Fix ICE with -g and -std=c23 related to incomplete types [PR114361]

We did not copy TYPE_CANONICAL to the incomplete variants when
completing a structure.

	PR c/114361

	gcc/c/
	* c-decl.cc (finish_struct): Set TYPE_CANONICAL when completing
	strucute types.

	gcc/testsuite/
	* gcc.dg/pr114361.c: New test.
	* gcc.dg/c23-tag-incomplete-1.c: New test.
	* gcc.dg/c23-tag-incomplete-2.c: New test.

gcc/c/c-decl.cc

@@ -9722,6 +9722,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       C_TYPE_VARIABLE_SIZE (x) = C_TYPE_VARIABLE_SIZE (t);
       C_TYPE_VARIABLY_MODIFIED (x) = C_TYPE_VARIABLY_MODIFIED (t);
       C_TYPE_INCOMPLETE_VARS (x) = NULL_TREE;
+      TYPE_CANONICAL (x) = TYPE_CANONICAL (t);
     }
 
   /* Update type location to the one of the definition, instead of e.g.

gcc/testsuite/gcc.dg/c23-tag-incomplete-1.c

@@ -0,0 +1,14 @@
+/* { dg-do compile }
+ * { dg-options "-std=c23 -g" } */
+
+struct a;
+typedef struct a b;
+
+void g() {
+    struct a { b* x; };
+}
+
+struct a { b* x; };
+
+
+

gcc/testsuite/gcc.dg/c23-tag-incomplete-2.c

@@ -0,0 +1,13 @@
+/* { dg-do compile }
+ * { dg-options "-std=c23 -g" } */
+
+struct a;
+typedef struct a b;
+
+void f() {
+	extern struct a { b* x; } t;
+}
+
+extern struct a { b* x; } t;
+
+

gcc/testsuite/gcc.dg/pr114361.c

@@ -0,0 +1,11 @@
+/* PR c/114361 */
+/* { dg-do compile } */
+/* { dg-options "-std=gnu23 -g" } */
+
+void f()
+{
+    typedef struct foo bar;
+    typedef __typeof( ({ (struct foo { bar *x; }){ }; }) ) wuz;
+    struct foo { wuz *x; };
+}
+