error when define a move assignment = default (if using type alias)

[twinstar6980]

For some reason, I used a type alias when defining the move constructor, which compiles successfully on MSVC and GCC, but Clang doesn't allow it
I tested other special member functions, copy constructor, move constructor, copy assignment function can all use the = default declaration explicitly in the case of using type alias, only move assignment function does not allow this, but it can use = delete explicitly, so is this a bug?
here is my test code

// constant reference
template <typename T>
using RC = T const &;

// non-constant reference
template <typename T>
using RV = T &;

// r-value reference
template <typename T>
using RM = T&&;

struct A {

	// default constructor
	A () = default;
	// copy constructor
	A (RC<A>) = default;
	// move constructor
	A (RM<A>) = default;
	
	// copy assignment
	auto operator = (RC<A>) -> RV<A> = default;
	// move assignment
	auto operator = (RM<A>) -> RV<A> = default; // error: only special member functions may be defaulted
	//auto operator = (RM<A>) -> RV<A> = delete;  // OK
	
};

int main () {
	return 0;
}

[shafik]

Looks like this is failing in Sema::SetDeclDefaulted, in particular it looks like Sema::getDefaultedFunctionKind is not doing the right thing because MD->isMoveAssignmentOperator() comes back false.

Looking at CXXMethodDecl::isCopyAssignmentOperator() is does the following:

  QualType ParamType = getParamDecl(0)->getType();
  if (const auto *Ref = ParamType->getAs<LValueReferenceType>())
    ParamType = Ref->getPointeeType();

while CXXMethodDecl::isMoveAssignmentOperator() does:

 QualType ParamType = getParamDecl(0)->getType();
  if (!isa<RValueReferenceType>(ParamType))
    return false;
  ParamType = ParamType->getPointeeType();

If I modify isMoveAssignmentOperator to work like isCopyAssignmentOperator it seems to fix the issue but I need to make sure this passes check-clang.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

That did not pass but modifying CXXMethodDecl::isMoveAssignmentOperator() this way does pass 'check-clang` cleanly:

 QualType ParamType = getParamDecl(0)->getType();
  if (!ParamType->isRValueReferenceType())
    return false;
  ParamType = ParamType->getPointeeType();

[twinstar6980]

That did not pass but modifying CXXMethodDecl::isMoveAssignmentOperator() this way does pass 'check-clang` cleanly:

 QualType ParamType = getParamDecl(0)->getType();
  if (!ParamType->isRValueReferenceType())
    return false;
  ParamType = ParamType->getPointeeType();

ok, as you said it is working now, thanks for your reply

[shafik]

Apologies, I was not clear. I have not put in a fix yet. I am just explaining the steps I have taken so far in looking into the problem. I still need to write some tests to cover this problem and post a PR for a fix. Once I have the PR I will post it to the bug.

[shafik]

PR: https://reviews.llvm.org/D129591

[twinstar6980]

PR: https://reviews.llvm.org/D129591

ok❤️

[EugeneZelenko]

@TwinKleS-C: patch is not merged into main yet.