C23: allow aliasing for types derived from structs with variable size

Previously, we set the aliasing set of structures with variable size

struct foo { int x[n]; char b; };

to zero. The reason is that such types can be compatible to diffrent
structure types which are incompatible.

struct foo { int x[2]; char b; };
struct foo { int x[3]; char b; };

But it is not enough to set the aliasing set to zero, because derived
types would then still end up in different equivalence classes even
though they might be compatible.  Instead those types should be set
to structural equivalency.  We also add checking assertions that
ensure that TYPE_CANONICAL is set correctly for all tagged types.

gcc/c/
	* c-decl.cc (finish_struct): Do not set TYPE_CANONICAL for
	structure or unions with variable size.
	* c-objc-common.cc (c_get_alias_set): Do not set alias set to zero.
	* c-typeck.cc (comptypes_verify): New function.
	(comptypes,comptypes_same_p,comptypes_check_enum_int): Add assertion.
	(comptypes_equiv_p): Add assertion that ensures that compatible
	types have the same equivalence class.
	(tagged_types_tu_compatible_p): Remove now unneeded special case.

gcc/testsuite/
	* gcc.dg/gnu23-tag-alias-8.c: New test.

1 files changed, 0 insertions, 5 deletions

diff --git a/gcc/c/c-objc-common.cc b/gcc/c/c-objc-common.cc
index 283f6a8..738e899 100644
--- a/gcc/c/c-objc-common.cc
+++ b/gcc/c/c-objc-common.cc
@@ -420,11 +420,6 @@ c_var_mod_p (tree x, tree fn ATTRIBUTE_UNUSED)
 alias_set_type
 c_get_alias_set (tree t)
 {
-  /* Structs with variable size can alias different incompatible
-     structs.  Let them alias anything.   */
-  if (RECORD_OR_UNION_TYPE_P (t) && C_TYPE_VARIABLE_SIZE (t))
-    return 0;
-
   return c_common_get_alias_set (t);
 }