[clang] [Clang][Sema] Diagnose function/variable templates that shadow their own template parameters (PR #78274)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian updated https://github.com/llvm/llvm-project/pull/78274

>From 90d66e29f3550d50bd114d68fc8b12156e26de18 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Tue, 16 Jan 2024 08:05:33 -0500
Subject: [PATCH] [Clang][Sema] Diagnose function/variable templates that
 shadow their own template parameters

---
 clang/docs/ReleaseNotes.rst                   |  1 +
 clang/lib/Sema/SemaDecl.cpp                   | 12 +++++------
 .../test/CXX/temp/temp.res/temp.local/p6.cpp  | 20 ++++++++++++++++---
 3 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ea57769a4a5795d..030e4ebce177638 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 function/variable templates that shadow their own template parameters, e.g. ``template<class T> void T();``.
 
 
 Improvements to Clang's time-trace
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 722e2ac9e4ff8da..6345e1d3208b990 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6371,12 +6371,6 @@ NamedDecl *Sema::HandleDeclarator(Scope *S, Declarator &D,
   } else if (DiagnoseUnexpandedParameterPack(NameInfo, UPPC_DeclarationType))
     return nullptr;
 
-  // The scope passed in may not be a decl scope.  Zip up the scope tree until
-  // we find one that is.
-  while ((S->getFlags() & Scope::DeclScope) == 0 ||
-         (S->getFlags() & Scope::TemplateParamScope) != 0)
-    S = S->getParent();
-
   DeclContext *DC = CurContext;
   if (D.getCXXScopeSpec().isInvalid())
     D.setInvalidType();
@@ -6529,6 +6523,12 @@ NamedDecl *Sema::HandleDeclarator(Scope *S, Declarator &D,
   if (getLangOpts().CPlusPlus)
     CheckExtraCXXDefaultArguments(D);
 
+  // The scope passed in may not be a decl scope.  Zip up the scope tree until
+  // we find one that is.
+  while ((S->getFlags() & Scope::DeclScope) == 0 ||
+         (S->getFlags() & Scope::TemplateParamScope) != 0)
+    S = S->getParent();
+
   NamedDecl *New;
 
   bool AddToScope = true;
diff --git a/clang/test/CXX/temp/temp.res/temp.local/p6.cpp b/clang/test/CXX/temp/temp.res/temp.local/p6.cpp
index e2aa0ff344291d2..0702966e5685480 100644
--- a/clang/test/CXX/temp/temp.res/temp.local/p6.cpp
+++ b/clang/test/CXX/temp/temp.res/temp.local/p6.cpp
@@ -127,16 +127,30 @@ template<int T> struct Z { // expected-note 16{{declared here}}
 template<typename T> // expected-note {{declared here}}
 void f(int T) {} // expected-error {{declaration of 'T' shadows template parameter}}
 
-// FIXME: These are ill-formed: a template-parameter shall not have the same name as the template name.
 namespace A {
   template<typename T> struct T {};  // expected-error{{declaration of 'T' shadows template parameter}}
                                      // expected-note at -1{{template parameter is declared here}}
+  template<typename T> struct U {
+    template<typename V> struct V {}; // expected-error{{declaration of 'V' shadows template parameter}}
+                                      // expected-note at -1{{template parameter is declared here}}
+  };
 }
 namespace B {
-  template<typename T> void T() {}
+  template<typename T> void T() {} // expected-error{{declaration of 'T' shadows template parameter}}
+                                   // expected-note at -1{{template parameter is declared here}}
+
+  template<typename T> struct U {
+    template<typename V> void V(); // expected-error{{declaration of 'V' shadows template parameter}}
+                                   // expected-note at -1{{template parameter is declared here}}
+  };
 }
 namespace C {
-  template<typename T> int T;
+  template<typename T> int T; // expected-error{{declaration of 'T' shadows template parameter}}
+                              // expected-note at -1{{template parameter is declared here}}
+  template<typename T> struct U {
+    template<typename V> static int V; // expected-error{{declaration of 'V' shadows template parameter}}
+                                       // expected-note at -1{{template parameter is declared here}}
+  };
 }
 
 namespace PR28023 {