clang rejects a musttail call when it's perfectly fine to tail call

[ahatanak]

$ cat test.cpp

void foobar() noexcept;

void dance() noexcept {
#ifdef MUSTTAIL
  [[clang::musttail]]
#endif
  return foobar();
}

$ clang++ -std=c++20 -c -o /dev/null -DMUSTTAIL test.cpp
test.cpp:7:9: error: cannot compile this tail call skipping over cleanups yet
    7 |         return foobar();
      |                ^~~~~~~~
1 error generated.

The attempt to tail call fails because the caller (dance) is marked as noexcept, which causes EHTerminateScope to be pushed onto the EHStack.

I think clang should be able to do the tail call as there's no cleanup code that has to be emitted after the call in this case.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Akira Hatanaka (ahatanak)

$ cat test.cpp ``` void foobar() noexcept;

void dance() noexcept {
#ifdef MUSTTAIL
[[clang::musttail]]
#endif
return foobar();
}

$ clang++ -std=c++20 -c -o /dev/null -DMUSTTAIL test.cpp
test.cpp:7:9: error: cannot compile this tail call skipping over cleanups yet
    7 |         return foobar();
      |                ^~~~~~~~
1 error generated.

The attempt to tail call fails because the caller (`dance`) is marked as `noexcept`, which causes `EHTerminateScope` to be pushed onto the EHStack.

I think clang should be able to do the tail call as there's no cleanup code that has to be emitted after the call in this case.
</details>

[efriedma-quic]

We need to be careful that we do actually emit the error if the caller is noexcept, and the callee is not.

But it should be fine if both are noexcept, I think.