[Clang][Sema] Abbreviated function templates do not append invented parameters to empty template parameter lists (#80864)

According to [dcl.fct] p23:
> An abbreviated function template can have a _template-head_. The
invented _template-parameters_ are appended to the
_template-parameter-list_ after the explicitly declared
_template-parameters_.

`template<>` is not a _template-head_ -- a _template-head_ must have at
least one _template-parameter_. This patch corrects our current behavior
of appending the invented template parameters to the innermost template
parameter list, regardless of whether it is empty. Example:
```
template<typename T>
struct A 
{ 
    void f(auto); 
};

template<>
void A<int>::f(auto); // ok

template<>
template<> // warning: extraneous template parameter list in template specialization
void A<int>::f(auto);
```

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -215,6 +215,8 @@ Bug Fixes to C++ Support
   Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
 - Fix a crash when trying to call a varargs function that also has an explicit object parameter.
   Fixes (`#80971 ICE when explicit object parameter be a function parameter pack`)
+- Fixed a bug where abbreviated function templates would append their invented template parameters to
+  an empty template parameter lists.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/DeclTemplate.h

@@ -134,6 +134,7 @@ class TemplateParameterList final
   const_iterator end() const { return begin() + NumParams; }
 
   unsigned size() const { return NumParams; }
+  bool empty() const { return NumParams == 0; }
 
   ArrayRef<NamedDecl *> asArray() { return llvm::ArrayRef(begin(), end()); }
   ArrayRef<const NamedDecl*> asArray() const {

clang/lib/AST/DeclPrinter.cpp

@@ -1215,6 +1215,10 @@ void DeclPrinter::printTemplateParameters(const TemplateParameterList *Params,
                                           bool OmitTemplateKW) {
   assert(Params);
 
+  // Don't print invented template parameter lists.
+  if (!Params->empty() && Params->getParam(0)->isImplicit())
+    return;
+
   if (!OmitTemplateKW)
     Out << "template ";
   Out << '<';

clang/lib/Sema/SemaDecl.cpp

@@ -9759,7 +9759,7 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
   SmallVector<TemplateParameterList *, 4> TemplateParamLists;
   llvm::append_range(TemplateParamLists, TemplateParamListsRef);
   if (TemplateParameterList *Invented = D.getInventedTemplateParameterList()) {
-    if (!TemplateParamLists.empty() &&
+    if (!TemplateParamLists.empty() && !TemplateParamLists.back()->empty() &&
         Invented->getDepth() == TemplateParamLists.back()->getDepth())
       TemplateParamLists.back() = Invented;
     else

clang/lib/Sema/SemaDeclCXX.cpp

@@ -19294,7 +19294,16 @@ void Sema::ActOnStartFunctionDeclarationDeclarator(
         ExplicitLists, /*IsFriend=*/false, IsMemberSpecialization, IsInvalid,
         /*SuppressDiagnostic=*/true);
   }
-  if (ExplicitParams) {
+  // C++23 [dcl.fct]p23:
+  //   An abbreviated function template can have a template-head. The invented
+  //   template-parameters are appended to the template-parameter-list after
+  //   the explicitly declared template-parameters.
+  //
+  // A template-head must have one or more template-parameters (read:
+  // 'template<>' is *not* a template-head). Only append the invented
+  // template parameters if we matched the nested-name-specifier to a non-empty
+  // TemplateParameterList.
+  if (ExplicitParams && !ExplicitParams->empty()) {
     Info.AutoTemplateParameterDepth = ExplicitParams->getDepth();
     llvm::append_range(Info.TemplateParams, *ExplicitParams);
     Info.NumExplicitTemplateParams = ExplicitParams->size();

clang/test/AST/ast-print-method-decl.cpp

@@ -32,8 +32,7 @@ struct DelegatingCtor2 {
 
 // CHECK: struct DelegatingCtor3 {
 struct DelegatingCtor3 {
-  // FIXME: template <> should not be output
-  // CHECK: template <> DelegatingCtor3(auto);
+  // CHECK: DelegatingCtor3(auto);
   DelegatingCtor3(auto);
 
   // FIXME: Implicitly specialized method should not be output

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p23.cpp

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -std=c++20 -pedantic-errors -verify %s
+
+// FIXME: This should be an error with -pedantic-errors.
+template<> // expected-warning {{extraneous template parameter list in template specialization}}
+void f(auto);
+
+template<typename>
+void f(auto);
+
+template<typename T>
+struct A {
+  void g(auto);
+};
+
+template<typename T>
+void A<T>::g(auto) { }
+
+template<>
+void A<int>::g(auto) { }
+
+// FIXME: This should be an error with -pedantic-errors.
+template<>
+template<> // expected-warning {{extraneous template parameter list in template specialization}}
+void A<long>::g(auto) { }

clang/test/OpenMP/for_loop_auto.cpp

@@ -10,7 +10,7 @@
 #ifndef HEADER
 #define HEADER
 
-// CHECK:      template <> void do_loop(const auto &v) {
+// CHECK:      void do_loop(const auto &v) {
 // CHECK-NEXT: #pragma omp parallel for
 // CHECK-NEXT:    for (const auto &i : v)
 // CHECK-NEXT:      ;