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

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/80864

According to [[dcl.fct] p23](http://eel.is/c++draft/dcl.fct#23):
> 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:
```cpp
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);
```


>From 570e25f19b428fb7d2936091726727f9633eec35 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Tue, 6 Feb 2024 11:08:04 -0500
Subject: [PATCH] [Clang][Sema] Abbreviated function templates do not append
 invented parameters to empty template parameter lists

---
 clang/docs/ReleaseNotes.rst                   |  2 ++
 clang/lib/Sema/SemaDecl.cpp                   |  2 +-
 clang/lib/Sema/SemaDeclCXX.cpp                | 11 ++++++++-
 .../CXX/dcl.decl/dcl.meaning/dcl.fct/p23.cpp  | 24 +++++++++++++++++++
 4 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p23.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 4d57ea4fd55b8..d0ba986601e92 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -204,6 +204,8 @@ Bug Fixes to C++ Support
   parameter where we did an incorrect specialization of the initialization of
   the default parameter.
   Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
+- Fixed a bug where abbreviated function templates would append their invented template parameters to
+  an empty template parameter lists.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 481d952d2389b..d9704e5bfd67c 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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()->size() &&
         Invented->getDepth() == TemplateParamLists.back()->getDepth())
       TemplateParamLists.back() = Invented;
     else
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index ab8a967b06a45..f34893d88a3b3 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/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->size()) {
     Info.AutoTemplateParameterDepth = ExplicitParams->getDepth();
     llvm::append_range(Info.TemplateParams, *ExplicitParams);
     Info.NumExplicitTemplateParams = ExplicitParams->size();
diff --git a/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p23.cpp b/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p23.cpp
new file mode 100644
index 0000000000000..469c4e091953c
--- /dev/null
+++ b/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) { }