[clang] [Clang][Parse] Diagnose member template declarations with multiple declarators (PR #78243)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/78243

According to [[temp.pre] p5](http://eel.is/c++draft/temp.pre#5):
> In a [template-declaration](http://eel.is/c++draft/temp.pre#nt:template-declaration), explicit specialization, or explicit instantiation the [init-declarator-list](http://eel.is/c++draft/dcl.decl.general#nt:init-declarator-list) in the declaration shall contain at most one declarator. 

A [member-declaration](http://eel.is/c++draft/class.mem.general#nt:member-declaration) that is a [template-declaration](http://eel.is/c++draft/temp.pre#nt:template-declaration) or [explicit-specialization](http://eel.is/c++draft/temp.expl.spec#nt:explicit-specialization) contains a [declaration](http://eel.is/c++draft/dcl.pre#nt:declaration), even though it declares a member. This means it _will_ contain an [init-declarator-list](http://eel.is/c++draft/dcl.decl.general#nt:init-declarator-list) (not a [member-declarator-list](http://eel.is/c++draft/class.mem.general#nt:member-declarator-list)), so [[temp.pre] p5](http://eel.is/c++draft/temp.pre#5) applies.

This diagnoses declarations such as:
```cpp
struct A
{
    template<typename T>
    static const int x = 0, f(); // error: a template declaration can only declare a single entity

    template<typename T>
    static const int g(), y = 0; // error: a template declaration can only declare a single entity
};
```
The diagnostic messages are the same as those of the equivalent namespace scope declarations.

Note: since we currently do not diagnose declarations with multiple abbreviated function template declarators at namespace scope e.g., `void f(auto), g(auto);`, I did not add diagnostics for the equivalent member declarations.

>From 44c582f0e23725b28b46c07bdb0d2991ebb7d204 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Fri, 12 Jan 2024 13:45:15 -0500
Subject: [PATCH] [Clang][Parse] Diagnose member template declarations with
 multiple declarators

---
 clang/docs/ReleaseNotes.rst                 | 1 +
 clang/lib/Parse/ParseDeclCXX.cpp            | 9 +++++++++
 clang/test/CXX/temp/p3.cpp                  | 6 ++++++
 clang/test/OpenMP/declare_simd_messages.cpp | 3 +--
 4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ea57769a4a5795..54debc4d40e945 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -555,6 +555,7 @@ Improvements to Clang's diagnostics
 - Clang now diagnoses unexpanded packs within the template argument lists of function template specializations.
 - Clang now diagnoses attempts to bind a bitfield to an NTTP of a reference type as erroneous
   converted constant expression and not as a reference to subobject.
+- Clang now diagnoses member template declarations with multiple declarators.
 
 
 Improvements to Clang's time-trace
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 5576be9e717a9b..36a9ef199a9588 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -3167,6 +3167,15 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
 
     DeclaratorInfo.complete(ThisDecl);
 
+    if (TemplateInfo.Kind != ParsedTemplateInfo::NonTemplate) {
+      if (Tok.is(tok::comma)) {
+        Diag(Tok, diag::err_multiple_template_declarators)
+          << (int)TemplateInfo.Kind;
+        SkipUntil(tok::semi, StopBeforeMatch);
+      }
+      break;
+    }
+
     // If we don't have a comma, it is either the end of the list (a ';')
     // or an error, bail out.
     SourceLocation CommaLoc;
diff --git a/clang/test/CXX/temp/p3.cpp b/clang/test/CXX/temp/p3.cpp
index b708c613d352d2..9e561d0b9a83b2 100644
--- a/clang/test/CXX/temp/p3.cpp
+++ b/clang/test/CXX/temp/p3.cpp
@@ -15,3 +15,9 @@ template<typename T> struct B { } f(); // expected-error {{expected ';' after st
 template<typename T> struct C { } // expected-error {{expected ';' after struct}}
 
 A<int> c;
+
+struct D {
+  template<typename T> static const int x = 0, f(); // expected-error {{can only declare a single entity}}
+
+  template<typename T> static const int g(), y = 0; // expected-error {{can only declare a single entity}}
+};
diff --git a/clang/test/OpenMP/declare_simd_messages.cpp b/clang/test/OpenMP/declare_simd_messages.cpp
index dd24322694b69f..fea045400e1faf 100644
--- a/clang/test/OpenMP/declare_simd_messages.cpp
+++ b/clang/test/OpenMP/declare_simd_messages.cpp
@@ -33,10 +33,9 @@ int main();
 int main();
 
 struct A {
-// expected-error at +1 {{function declaration is expected after 'declare simd' directive}}
   #pragma omp declare simd
   template<typename T>
-  T infunc1(T a), infunc2(T a);
+  T infunc1(T a);
 };
 
 // expected-error at +1 {{single declaration is expected after 'declare simd' directive}}