[Bug c++/94922] New: Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[Bug c++/94922] New: Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94922

            Bug ID: 94922
           Summary: Functional cast from base class to derived class
                    incorrectly accepted with -std=c++2a
           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
struct base {};
struct derived : base {};

template <typename T, typename R, typename = decltype(T(R()))>
void foo();

void
bar()
{
  foo<derived, base>();
}

$ g++-10 -std=c++2a testcase.C
$ g++-10 -std=c++17 testcase.C
testcase.C: In function ‘void bar()’:
testcase.C:10:22: error: no matching function for call to ‘foo<derived,
base>()’
   13 |   foo<derived, base>();
      |                      ^
...

[Bug c++/94922] Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94922

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The above appears to be a 10/11 regression, but note that all versions of GCC
>= 4.7 also incorrectly accept the following testcase:

struct base {};
struct derived : base {};

template <typename = decltype(derived(base()))>
void foo();

void
bar()
{
  foo();
}

[Bug c++/94922] Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94922

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Another testcase that exhibits the 10/11 regression:

struct base {};
struct derived : base {};

void
foo()
{
  derived a = derived(base());
}

[Bug c++/94922] Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94922

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Upon closer investigation I think we're correct to accept these testcases.  As
I understand it the construction 'Derived(Base())' is valid in C++20 as a
direct consequence of C++17's aggregate init of classes with base classes
(P0017R1) in conjunction with C++20's parenthesized init of aggregates
(P0960R3).

Thus closing this bug as invalid, sorry for the noise.