[Bug c/106797] New: Improvement: diagnose undefined behavior: not all declarations that refer to the same object or function have compatible type

[Bug c/106797] New: Improvement: diagnose undefined behavior: not all declarations that refer to the same object or function have compatible type

[pavel.morozkin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106797

            Bug ID: 106797
           Summary: Improvement: diagnose undefined behavior: not all
                    declarations that refer to the same object or function
                    have compatible type
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pavel.morozkin at gmail dot com
  Target Milestone: ---

This code:
static int (*x)[];

void f1(void)
{
    extern int (*x)[3];
}

void f2(void)
{
    extern int (*x)[5];
}

compiled with -std=c11 -pedantic -Wall -Wextra -Wno-unused-variable triggers UB
(see below) and leads to no diagnostics. It is proposed to produce this (or
similar) diagnostics: "not all declarations that refer to the same object x
have compatible type" OR "declarations that refer to the same object x have
types 'int (*)[5]' and 'int (*)[3]' which are not compatible".

Relevant quote from C11, 6.2.7 Compatible type and composite type, 2:
> All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

In the program above types 'int (*)[5]' and 'int (*)[3]' are not compatible. It
is useful to diagnose that.

[Bug c/106797] Improvement: diagnose undefined behavior: not all declarations that refer to the same object or function have compatible type

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106797

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
See bug 21343.

*** This bug has been marked as a duplicate of bug 21343 ***

[Bug c/106797] Improvement: diagnose undefined behavior: not all declarations that refer to the same object or function have compatible type

[pavel.morozkin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106797

--- Comment #2 from Pavel M <pavel.morozkin at gmail dot com> ---
Observation: if "static" is removed, then GCC generates:
<source>: In function 'f2':
<source>:10:18: error: conflicting types for 'x'; have 'int (*)[5]'
   10 |     extern int (*x)[5];
      |                  ^
<source>:5:18: note: previous declaration of 'x' with type 'int (*)[3]'
    5 |     extern int (*x)[3];
      |