Reland [Clang] skip default argument instantiation for non-defining friend declarations to meet [dcl.fct.default] p4

clang/docs/ReleaseNotes.rst

@@ -726,6 +726,8 @@ Bug Fixes to C++ Support
   missing placeholder return type. (#GH78694)
 - Fixed a bug where bounds of partially expanded pack indexing expressions were checked too early. (#GH116105)
 - Fixed an assertion failure caused by using ``consteval`` in condition in consumed analyses. (#GH117385)
+- Fixed an assertion failure caused by invalid default argument substitutions in non-defining
+  friend declarations. (#GH113324)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -4692,6 +4692,17 @@ bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                       ParmVarDecl *Param) {
   assert(Param->hasUninstantiatedDefaultArg());
 
+  // FIXME: We don't track member specialization info for non-defining
+  // friend declarations, so we will not be able to later find the function
+  // pattern. As a workaround, don't instantiate the default argument in this
+  // case. This is correct per the standard and only an issue for recovery
+  // purposes. [dcl.fct.default]p4:
+  //   if a friend declaration D specifies a default argument expression,
+  //   that declaration shall be a definition.
+  if (FD->getFriendObjectKind() != Decl::FOK_None &&
+      !FD->getTemplateInstantiationPattern())
+    return true;
+
   // Instantiate the expression.
   //
   // FIXME: Pass in a correct Pattern argument, otherwise

clang/test/CXX/temp/temp.res/p4.cpp

@@ -185,3 +185,23 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct S1 {
+  friend void f1(S1, int = 0); // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend void f2(S1 a, S1 = decltype(a){}); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template <class T> using alias = int;
+template <typename T> struct S2 {
+  // FIXME: We miss diagnosing the default argument instantiation failure
+  // (forming reference to void)
+  friend void f3(S2, int a = alias<T &>(1)); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+void test() {
+  f1(S1<>{});
+  f2(S1<>{});
+  f3(S2<void>());
+}
+} // namespace GH113324

clang/test/CodeGenCXX/default-arguments.cpp

@@ -12,6 +12,17 @@ void g() {
 }
 }
 
+namespace GH113324 {
+struct S1 {
+  friend void f(S1, int = 42) {}
+};
+
+void test() {
+  S1 s1;
+  f(s1);
+}
+};
+
 struct A1 {
  A1();
  ~A1();