[C++] Side effect get removed if the return type of a unary operator is `std::nullptr_t`

[ChuanqiXu9]

This comes from #58459 but I find it is not related to coroutines after I reduce it.

Reproducer: https://godbolt.org/z/TGefxv5e5

From the generated code, we can see the call to DoAnotherThing() is elided incorrectly.

And if we change the return type of operator++, we can find the problem is gone: https://godbolt.org/z/TGefxv5e5

While I tagged codegen, I feel like this is a frontend issue.

CC people since I know not so many people subscribed to the c++ tag: @AaronBallman @cor3ntin @Endilll @erichkeane

[llvmbot]

@llvm/issue-subscribers-c-1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[jacobsa]

Thanks for reducing! Here is a corrected link for what I think you meant about changing the return type (your post currently has the same link twice).

[tbaederr]

Most likely a problem in the constant interpreter, since this works with -fexperimental-new-constant-interpreter: https://godbolt.org/z/c3zhjdv5r

[cor3ntin]

We don't bother evaluating either side of the expression when we know the type is nullptr_t
https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/ExprConstant.cpp#L13307-L13313

Fix shortly, i just need to write a test!

[cor3ntin]

https://reviews.llvm.org/D158601

[jacobsa]

Thank you for the quick fix!