[Clang] Ensure default arguments in friend declarations are only allowed in defining declarations to prevent multiple reachable declarations

[a-tarasyuk]

Fixes #113324

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #113324

---

Full diff: https://github.com/llvm/llvm-project/pull/113777.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+4-4)
• (modified) clang/test/CXX/temp/temp.res/p4.cpp (+7)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6a95337815174b..428ec8c87a432e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -561,6 +561,8 @@ Bug Fixes to C++ Support
   const-default-constructible even if a union member has a default member initializer.
   (#GH95854).
 - Fixed an assertion failure when evaluating an invalid expression in an array initializer (#GH112140)
+- Fixed an assertion failure caused by an invalid template instantiation pattern
+  for adding instantiated parameters to the scope in friend functions with defaulted parameters. (#GH113324).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 6a55861fe5af3b..cddfcc48312042 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -3435,10 +3435,10 @@ bool Sema::SubstDefaultArgument(
       //   template<typename T> void f(T a, int = decltype(a)());
       //   void g() { f(0); }
       LIS = std::make_unique<LocalInstantiationScope>(*this);
-      FunctionDecl *PatternFD = FD->getTemplateInstantiationPattern(
-          /*ForDefinition*/ false);
-      if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, TemplateArgs))
-        return true;
+      if (FunctionDecl *PatternFD =
+              FD->getTemplateInstantiationPattern(/*ForDefinition*/ false))
+        if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, TemplateArgs))
+          return true;
     }
 
     runWithSufficientStackSpace(Loc, [&] {
diff --git a/clang/test/CXX/temp/temp.res/p4.cpp b/clang/test/CXX/temp/temp.res/p4.cpp
index f54d8649f5da88..62bd766e7e1140 100644
--- a/clang/test/CXX/temp/temp.res/p4.cpp
+++ b/clang/test/CXX/temp/temp.res/p4.cpp
@@ -185,3 +185,10 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct ct {
+  friend void f(ct, int = 0); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+void test() { f(ct<>{}); }
+}

[mizvekov]

LGTM except for a couple of nits.

Thanks!

[mizvekov]

Suggested change

	friend void f3(C, int a = alias<T&>(1)); // expected-error {{friend declaration specifying a default argument must be a definition}}
	// FIXME: We miss diagnosing the default argument instantiation failure (forming reference to void)
	friend void f3(C, int a = alias<T&>(1)); // expected-error {{friend declaration specifying a default argument must be a definition}}

[mizvekov]

(It's missing this change by the way)