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

clang/docs/ReleaseNotes.rst

@@ -670,6 +670,8 @@ Bug Fixes to C++ Support
   assumption if they also occur inside of a dependent lambda. (#GH114787)
 - Clang now uses valid deduced type locations when diagnosing functions with trailing return type
   missing placeholder return type. (#GH78694)
+- 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 wording and only an error recovery issue, as per
+  // [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 +185,46 @@ 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}}
+};
+
+struct S3 {
+  friend void f4(S3, int = 42) { }
+};
+
+template <bool, class> using __enable_if_t = int;
+template <int v> struct S4 {
+  static const int value = v;
+};
+struct S5 {
+  template <__enable_if_t<S4<true>::value, int> = 0>
+  S5(const char *);
+};
+struct S6 {
+  template <typename a, typename b>
+  friend void f5(int, S6, a, b, S5 = "") { }
+};
+
+void test() {
+  f1(S1<>{});
+  f2(S1<>{});
+  f3(S2<void>());
+
+  S3 s3;
+  f4(s3);
+
+  S6 s6;
+  auto result = f5(0, s6, [] {}, [] {}); // expected-error {{variable has incomplete type 'void}}
+}
+} // namespace GH113324