[Bug c++/95066] New: [C++ 20] Incorrect valid compilation with a conditional explicit

[Bug c++/95066] New: [C++ 20] Incorrect valid compilation with a conditional explicit

[ojman101 at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

            Bug ID: 95066
           Summary: [C++ 20] Incorrect valid compilation with a
                    conditional explicit
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ojman101 at protonmail dot com
  Target Milestone: ---

The code below is invalid C++, the line "Foo<int> b = a;" should fail to
compile as implicitly casting is made illegal by the conditional explicit using
the "IsSafelyCastable" predicate.

----------------------------------------------------------------
#include <type_traits>

template <typename, typename>
class IsSafelyCastable : public std::false_type {};

template <>
class IsSafelyCastable<int, float> : public std::true_type {};

template <typename T>
struct Foo {
    template <typename U>
    explicit(!IsSafelyCastable<T, U>::value) operator Foo<U>();
};

template <typename T>
template <typename U>
Foo<T>::operator Foo<U>() {
  return {};
}

int main() {
    Foo<float> a;
    Foo<int> b = a;
}
----------------------------------------------------------------

Clang 10 correctly evaluates the explicit condition to be true and blocks the
implicit cast. However, GCC 9.3.0 successfully compiles without any errors. I
believe this to be a GCC bug as subtle changes can make GCC produce the correct
error. For example, moving the definition of the function to be inline with the
declaration.

[Bug c++/95066] [C++ 20] Incorrect valid compilation with a conditional explicit

[ojman101 at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

--- Comment #1 from Owen Smith <ojman101 at protonmail dot com> ---
----------------------------------------------------------------
#include <type_traits>

template <typename, typename>
class IsSafelyCastable : public std::false_type {};

template <>
class IsSafelyCastable<int, float> : public std::true_type {};

template <typename T>
struct Foo {
    template <typename U>
    explicit(!IsSafelyCastable<T, U>::value) operator Foo<U>() { return {}; }
};

int main() {
    Foo<float> a;
    Foo<int> b = a;
}
----------------------------------------------------------------

Inlining the definition yields the correct error with GCC 9.3.0:

----------------------------------------------------------------
main.cpp: In function 'int main()':
main.cpp:17:18: error: conversion from 'Foo<float>' to non-scalar type
'Foo<int>' requested
   17 |     Foo<int> b = a;
      | 
----------------------------------------------------------------

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2020-05-11
           Keywords|                            |accepts-invalid
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Thanks for the report.  In the first case we don't seem to ever substitute the
explicit-specifier.  I think we forget to lookup the explicit-specifier when
instantiating the out-of-line definition of operator Foo.

Since I've implemented explicit(bool), mine.

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Reduced:

template <typename T>
struct Foo {
  template <typename U>
  explicit(static_cast<U>(true)) operator Foo<U>();
};

template <typename T>
template <typename U>
Foo<T>::operator Foo<U>() {
  return {};
}

int
main ()
{
  Foo<float> a;
  Foo<int> b = a;
}

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Patch posted: https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545565.html

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:661232da72d29f8f2385d5f588727beb74360144

commit r11-371-g661232da72d29f8f2385d5f588727beb74360144
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon May 11 18:28:19 2020 -0400

    c++: explicit(bool) malfunction with dependent expression [PR95066]

    I forgot to set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P when merging two
    function declarations and as a sad consequence, we never tsubsted
    the dependent explicit-specifier in tsubst_function_decl, leading to
    disregarding the explicit-specifier altogether, and wrongly accepting
    this test.

            PR c++/95066
            * decl.c (duplicate_decls): Set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P.

            * g++.dg/cpp2a/explicit16.C: New test.

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:ae275b986b8cc747a5b4f389cb05a71fdee1f886

commit r10-8280-gae275b986b8cc747a5b4f389cb05a71fdee1f886
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Jun 11 16:33:13 2020 -0400

    c++: explicit(bool) malfunction with dependent expression [PR95066]

    I forgot to set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P when merging two
    function declarations and as a sad consequence, we never tsubsted
    the dependent explicit-specifier in tsubst_function_decl, leading to
    disregarding the explicit-specifier altogether, and wrongly accepting
    this test.

            PR c++/95066
            * decl.c (duplicate_decls): Set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P.

            * g++.dg/cpp2a/explicit16.C: New test.

[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.