[Bug c++/77540] New: Confusing diagnostics due to stray comma in ctor-init-list

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77540

            Bug ID: 77540
           Summary: Confusing diagnostics due to stray comma in
                    ctor-init-list
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

struct A { };

struct B : A
{
  B() : A(),  // N.B. erroneous comma here
  {
  }

  typedef int C;

  C c;
};

This says:

e.cc:11:3: error: ‘C’ does not name a type
   C c;
   ^
e.cc: In constructor ‘B::B()’:
e.cc:6:3: error: expected identifier before ‘{’ token
   {
   ^

This doesn't help find the error in the code.

The first diagnostic is a consequence of the second one, so a user who tries to
fix the errors in the order they appear (usually the best approach to fixing
compiler errors) is stuck, because C certainly *is* a type from the user's
perspective. They have to ignore the "does not name a type" error and fix the
next one instead. If there are several uses of C then there could be several
errors before the important "expected identifier" one.

The second diagnostic is quite vague, and could have a better location. A
better diagnostic, with fixit hint, would be:

e.cc: In constructor ‘B::B()’:
e.cc:6:3: error: expected identifier before ‘{’ token. Remove the comma?
  B() : A(),  // N.B. erroneous comma here
           ^

Ideally this would also be printed first, but I realise that the "does not name
a type" error comes when compiling the class body, which happens before the
constructor body. I assume the parser consumes everything up to the semi-colon
after the typedef declaration as part of the constructor body, and so the
declaration isn't seen before the uses of C.