[Bug c/91815] New: questionable error on type definition at file scope

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91815

            Bug ID: 91815
           Summary: questionable error on type definition at file scope
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ebotcazou at gcc dot gnu.org
  Target Milestone: ---

The C compiler currently rejects the following code:

int f (void)
{
  extern int t;
  extern float v;   

  return (v > 0.0f);
}

typedef float t;

t v = 4.5f;

with the message:

t.c:9:15: error: 't' redeclared as different kind of symbol
    9 | typedef float t;
      |               ^
t.c:3:14: note: previous declaration of 't' was here
    3 |   extern int t;
      |

This behavior looks questionable and both Clang and MSVC disagree with GCC.

And a cursory look at the relevant code in pushdecl seems to point to an
overzealous implementation of C99 6.2.7p2, leading to a possible simple fix:

Index: c-decl.c
===================================================================
--- c-decl.c    (revision 275746)
+++ c-decl.c    (working copy)
@@ -3131,7 +3131,8 @@ pushdecl (tree x)
      what scope they are in; this is what we do here.  (C99 6.2.7p2:
      All declarations that refer to the same object or function shall
      have compatible type; otherwise, the behavior is undefined.)  */
-  if (DECL_EXTERNAL (x) || scope == file_scope)
+  if ((DECL_EXTERNAL (x) || scope == file_scope)
+      && VAR_OR_FUNCTION_DECL_P (x))
     {
       tree type = TREE_TYPE (x);
       tree vistype = NULL_TREE;