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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99546] Weird return value of C++20 requires expression
Date: Tue, 20 Sep 2022 10:15:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99546

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2021-08-16 00:00:00         |2022-9-20

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to =E5=BA=B7=E6=A1=93=E7=91=8B from comment #1)
> You can see more weird cases in this:=20
> https://stackoverflow.com/questions/66578966/weird-return-value-of-c20-
> requires-expression.

Copying that here instead of linking to impermanent SO comments:

template <bool b>
void foo() {
    static_assert(b);
}

int main() {
  constexpr bool b =3D requires { []{}; };
  foo<!b>();
  foo<b>();
}

The bool appears to not be a constant expression:

x.C: In function 'int main()':
x.C:8:10: error: no matching function for call to 'foo<(!(bool)b)>()'
    8 |   foo<!b>();
      |   ~~~~~~~^~
x.C:2:6: note: candidate: 'template<bool b> void foo()'
    2 | void foo() {
      |      ^~~
x.C:2:6: note:   template argument deduction/substitution failed:
x.C:9:9: error: no matching function for call to 'foo<b>()'
    9 |   foo<b>();
      |   ~~~~~~^~
x.C:2:6: note: candidate: 'template<bool b> void foo()'
    2 | void foo() {
      |      ^~~
x.C:2:6: note:   template argument deduction/substitution failed:=