c: Avoid ICE with _BitInt(N) : 0 bitfield [PR113740]

finish_struct already made sure not to call build_bitint_type for
signed _BitInt(2) : 1;
or
signed _BitInt(2) : 0;
bitfields (but instead build a zero precision integral type,
we remove it later), this patch makes sure we do it also for
unsigned _BitInt(1) : 0;
because of the build_bitint_type assertion that precision is
>= (unsigned ? 1 : 2).

2024-02-05  Jakub Jelinek  <[email protected]>

	PR c/113740
	* c-decl.cc (finish_struct): Only use build_bitint_type if
	bit-field has width larger or equal to minimum _BitInt
	precision.

	* gcc.dg/bitint-85.c: New test.

Author: jakubjelinek

gcc/c/c-decl.cc

@@ -9555,7 +9555,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 	  if (width != TYPE_PRECISION (type))
 	    {
 	      if (TREE_CODE (type) == BITINT_TYPE
-		  && (width > 1 || TYPE_UNSIGNED (type)))
+		  && width >= (TYPE_UNSIGNED (type) ? 1 : 2))
 		TREE_TYPE (field)
 		  = build_bitint_type (width, TYPE_UNSIGNED (type));
 	      else

gcc/testsuite/gcc.dg/bitint-85.c

@@ -0,0 +1,5 @@
+/* PR c/113740 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-std=c23" } */
+
+struct S { unsigned _BitInt(32) : 0; };