[Clang] skip default argument instantiation for non-defining friend declarations without specialization info to meet [dcl.fct.default]p4

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -595,6 +595,8 @@ Bug Fixes to C++ Support
   an implicitly instantiated class template specialization. (#GH51051)
 - Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
 - Name independent data members were not correctly initialized from default member initializers. (#GH114069)
+- Fixed an assertion failure caused by invalid default argument substitutions in non-defining
+  friend declarations. (#GH113324).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -6018,6 +6018,17 @@ bool Sema::GatherArgumentsForCall(SourceLocation CallLoc, FunctionDecl *FDecl,
     } else {
       assert(Param && "can't use default arguments without a known callee");
 
+      // 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 (FDecl->getFriendObjectKind() != Decl::FOK_None &&
+          FDecl->getMemberSpecializationInfo() == nullptr)
+        return true;
+
       ExprResult ArgExpr = BuildCXXDefaultArgExpr(CallLoc, FDecl, Param);
       if (ArgExpr.isInvalid())
         return true;

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

@@ -185,3 +185,22 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct ct {
+  friend void f1(ct, int = 0);               // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend void f2(ct a, ct = decltype(a){ }); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template<class T> using alias = int;
+template<typename T> struct C {
+  // 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}}
+};
+
+void test() {
+  f1(ct<>{});
+  f2(ct<>{});
+  f3(C<void>());
+}
+} // namespace GH113324