[Bug c++/94592] New: ICE in non-type template parameter with constexpr constructor

[Bug c++/94592] New: ICE in non-type template parameter with constexpr constructor

[pacoarjonilla at yahoo dot es]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

            Bug ID: 94592
           Summary: ICE in non-type template parameter with constexpr
                    constructor
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pacoarjonilla at yahoo dot es
  Target Milestone: ---

GCC10 --std=c++20

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

struct A {
    constexpr A() {}
};

template <A> struct B { };

template<typename> void bar () {
    B<{}> var;
}

void fu() {
    bar<int>();
}

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


meta.cc: In instantiation of ‘struct B<((A)<brace-enclosed initializer
list>())>’:
meta.cc:37:11:   required from ‘void bar() [with <template-parameter-1-1> =
int]’
meta.cc:41:14:   required from here
meta.cc:34:23: internal compiler error: unexpected expression
‘(A)<brace-enclosed initializer list>()’ of kind implicit_conv_expr
   34 | template <A> struct B { };
      |                       ^
0x8c3f37 cxx_eval_constant_expression
        /home/paco/git/gcc/gcc/cp/constexpr.c:6301
0x8c4366 cxx_eval_outermost_constant_expr
        /home/paco/git/gcc/gcc/cp/constexpr.c:6502
0x8c8654 maybe_constant_value(tree_node*, tree_node*, bool, bool)
        /home/paco/git/gcc/gcc/cp/constexpr.c:6774
0x9f860c convert_nontype_argument
        /home/paco/git/gcc/gcc/cp/pt.c:7099
0x9f860c convert_template_argument
        /home/paco/git/gcc/gcc/cp/pt.c:8331
0x9f9893 coerce_template_parms
        /home/paco/git/gcc/gcc/cp/pt.c:8810
0xa1bf61 lookup_template_class_1
        /home/paco/git/gcc/gcc/cp/pt.c:9648
0xa1e363 lookup_template_class(tree_node*, tree_node*, tree_node*, tree_node*,
int, int)
        /home/paco/git/gcc/gcc/cp/pt.c:10020
0xa1e363 tsubst_aggr_type
        /home/paco/git/gcc/gcc/cp/pt.c:13301
0xa0e44f tsubst_decl
        /home/paco/git/gcc/gcc/cp/pt.c:14294
0xa3079d instantiate_class_template_1
        /home/paco/git/gcc/gcc/cp/pt.c:11748
0xa31ab2 instantiate_class_template(tree_node*)
        /home/paco/git/gcc/gcc/cp/pt.c:12007
0xa77ff9 complete_type(tree_node*)
        /home/paco/git/gcc/gcc/cp/typeck.c:137
0xa0e70b tsubst_decl
        /home/paco/git/gcc/gcc/cp/pt.c:14415
0xa063e0 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
        /home/paco/git/gcc/gcc/cp/pt.c:17768
0xa0449e tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
        /home/paco/git/gcc/gcc/cp/pt.c:17985
0xa01733 instantiate_decl(tree_node*, bool, bool)
        /home/paco/git/gcc/gcc/cp/pt.c:25509
0xa31d7b instantiate_pending_templates(int)
        /home/paco/git/gcc/gcc/cp/pt.c:25625
0x93b9f3 c_parse_final_cleanups()
        /home/paco/git/gcc/gcc/cp/decl2.c:4874
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ice-on-valid-code

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Huh, constexprs are valid template parameters?

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org
           Keywords|ice-on-valid-code           |ice-on-invalid-code

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
Both clang and ICC reject that. So I bet it's an invalid code.

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[pacoarjonilla at yahoo dot es]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

Paco Arjonilla <pacoarjonilla at yahoo dot es> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|ice-on-invalid-code         |ice-on-valid-code

--- Comment #3 from Paco Arjonilla <pacoarjonilla at yahoo dot es> ---
It should be valid code.
https://wg21.link/P0732R2

The reason why other compilers reject it is because they have not implemented
it yet.
https://en.cppreference.com/w/cpp/compiler_support
Class types in non-type template parameters (Around halfway of C++20 language
features)

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-14
     Ever confirmed|0                           |1
   Target Milestone|---                         |10.0
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
           Priority|P3                          |P1
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r10-7096-gd417b4f5414d9076300ab41974a14424f722688c so mine.

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
{} as a template argument is currently only supported by GCC as an extension,
but I raised this on the core C++ list and it seems that the the conclusion is
that we want this to work, though there's no CWG for it yet.

[Bug c++/94592] ICE in non-type template parameter with constexpr constructor

[pacoarjonilla at yahoo dot es]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

--- Comment #6 from Paco Arjonilla <pacoarjonilla at yahoo dot es> ---
Thanks for supporting this feature. I think it is one of the core features that
modern C++ should have.

[Bug c++/94592] [10 Regression] ICE in non-type template parameter with constexpr constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Another problematical testcase:

struct A {
    int i;
    constexpr A(int n) : i(n) {}
};

template <A a> struct B { int i; constexpr B() : i(a.i) { } };

template<typename> void bar () {
    B<{1}> var;
}

void fu() {
    bar<int>();
}

[Bug c++/94592] [10 Regression] ICE in non-type template parameter with constexpr constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

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

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

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Patch posted: https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544068.html

[Bug c++/94592] [10 Regression] ICE in non-type template parameter with constexpr constructor

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |9.3.0

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Indeed works with GCC 9.

[Bug c++/94592] [10 Regression] ICE in non-type template parameter with constexpr constructor

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

--- Comment #10 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:d419e176d74162af6513be0b3bc1031726543993

commit r10-7836-gd419e176d74162af6513be0b3bc1031726543993
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Apr 17 15:47:15 2020 -0400

    c++: Fix ICE with { } as template argument [PR94592]

    As an extension (there should be a CWG about this though), we support
    braced-init-list as a template argument, but convert_nontype_argument
    had trouble digesting them.  We ICEd because of the double coercion we
    perform for template arguments: convert_nontype_argument called from
    finish_template_type got a { }, and since a class type was involved and
    we were in a template, convert_like created an IMPLICIT_CONV_EXPR.  Then
    the second conversion of the same argument crashed in constexpr.c
    because the IMPLICIT_CONV_EXPR had gotten wrapped in a TARGET_EXPR.
    Another issue was that an IMPLICIT_CONV_EXPR leaked to constexpr.c when
    building an aggregate init.

    We should have instantiated the IMPLICIT_CONV_EXPR in the first call to
    convert_nontype_argument, but we didn't, because the call to
    is_nondependent_constant_expression returned false because it checks
    !BRACE_ENCLOSED_INITIALIZER_P.  Then non_dep was false even though the
    expression didn't contain anything dependent and we didn't instantiate
    it in convert_nontype_argument.  To fix this, check
    BRACE_ENCLOSED_INITIALIZER_P in cxx_eval_outermost_constant_expr rather
    than in is_nondependent_*.

            PR c++/94592
            * constexpr.c (cxx_eval_outermost_constant_expr): Return when T is
            a BRACE_ENCLOSED_INITIALIZER_P.
            (is_nondependent_constant_expression): Don't check
            BRACE_ENCLOSED_INITIALIZER_P.
            (is_nondependent_static_init_expression): Likewise.

            * g++.dg/cpp2a/nontype-class34.C: New test.
            * g++.dg/cpp2a/nontype-class35.C: New test.

[Bug c++/94592] [10 Regression] ICE in non-type template parameter with constexpr constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94592

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

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

--- Comment #11 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.