Requires expression with local parameter of void type

[Fedr]

This program is accepted by Clang (requires is evaluated as true):

static_assert( requires(void t) {t;} );

but rejected by GCC and MSVC with hard error: https://gcc.godbolt.org/z/W7f3zqbMb

If it were evaluated as false, it would be more understandable.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Fedor Chelnokov (Fedr)

This program is accepted by Clang (`requires` is evaluated as `true`): ``` static_assert( requires(void t) {t;} ); ``` but rejected by GCC and MSVC with hard error: https://gcc.godbolt.org/z/W7f3zqbMb

If it were evaluated as false, it would be more understandable.

[shafik]

Looks like a clang bug: https://godbolt.org/z/E7a4eEeKs

I feel like we had another bug like this but can't find it.

CC @erichkeane

[zyn0217]

This seems to be an oversight in Sema::ActOnStartRequiresExpr() and we forgot to ensure the non-void parameter types in a RequiresExpr.

Like what we did in Sema::BuildFunctionType(), we probably should check if any ParmVarDecl in LocalParameters is of void type, and if so, we should diagnose it with diag::err_param_with_void_type.

Further, we should set the RequiresExprBodyDecl to invalid in that event; we also want to set the satisfaction flag to false in RequiresExpr's constructor so that the expression could be evaluated to false.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Fedor Chelnokov (Fedr)

This program is accepted by Clang (`requires` is evaluated as `true`): ``` static_assert( requires(void t) {t;} ); ``` but rejected by GCC and MSVC with hard error: https://gcc.godbolt.org/z/W7f3zqbMb

If it were evaluated as false, it would be more understandable.

[c8ef]

I would like to work on this issue. 💪

[erichkeane]

I would like to work on this issue. 💪

I've assigned it to you, have fun! Make sure you add a few of us (myself and @zyn0217 included!) to the review, and feel free to ask questions.