c: Implement new -Wenum-int-mismatch warning [PR105131]

In C, an enumerated type is compatible with char, a signed integer type,
or an unsigned integer type (6.7.2.2/5).  Therefore this code compiles:

  enum E { l = -1, z = 0, g = 1 };
  int foo(void);
  enum E foo(void) { return z; }

if the underlying type of 'enum E' is 'int' (if not, we emit an error).
This is different for typedefs, where C11 permits typedefs to be
redeclared to the same type, but not to compatible types.  In C++, the
code above is invalid.

It seems desirable to emit a warning in the C case, because it is
probably a mistake and definitely a portability error, given that the
choice of the underlying type is implementation-defined.

To that end, this patch implements a new -Wenum-int-mismatch warning.
Conveniently, we already have comptypes_check_enum_int to detect such
mismatches.  This warning is enabled by either -Wall or -Wc++-compat.

	PR c/105131

gcc/c-family/ChangeLog:

	* c.opt (Wenum-int-mismatch): New.

gcc/c/ChangeLog:

	* c-decl.cc (diagnose_mismatched_decls): Warn about enum/integer type
	mismatches.
	* c-tree.h (comptypes_check_enum_int): Declare.
	* c-typeck.cc (comptypes): No longer static.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wenum-int-mismatch.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wenum-int-mismatch-1.c: New test.
	* gcc.dg/Wenum-int-mismatch-2.c: New test.
	* gcc.dg/Wenum-int-mismatch-3.c: New test.
	* gcc.dg/Wenum-int-mismatch-4.c: New test.
	* gcc.dg/Wenum-int-mismatch-5.c: New test.

3 files changed, 13 insertions, 3 deletions

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 8365554..5266a61 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -1993,9 +1993,12 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 
   bool pedwarned = false;
   bool warned = false;
+  bool enum_and_int_p = false;
   auto_diagnostic_group d;
 
-  if (!comptypes (oldtype, newtype))
+  int comptypes_result = comptypes_check_enum_int (oldtype, newtype,
+						   &enum_and_int_p);
+  if (!comptypes_result)
     {
       if (TREE_CODE (olddecl) == FUNCTION_DECL
 	  && fndecl_built_in_p (olddecl, BUILT_IN_NORMAL)
@@ -2137,6 +2140,13 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	  return false;
 	}
     }
+  /* Warn about enum/integer type mismatches.  They are compatible types
+     (C2X 6.7.2.2/5), but may pose portability problems.  */
+  else if (enum_and_int_p && TREE_CODE (newdecl) != TYPE_DECL)
+    warned = warning_at (DECL_SOURCE_LOCATION (newdecl),
+			 OPT_Wenum_int_mismatch,
+			 "conflicting types for %q+D due to enum/integer "
+			 "mismatch; have %qT", newdecl, newtype);
 
   /* Redeclaration of a type is a constraint violation (6.7.2.3p1),
      but silently ignore the redeclaration if either is in a system
@@ -2146,7 +2156,6 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
   if (TREE_CODE (newdecl) == TYPE_DECL)
     {
       bool types_different = false;
-      int comptypes_result;
 
       comptypes_result
 	= comptypes_check_different_types (oldtype, newtype, &types_different);
diff --git a/gcc/c/c-tree.h b/gcc/c/c-tree.h
index c70f0ba..2bcb966 100644
--- a/gcc/c/c-tree.h
+++ b/gcc/c/c-tree.h
@@ -685,6 +685,7 @@ extern tree require_complete_type (location_t, tree);
 extern bool same_translation_unit_p (const_tree, const_tree);
 extern int comptypes (tree, tree);
 extern int comptypes_check_different_types (tree, tree, bool *);
+extern int comptypes_check_enum_int (tree, tree, bool *);
 extern bool c_vla_type_p (const_tree);
 extern bool c_mark_addressable (tree, bool = false);
 extern void c_incomplete_type_error (location_t, const_tree, const_tree);
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index bcfe08b..4f3611f 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -1055,7 +1055,7 @@ comptypes (tree type1, tree type2)
 /* Like comptypes, but if it returns non-zero because enum and int are
    compatible, it sets *ENUM_AND_INT_P to true.  */
 
-static int
+int
 comptypes_check_enum_int (tree type1, tree type2, bool *enum_and_int_p)
 {
   const struct tagged_tu_seen_cache * tagged_tu_seen_base1 = tagged_tu_seen_base;