[Bug c++/114265] New: Unhelpful message when var name is also a struct name

[Bug c++/114265] New: Unhelpful message when var name is also a struct name

[denis.campredon at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114265

            Bug ID: 114265
           Summary: Unhelpful message when var name is also a struct name
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: denis.campredon at gmail dot com
  Target Milestone: ---

While working with legacy code, I encountered the following problem.

The code:
------------
struct Struct;

int fn(Struct *);

Struct *Struct;

int fn(Struct *)
{ 
    return 0;
}
-------------
produces the following error message:
-------------
aaaaaa.c:7:8: error: 'int fn' redeclared as different kind of entity
    7 | int fn(Struct *)
      |        ^~~~~~
aaaaaa.c:3:5: note: previous declaration 'int fn(Struct*)'
    3 | int fn(Struct *);
      |     ^~
aaaaaa.c:7:16: error: expected primary-expression before ')' token
    7 | int fn(Struct *)
--------------

This message does not really explain what the real problem is.
A message saying that `Struct` has been redefined as a variable would help
more.

[Bug c++/114265] Unhelpful message when var name is also a struct name

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114265

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-03-07
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But it's valid to define Struct as a variable after also defining it as a type,
the problem is only when you later refer to it as a type. That's why the error
talks about the 'fn' being redeclared. When it sees `int fn(Struct*)` the
second time, the language grammar says you are declaring an int variable, and
initializing it with an arithmetic expression involving the Struct variable
multiplied by a missing operand.

The code would be valid if the use of Struct after its redefinition used an
elaborated-type-specifier to refer to it (i.e. included the 'struct' keyword):

int fn(struct Struct *)
{ 
    return 0;
}

Maybe it would be possible for the "redeclared as a different kind of entity"
to try to parse `int fn(Struct*)` again, excluding non-types from name lookup
(except in unevaluated operands and default arguments?) and then if that
succeeds, adda fix-it hint for the elaborated-type-specifier (with either
'struct', 'class', 'union', or 'enum' as appropriate):

note: use 'struct Struct' to refer to the type
int fn(struct Struct *)
       ^~~~~~



For comparison, other compilers give very similar results:

Clang gives:

s.cc:7:5: error: redefinition of 'fn' as different kind of symbol
int fn(Struct *)
    ^
s.cc:3:5: note: previous definition is here
int fn(Struct *);
    ^
s.cc:7:16: error: expected expression
int fn(Struct *)
               ^
s.cc:7:17: error: expected ';' after top level declarator
int fn(Struct *)
                ^
                ;
3 errors generated.


EDG gives:


"s.cc", line 7: error: declaration is incompatible with "int fn(Struct *)"
          (declared at line 3)
  int fn(Struct *)
      ^

"s.cc", line 7: error: expected an expression
  int fn(Struct *)
                 ^

"s.cc", line 8: error: expected a ";"
  { 
  ^

3 errors detected in the compilation of "s.cc".



MSVC seems the least helpful:

<source>(7): error C2059: syntax error: ')'
<source>(8): error C2143: syntax error: missing ';' before '{'
<source>(8): error C2447: '{': missing function header (old-style formal list?)