];
         template $Bracket[[<]]$Bracket[[>]]
         class $Class[[S]]$Bracket[[<]]float$Bracket[[>]]::$Class_def[[Nested]]$Bracket[[<]]float$Bracket[[>]] {};
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]] void $Function_decl[[foo]]();
         void $Function_def[[bar]]() {
           $Function[[foo]]$Bracket[[<]]int$Bracket[[>]]();
         }
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]] constexpr int $Variable_def_readonly[[V]] = 42;
         constexpr int $Variable_def_readonly[[Y]] = $Variable_readonly[[V]]$Bracket[[<]]char$Bracket[[>]];
-        template <>
+        template $Bracket[[<]]$Bracket[[>]]
         constexpr int $Variable_def_readonly[[V]]$Bracket[[<]]int$Bracket[[>]] = 5;
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]]
         constexpr int $Variable_def_readonly[[V]]$Bracket[[<]]$TemplateParameter[[T]]*$Bracket[[>]] = 6;
         template $Bracket[[<]]typename$Bracket[[>]]
         class $Class_def[[A]] {
           enum class $Enum_decl[[E]];
         };
         template $Bracket[[<]]typename $TemplateParameter_def[[T]]$Bracket[[>]]

clang/lib/Sema/SemaDecl.cpp

if (D.isInvalidType() || Invalid) {
       if (NewTemplate)
         NewTemplate->setInvalidDecl();
     }
 
     SetNestedNameSpecifier(*this, NewVD, D);
 
     // If we have any template parameter lists that don't directly belong to
     // the variable (matching the scope specifier), store them.
-    unsigned VDTemplateParamLists = TemplateParams ? 1 : 0;
+    // An explicit variable template specialization does not own any template
+    // parameter lists.
+    bool IsExplicitSpecialization =
+        IsVariableTemplateSpecialization && !IsPartialSpecialization;
+    unsigned VDTemplateParamLists =
+        (TemplateParams && !IsExplicitSpecialization) ? 1 : 0;
     if (TemplateParamLists.size() > VDTemplateParamLists)
       NewVD->setTemplateParameterListsInfo(
           Context, TemplateParamLists.drop_back(VDTemplateParamLists));
   }
 
   if (D.getDeclSpec().isInlineSpecified()) {
     if (!getLangOpts().CPlusPlus) {
       Diag(D.getDeclSpec().getInlineSpecLoc(), diag::err_inline_non_function)