[clang] Fix crash on template-specialization (PR #142338)

[Mark de Wever via cfe-commits]

https://github.com/mark-de-wever-sonarsource created 
https://github.com/llvm/llvm-project/pull/142338

This applies the name restoration as suggested by Richard Smith.

Fixes: #54279

>From 930e073e4d5bfbde8028fbb8330e6b91348cc20b Mon Sep 17 00:00:00 2001
From: Mark de Wever <mark.dewe...@sonarsource.com>
Date: Mon, 2 Jun 2025 08:57:59 +0200
Subject: [PATCH] Fix crash on template-specialization

This applies the name restoration as suggested by Richard Smith.

Fixes: #54279
---
 clang/lib/Sema/SemaTemplateInstantiateDecl.cpp | 5 +++++
 clang/test/CodeGenCXX/constructor-init.cpp     | 2 +-
 clang/test/SemaTemplate/default-arguments.cpp  | 9 +++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp 
b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 174c8fc59e4fa..9853466e01496 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -34,6 +34,7 @@
 #include "clang/Sema/SemaSwift.h"
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateInstCallback.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/TimeProfiler.h"
 #include <optional>
 
@@ -5120,6 +5121,10 @@ bool Sema::addInstantiatedParametersToScope(
       // Simple case: not a parameter pack.
       assert(FParamIdx < Function->getNumParams());
       ParmVarDecl *FunctionParam = Function->getParamDecl(FParamIdx);
+      DeclarationName name = FunctionParam->getDeclName();
+      auto _ = llvm::make_scope_exit([&]() {
+        FunctionParam->setDeclName(name);
+      });
       FunctionParam->setDeclName(PatternParam->getDeclName());
       // If the parameter's type is not dependent, update it to match the type
       // in the pattern. They can differ in top-level cv-qualifiers, and we 
want
diff --git a/clang/test/CodeGenCXX/constructor-init.cpp 
b/clang/test/CodeGenCXX/constructor-init.cpp
index f191599f360e7..b5f4cc0b3d6c8 100644
--- a/clang/test/CodeGenCXX/constructor-init.cpp
+++ b/clang/test/CodeGenCXX/constructor-init.cpp
@@ -160,7 +160,7 @@ template<typename T> struct X;
 
 // Make sure that the instantiated constructor initializes start and
 // end properly.
-// CHECK-LABEL: define linkonce_odr void @_ZN1XIiEC2ERKS0_(ptr {{[^,]*}} 
%this, ptr noundef nonnull align {{[0-9]+}} dereferenceable({{[0-9]+}}) %other) 
unnamed_addr
+// CHECK-LABEL: define linkonce_odr void @_ZN1XIiEC2ERKS0_(ptr {{[^,]*}} 
%this, ptr noundef nonnull align {{[0-9]+}} dereferenceable({{[0-9]+}}) %0) 
unnamed_addr
 // CHECK: {{store.*null}}
 // CHECK: {{store.*null}}
 // CHECK: ret
diff --git a/clang/test/SemaTemplate/default-arguments.cpp 
b/clang/test/SemaTemplate/default-arguments.cpp
index 5ea34c0254ec1..a366c3d8ab722 100644
--- a/clang/test/SemaTemplate/default-arguments.cpp
+++ b/clang/test/SemaTemplate/default-arguments.cpp
@@ -283,3 +283,12 @@ static_assert(S<short *>().SizeOfT<char>() == sizeof(short 
*), "");
 } // namespace GH68490
 
 #endif
+
+namespace PR54279 {
+// Using a different name for the argument when there is a default argument
+// caused a crash.
+template <typename T> void f(const T &a, int c = 0);
+template <> void f(const int &unused, int) {
+  f(42);
+}
+}

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