[Bug c++/89220] New: Inconsistent behaviour of class template type deduction

[matt at ookypooky dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89220

            Bug ID: 89220
           Summary: Inconsistent behaviour of class template type
                    deduction
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at ookypooky dot com
  Target Milestone: ---

Demonstrated by the following code:

  template<typename T>
  struct Foo
  {
    Foo (T) {}
    auto operator () (int) { return 0; }
  };

  // this compiles
  auto x = Foo(123)(0);

  // this doesn't
  auto y = (Foo(123))(0);

When compiled with "g++ -std=c++17". See here https://godbolt.org/z/Xrmf_8

Interestingly with "-std=c++1y" both x and y fail to compile, which is at least
consistent (even if I'm not sure it's correct...)