[Bug c++/94521] New: Infinite loop with decltype of function parameter of type decltype([]{})

[Bug c++/94521] New: Infinite loop with decltype of function parameter of type decltype([]{})

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

            Bug ID: 94521
           Summary: Infinite loop with decltype of function parameter of
                    type decltype([]{})
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

$ cat testcase.C
template <typename>
void spam(decltype([]{}) *s)
{
  static_assert(__is_same(int, decltype(s)));
}

void foo()
{
  spam<int>(nullptr);
}

$ g++ -std=c++2a testcase.C
^C

[Bug c++/94521] Infinite loop with decltype of function parameter of type decltype([]{})

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-07
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.  Looks like it never worked.

[Bug c++/94521] Infinite loop with decltype of function parameter of type decltype([]{})

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

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++/94521] Infinite loop with decltype of function parameter of type decltype([]{})

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

--- Comment #2 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:1dcb779916502a44b4ae67d6bf60eb59474bd78c

commit r10-7705-g1dcb779916502a44b4ae67d6bf60eb59474bd78c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Apr 13 16:53:41 2020 -0400

    c++: Infinite diagnostic loop with decltype([]{}) [PR94521]

    We are hitting a recursive loop when printing the signature of a function
    containing a decltype([]{}) type.  The loop is

      dump_function_decl -> dump_substitution
                         -> dump_template_bindings
                         -> dump_type
                         -> dump_aggr_type
                         -> dump_scope -> dump_function_decl

    and we loop because dump_template_bindings wants to print the resolved type
of
    decltype([]{}) (i.e. just a lambda type), so it calls dump_aggr_type, which
    wants to print the function scope of the lambda type.  But the function
scope of
    the lambda type is the function which we're in the middle of printing.

    This patch breaks the loop by passing TFF_NO_FUNCTION_ARGUMENTS to
    dump_function_decl from dump_scope, so that we avoid recursing into
    dump_substitution and ultimately looping.

    This also means we no longer emit the "[with ...]" clause when printing a
    function template scope, and we instead just emit its template argument
list in
    a more natural way, e.g. instead of
        foo(int, char) [with T=bool]::x
    we would now print
        foo<bool>::x
    which seems like an improvement on its own.

    The full signature of the function 'spam' in the below testcase is now
      void spam(decltype (<lambda>)*) [with T = int; decltype (<lambda>) =
spam<int>::<lambda()>]

    gcc/cp/ChangeLog:

            PR c++/94521
            * error.c (dump_scope): Pass TFF_NO_FUNCTION_ARGUMENTS to
            dump_function_decl when printing a function template instantiation
as a
            scope.

    gcc/testsuite/ChangeLog:

            PR c++/94521
            * g++.dg/cpp2a/lambda-uneval12.C: New test.

[Bug c++/94521] Infinite loop with decltype of function parameter of type decltype([]{})

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |10.0

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 10.

[Bug c++/94521] Infinite loop with decltype of function parameter of type decltype([]{})

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94521

--- Comment #4 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:7fb9a1e929db520fd741e60d84ec1a58581a8299

commit r11-7406-g7fb9a1e929db520fd741e60d84ec1a58581a8299
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Feb 25 16:44:34 2021 -0500

    c++: Fix pretty printing the context of local class [PR99213]

    My r10-7705 patch for PR94521 made us set TFF_NO_FUNCTION_ARGUMENTS when
    pretty printing the function scope of a local class type in order to
    eliminate infinite recursion with a function signature that contains
    decltype([]{}).  But due to the way dump_function_decl works, this
    change regressed our pretty printing of local class types whose context
    contains a class template specialization, as in the testcase below, in
    which we wrongly pretty print the two local types as 'A<T>::f<char>::S1'
    and 'B<T>::f<int>::S2'.

    This patch makes dump_scope pass TFF_NO_TEMPLATE_BINDINGS instead of
    TFF_NO_FUNCTION_ARGUMENTS when pretty printing a function scope.  It
    appears this is the strictly better flag to use: it avoids the infinite
    recursion issue, it restores pretty printing of the function parameter
    list, and it stops dump_function_decl from trying to print a function
    template specialization in its own weird way.

    Summary of pretty printing differences for the below testcase:

      r10-7704:   A<T>::f() [with U = char; T = int]::S1
                  B<T>::f() [with T = int]::S2

      r10-7705:   A<T>::f<char>::S1
                  B<T>::f<int>::S2

      this patch: A<int>::f<char>()::S1
                  B<int>::f()::S2

    gcc/cp/ChangeLog:

            PR c++/99213
            PR c++/94521
            * error.c (dump_scope): Pass TFF_NO_TEMPLATE_BINDINGS instead of
            TFF_NO_FUNCTION_ARGUMENTS when dumping a function scope.

    gcc/testsuite/ChangeLog:

            PR c++/99213
            PR c++/94521
            * g++.dg/diagnostic/local1.C: New test.