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From: "barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97518] New: Improving static_assert diagnostics
Date: Wed, 21 Oct 2020 14:57:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97518

            Bug ID: 97518
           Summary: Improving static_assert diagnostics
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider the following:

template <typename T, typename U> struct is_same { static constexpr bool va=
lue
=3D false; };
template <typename T> struct is_same<T, T> { static constexpr bool value =3D
true; };

template <typename T> using some_metafunction_t =3D T;

template <typename T>
void foo(T ) {
    using X =3D T*;
    using Y =3D some_metafunction_t<T>;

    static_assert(is_same<X, Y>::value);
}

void bar() {
    foo(0);
}

gcc emits:

<source>: In instantiation of 'void foo(T) [with T =3D int]':
<source>:15:10:   required from here
<source>:11:34: error: static assertion failed
   11 |     static_assert(is_same<X, Y>::value);
      |=20=20=20=20=20=20=20=20

Notably, it does not tell me what either X or Y are. All I know is that the=
y're
not the same. I get T, but the computation of X and Y could be fairly
complicated and T may not help (or even be relevant, necessarily). This end=
s up
being useless for me, to the point where I actually created my own verify_s=
ame
type such that verify_same<T, U> is only defined when T =3D=3D U, and creat=
e a
variable like:

[[maybe_unused]] verify_same<T, U> _;

It would be a lot cooler if gcc could diagnose all the types and values that
were used in a static_assert condition.=20

clang, for instance, gives me:

<source>:11:5: error: static_assert failed due to requirement 'is_same<int =
*,
int>::value'
    static_assert(is_same<X, Y>::value);
    ^             ~~~~~~~~~~~~~~~~~~~~
<source>:15:5: note: in instantiation of function template specialization
'foo<int>' requested here
    foo(0);
    ^

Which, while it doesn't tell me that X=3Dint* and Y=3Dint, at least clearly
illustrates both types, and is a much more useful error diagnostic.=