[Bug c++/94252] New: Can't use a lambda in a requires expression

[Bug c++/94252] New: Can't use a lambda in a requires expression

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

            Bug ID: 94252
           Summary: Can't use a lambda in a requires expression
           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: ---

This program:

    auto f = []{ return 0; };
    static_assert(requires { f(); });

Fails to compile on trunk (https://godbolt.org/z/DEuoVM), with the error:

<source>:2:15: error: static assertion failed
    2 | static_assert(requires { f(); });
      |               ^~~~~~~~~~~~~~~~~

[Bug c++/94252] Can't use a lambda in a requires expression

[julius_huelsmann at freenet dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

--- Comment #1 from Julius Hülsmann <julius_huelsmann at freenet dot de> ---
This seems to apply to member functions aswell; the following assertion fails
aswell on gcc-trunk on [compilerExplorer](https://godbolt.org/z/NxgkH5)

struct T { int f(int) noexcept; };

int main() {
  static_assert(requires(T o, int i) {
    o.f(i);
    //{ o.f(i) } noexcept->std::same_as<int>; //< also fails.
  });
}

[Bug c++/94252] Can't use a lambda in a requires expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.0
      Known to fail|                            |10.0
   Last reconfirmed|                            |2020-03-24
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.

[Bug c++/94252] Can't use a lambda in a requires expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org

[Bug c++/94252] Can't use a lambda in a requires expression

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:a7ea3d2ced786c4544fa625f34f515d89ed074fe

commit r10-7441-ga7ea3d2ced786c4544fa625f34f515d89ed074fe
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Mar 28 08:56:59 2020 -0400

    c++: requires-expression outside of a template is misevaluated [PR94252]

    This PR shows that a REQUIRES_EXPR outside of a template can sometimes be
    misevaluated.  This happens because the evaluation routine
tsubst_requires_expr
    (and diagnose_requires_expr) assumes the REQUIRES_EXPR's subtrees are
templated
    trees and that therefore it's safe to call tsubst_expr on them.  But this
    assumption isn't valid when we've parsed a REQUIRES_EXPR outside of a
template
    context.  In order to make this assumption valid here, this patch sets
    processing_template_decl to non-zero before parsing the body of a
REQUIRES_EXPR
    so that its subtrees are indeed always templated trees.

    gcc/cp/ChangeLog:

            PR c++/94252
            * constraint.cc (tsubst_compound_requirement): Always suppress
errors
            from type_deducible_p and expression_convertible_p, as they're not
            substitution errors.
            (diagnose_atomic_constraint) <case INTEGER_CST>: Remove this case
so
            that we diagnose INTEGER_CST expressions of non-bool type via the
            default case.
            * cp-gimplify.c (cp_genericize_r) <case REQUIRES_EXPR>: New case.
            * parser.c (cp_parser_requires_expression): Always parse the
requirement
            body as if we're processing a template, by temporarily incrementing
            processing_template_decl.  Afterwards, if we're not actually in a
            template context, perform semantic processing to diagnose any
invalid
            types and expressions.
            * pt.c (tsubst_copy_and_build) <case REQUIRES_EXPR>: Remove dead
code.
            * semantics.c (finish_static_assert): Explain an assertion failure
            when the condition is a REQUIRES_EXPR like we do when it is a
concept
            check.

    gcc/testsuite/ChangeLog:

            PR c++/94252
            * g++.dg/concepts/diagnostic7.C: New test.
            * g++.dg/concepts/pr94252.C: New test.
            * g++.dg/cpp2a/concepts-requires18.C: Adjust to expect an
additional
            diagnostic.

[Bug c++/94252] Can't use a lambda in a requires expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94252

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 10.