[Bug c++/92438] New: Function declaration parsed incorrectly with `-std=c++1z`

[sagebar at web dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92438

            Bug ID: 92438
           Summary: Function declaration parsed incorrectly with
                    `-std=c++1z`
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sagebar at web dot de
  Target Milestone: ---

Created attachment 47204
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47204&action=edit
Demonstration of incorrect parsing + several work-arounds

Functions declarations returning a typedef'ed struct-type, and having their
names wrapped by parenthesis, as well as a calling-convention attribute within
those parenthesis, causes g++ to think that it's a variable declaration, rather
than a function declaration.

Essentially, given something like this:
```
typedef struct myint_struct { int x; } MYINT;
```

All of the following declarations get parsed correctly:
```
MYINT (myabs1)(MYINT x);
MYINT __attribute__((stdcall)) myabs2(MYINT x);
struct myint_struct (__attribute__((stdcall)) myabs3)(MYINT x);
```

However, this declaration (incorrectly) causes a parser error:
```
MYINT (__attribute__((stdcall)) myabs4)(MYINT x);
```

Note that the problem only appears when `-std=c++1z` or `-std=gnu++1z` is
passed on the commandline, such that you can re-produce the problem with the
attached file and:
```
g++ -c -std=c++1z attached-file.cc
g++ -c -std=gnu++1z attached-file.cc
```

However, compiling it without any special std=... flag doesn't produce any
errors:
```
g++ -c attached-file.cc
```