[Clang] Fix crash on template-specialization

clang/docs/ReleaseNotes.rst

@@ -805,6 +805,7 @@ Bug Fixes to C++ Support
 - Clang modules now allow a module and its user to differ on TrivialAutoVarInit*
 - Fixed an access checking bug when initializing non-aggregates in default arguments (#GH62444), (#GH83608)
 - Fixed a pack substitution bug in deducing class template partial specializations. (#GH53609)
+- Fixed a crash when specializing a template function whose primary template has a default argument (#GH54279) (#GH95420)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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

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

clang/test/SemaTemplate/default-arguments.cpp

@@ -283,3 +283,52 @@ 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);
+}
+}
+
+namespace PR54279 {
+template <int> struct a {};
+namespace b {
+template <int c>
+void e(a<c> &, const int &center, double, double, unsigned, bool = 0);
+template <int c> void d(a<c> &, double, double, double, unsigned, bool);
+} // namespace b
+struct g {
+  g(int center = 0);
+  int center;
+};
+namespace b {
+template <> void e(a<3> &, const int &f, double, double, unsigned, bool) {
+  a<3> h;
+  e(h, 0, 0, 0, 0);
+}
+template <> void d(a<0> &, double, double, double, unsigned, bool);
+} // namespace b
+}
+
+namespace PR95420 {
+template <typename _Container>
+constexpr auto
+size(const _Container &__cont) noexcept -> decltype(__cont.size0);
+
+struct A {
+  A* data[2];
+};
+
+template <typename T>
+void f1(unsigned long, const A& f, const A& b = {});
+
+template <>
+void f1<A>(unsigned long size, const A& f, const A& b) { // We also need b
+  f1<A>(0, *f.data[0]); // We still need this line
+
+  size; // Changing the name of size prevents crash
+}
+}