[Clang] skip default argument instantiation for non-defining friend declarations (#113777)

This fixes a crash when instantiating default arguments for templated
friend function declarations which lack a definition.
There are implementation limits which prevents us from finding the
pattern for such functions, and this causes difficulties
setting up the instantiation scope for the function parameters.

This patch skips instantiating the default argument in these cases,
which causes a minor regression in error recovery, but otherwise avoids
the crash.

Fixes #113324

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -613,6 +613,8 @@ 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