[Clang][Sema] fix outline member function template with default align crash

clang/docs/ReleaseNotes.rst

@@ -115,6 +115,10 @@ Bug Fixes to C++ Support
 
 - Fix crash when using lifetimebound attribute in function with trailing return.
   Fixes (`#73619 <https://github.com/llvm/llvm-project/issues/73619>`_)
+- Fix a crash when specializing an out-of-line member function with a default
+  parameter where we did an incorrect specialization of the initialization of
+  the default parameter.
+  Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -3049,6 +3049,7 @@ bool Sema::SubstDefaultArgument(
     //   default argument expression appears.
     ContextRAII SavedContext(*this, FD);
     std::unique_ptr<LocalInstantiationScope> LIS;
+    MultiLevelTemplateArgumentList NewTemplateArgs = TemplateArgs;
 
     if (ForCallExpr) {
       // When instantiating a default argument due to use in a call expression,
@@ -3061,11 +3062,19 @@ bool Sema::SubstDefaultArgument(
           /*ForDefinition*/ false);
       if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, TemplateArgs))
         return true;
+      if (FD->isOutOfLine()) {
+        TemplateArgumentList *CurrentTemplateArgumentList =
+            TemplateArgumentList::CreateCopy(getASTContext(),
+                                             TemplateArgs.getInnermost());
+        NewTemplateArgs = getTemplateInstantiationArgs(
+            FD, FD->getDeclContext(), /*Final=*/false,
+            CurrentTemplateArgumentList, /*RelativeToPrimary=*/true);
+      }
     }
 
     runWithSufficientStackSpace(Loc, [&] {
-      Result = SubstInitializer(PatternExpr, TemplateArgs,
-                                /*DirectInit*/false);
+      Result = SubstInitializer(PatternExpr, NewTemplateArgs,
+                                /*DirectInit*/ false);
     });
   }
   if (Result.isInvalid())

clang/test/SemaTemplate/default-parm-init.cpp

@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+// expected-no-diagnostics
+
+template<typename TemplateParam>
+struct Problem{
+  template<typename FunctionTemplateParam>
+  constexpr int FuncAlign(int param = alignof(FunctionTemplateParam));
+
+  template<typename FunctionTemplateParam>
+  constexpr int FuncSizeof(int param = sizeof(FunctionTemplateParam));
+
+  template<typename FunctionTemplateParam>
+  constexpr int FuncAlign2(int param = alignof(TemplateParam));
+
+  template<typename FunctionTemplateParam>
+  constexpr int FuncSizeof2(int param = sizeof(TemplateParam));
+};
+
+template <>
+template<typename FunctionTemplateParam>
+constexpr int Problem<int>::FuncAlign(int param) {
+	return param;
+}
+
+template <>
+template<typename FunctionTemplateParam>
+constexpr int Problem<int>::FuncSizeof(int param) {
+	return param;
+}
+
+template <>
+template<typename FunctionTemplateParam>
+constexpr int Problem<int>::FuncAlign2(int param) {
+	return param;
+}
+
+template <>
+template<typename FunctionTemplateParam>
+constexpr int Problem<int>::FuncSizeof2(int param) {
+	return param;
+}
+
+int main(){
+    Problem<int> p = {};
+    static_assert(p.FuncAlign<char>() == alignof(char));
+    static_assert(p.FuncSizeof<char>() == sizeof(char));
+    static_assert(p.FuncAlign2<char>() == alignof(int));
+    static_assert(p.FuncSizeof2<char>() == sizeof(int));
+}