[Bug c++/90849] New: Gcc accepts invalid code

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90849

            Bug ID: 90849
           Summary: Gcc accepts invalid code
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

My gcc is 10.0.0, and the code is:

template < typename = void > 
class A 
{
public:
 A () : f0 ( 1 ) { }
 A (int);

private:
 typedef A<> f0;
};

A<> a;

The code comes from a previous bug report:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70468

It is said to be invalid and it triggers a ICE. The bug is marked as fixed. I
tried gcc 10.0.0, it accepts the code. If it is invalid, gcc shall reject it.
BTW, clang reject it:

<source>:5:9: error: type 'A::f0' (aka 'A<>') is not a direct or virtual base
of 'A<type-parameter-0-0>'

 A () : f0 ( 1 ) { }

        ^~

1 error generated.

Compiler returned: 1