[Bug c++/102156] New: `cannot call constructor` error during member access

[Bug c++/102156] New: `cannot call constructor` error during member access

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102156

            Bug ID: 102156
           Summary: `cannot call constructor` error during member access
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The following valid code accesses a field in a base class:
```
struct A { int v; };
template <typename T> struct B : T {};
struct C : B<A> {};
int main() { return C{}.B::B<A>::v; }

```

but GCC prints an error:
```
cannot call constructor 'B<A>::B' directly
```
https://gcc.godbolt.org/z/hchqG7Tq8

[Bug c++/102156] `cannot call constructor` error during member access

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102156

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There was an C++ old defect report about B::B being the name of the constructor
or the class name.

[Bug c++/102156] [9/10/11/12 Regression] `cannot call constructor` error during member access

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102156

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
            Summary|`cannot call constructor`   |[9/10/11/12 Regression]
                   |error during member access  |`cannot call constructor`
                   |                            |error during member access
   Target Milestone|---                         |9.5
      Known to work|                            |4.4.7
   Last reconfirmed|                            |2021-09-01
             Status|UNCONFIRMED                 |NEW
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=11764
           Keywords|                            |rejects-valid
      Known to fail|                            |4.5.4

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> There was an C++ old defect report about B::B being the name of the
> constructor or the class name.

But it looks like it is GCC getting confused because of that defect report
(PR11764).

And here is a testcase which makes this a regression from GCC 4.4.0 (valid
C++98)

struct C{int tt;};
template<class T >
struct B :T { int t;};
struct A :B<C>
{
};

int main()
{
    A t;
    return t.B<C>::B<C>::tt;
}

[Bug c++/102156] [9/10/11/12 Regression] `cannot call constructor` error during member access

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102156

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/102156] [9/10/11/12 Regression] `cannot call constructor` error during member access

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102156

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |jason at gcc dot gnu.org
             Status|NEW                         |RESOLVED

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Why do you think that DR147 doesn't apply to this testcase?  We see B::B<, look
up B in B, and DR147 says we find the constructor.

The rule mentioned in PR11764, that lookup in a nested-name-specifier only
considers types and namespaces, doesn't apply here because the name B isn't
followed directly by ::.  The current draft even has an example to illustrate
this:

template<int> struct B : A {};
namespace N {
  template<int> void B();
  int f() {
    return B<0>::n;     // error: N​::​B<0> is not a type
  }
}

EDG also rejects this testcase.