[all-commits] [llvm/llvm-project] 858b56: [Clang] Preserve coroutine parameter referenced st...

[Yuxuan Chen via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 858b56e4962749013ded409ff43370b542c8b6cb
      https://github.com/llvm/llvm-project/commit/858b56e4962749013ded409ff43370b542c8b6cb
  Author: Yuxuan Chen <yuxuanchen1997 at outlook.com>
  Date:   2023-11-02 (Thu, 02 Nov 2023)

  Changed paths:
    M clang/lib/Sema/SemaCoroutine.cpp
    A clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp

  Log Message:
  -----------
  [Clang] Preserve coroutine parameter referenced state (#70973)

This PR is proposing a fix for
https://github.com/llvm/llvm-project/issues/65971.

Previously, given a coroutine like this
```
task foo(int a) {
  co_return;
}
```
Parameter `a` is never used. However, because C++ coroutines move
constructs the variable to a heap allocated coroutine activation frame,
we considered all parameters referenced. When diagnosing unused
parameters, we cannot distinguish if the variable reference was due to
coroutine parameter moves.

Compiler Explorer shows that GCC warns against this case correctly, but
clang does not: https://godbolt.org/z/Wo7dfqeaf

This patch addresses this issue by preserving the original
`ParmVarDecl`'s `Referenced` state.