[Bug c++/91760] New: Trailing return type breaks final specifier

[john.boyer at tutanota dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91760

            Bug ID: 91760
           Summary: Trailing return type breaks final specifier
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john.boyer at tutanota dot com
  Target Milestone: ---

The following code:

struct final;
struct test
{
    virtual auto foo() -> int final;
};

Generates this error:

error: two or more data types in declaration of 'type name'

Example: https://godbolt.org/z/4GSoW_

The same code works fine on clang (https://godbolt.org/z/4d3J85), MSVC
(https://godbolt.org/z/BYq2yW) and ICC (https://godbolt.org/z/_V9Qvd).

Since final and override are "identifiers with special meaning", this code
should compile as there is no ambiguity on whether or not "final" can be an
identifier in this context.

If the trailing return type is changed to a normal return type, this of course
will compile as expected (https://godbolt.org/z/ReWHag):

struct final;
struct test
{
    virtual int foo() final;
};