diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 64d3a941cb9..194dd595334 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -2039,6 +2039,28 @@ locate_old_decl (tree decl)
 	    decl, TREE_TYPE (decl));
 }
 
+
+/* Subroutine of finish_struct.  For a tagged type, it finds the
+   declaration for a visible tag declared in the the same scope
+   if such a declaration exists.  */
+static tree
+previous_tag (tree type)
+{
+  struct c_binding *b = NULL;
+  tree name = TYPE_NAME (type);
+
+  if (name)
+    b = I_TAG_BINDING (name);
+
+  if (b)
+    b = b->shadowed;
+
+  if (b && B_IN_CURRENT_SCOPE (b))
+    return b->decl;
+
+  return NULL_TREE;
+}
+
 /* Subroutine of duplicate_decls.  Compare NEWDECL to OLDDECL.
    Returns true if the caller should proceed to merge the two, false
    if OLDDECL should simply be discarded.  As a side effect, issues
@@ -8573,11 +8595,14 @@ get_parm_info (bool ellipsis, tree expr)
 	  if (TREE_CODE (decl) != UNION_TYPE || b->id != NULL_TREE)
 	    {
 	      if (b->id)
-		/* The %s will be one of 'struct', 'union', or 'enum'.  */
-		warning_at (b->locus, 0,
-			    "%<%s %E%> declared inside parameter list"
-			    " will not be visible outside of this definition or"
-			    " declaration", keyword, b->id);
+		{
+		  /* The %s will be one of 'struct', 'union', or 'enum'.  */
+		  if (!flag_isoc23)
+		    warning_at (b->locus, 0,
+				"%<%s %E%> declared inside parameter list"
+				" will not be visible outside of this definition or"
+				" declaration", keyword, b->id);
+		}
 	      else
 		/* The %s will be one of 'struct', 'union', or 'enum'.  */
 		warning_at (b->locus, 0,
@@ -8782,6 +8807,14 @@ start_struct (location_t loc, enum tree_code code, tree name,
 
   if (name != NULL_TREE)
     ref = lookup_tag (code, name, true, &refloc);
+
+  /* For C23, even if we already have a completed definition,
+     we do not use it. We will check for consistency later.
+     If we are in a nested redefinition the type is not
+     complete. We will then detect this below.  */
+  if (flag_isoc23 && ref && TYPE_SIZE (ref))
+    ref = NULL_TREE;
+
   if (ref && TREE_CODE (ref) == code)
     {
       if (TYPE_STUB_DECL (ref))
@@ -9576,6 +9609,25 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       warning_at (loc, 0, "union cannot be made transparent");
     }
 
+  /* Check for consistency with previous definition */
+  if (flag_isoc23)
+    {
+      tree vistype = previous_tag (t);
+      if (vistype
+	  && TREE_CODE (vistype) == TREE_CODE (t)
+	  && !C_TYPE_BEING_DEFINED (vistype))
+	{
+	  TYPE_STUB_DECL (vistype) = TYPE_STUB_DECL (t);
+	  if (c_type_variably_modified_p (t))
+	    error ("redefinition of struct or union %qT with variably "
+		   "modified type", t);
+	  else if (!comptypes_same_p (t, vistype))
+	    error ("redefinition of struct or union %qT", t);
+	}
+    }
+
+  C_TYPE_BEING_DEFINED (t) = 0;
+
   tree incomplete_vars = C_TYPE_INCOMPLETE_VARS (TYPE_MAIN_VARIANT (t));
   for (x = TYPE_MAIN_VARIANT (t); x; x = TYPE_NEXT_VARIANT (x))
     {
diff --git a/gcc/c/c-tree.h b/gcc/c/c-tree.h
index c6f38ec94a0..7df4d65bf7a 100644
--- a/gcc/c/c-tree.h
+++ b/gcc/c/c-tree.h
@@ -756,6 +756,7 @@ extern tree c_objc_common_truthvalue_conversion (location_t, tree);
 extern tree require_complete_type (location_t, tree);
 extern bool same_translation_unit_p (const_tree, const_tree);
 extern int comptypes (tree, tree);
+extern bool comptypes_same_p (tree, tree);
 extern int comptypes_check_different_types (tree, tree, bool *);
 extern int comptypes_check_enum_int (tree, tree, bool *);
 extern bool c_mark_addressable (tree, bool = false);
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 1dbb4471a88..dc8a16df272 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -1080,6 +1080,23 @@ comptypes (tree type1, tree type2)
   return ret ? (data.warning_needed ? 2 : 1) : 0;
 }
 
+
+/* Like comptypes, but it returns non-zero only for identical
+   types.  */
+
+bool
+comptypes_same_p (tree type1, tree type2)
+{
+  struct comptypes_data data = { };
+  bool ret = comptypes_internal (type1, type2, &data);
+
+  if (data.different_types_p)
+    return false;
+
+  return ret;
+}
+
+
 /* Like comptypes, but if it returns non-zero because enum and int are
    compatible, it sets *ENUM_AND_INT_P to true.  */
 
@@ -1266,11 +1283,11 @@ comptypes_internal (const_tree type1, const_tree type2,
     case ENUMERAL_TYPE:
     case RECORD_TYPE:
     case UNION_TYPE:
-      if (false)
-	{
-	  return tagged_types_tu_compatible_p (t1, t2, data);
-	}
-      return false;
+
+      if (!flag_isoc23)
+	return false;
+
+      return tagged_types_tu_compatible_p (t1, t2, data);
 
     case VECTOR_TYPE:
       return known_eq (TYPE_VECTOR_SUBPARTS (t1), TYPE_VECTOR_SUBPARTS (t2))
@@ -7031,7 +7048,7 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
   /* Aggregates in different TUs might need conversion.  */
   if ((codel == RECORD_TYPE || codel == UNION_TYPE)
       && codel == coder
-      && comptypes (type, rhstype))
+      && comptypes (TYPE_MAIN_VARIANT (type), TYPE_MAIN_VARIANT (rhstype)))
     return convert_and_check (expr_loc != UNKNOWN_LOCATION
 			      ? expr_loc : location, type, rhs);
 
@@ -8401,6 +8418,13 @@ digest_init (location_t init_loc, tree type, tree init, tree origtype,
 	   conversion.  */
 	inside_init = convert (type, inside_init);
 
+      if ((code == RECORD_TYPE || code == UNION_TYPE)
+	  && !comptypes (TYPE_MAIN_VARIANT (type), TYPE_MAIN_VARIANT (TREE_TYPE (inside_init))))
+	{
+	  error_init (init_loc, "invalid initializer");
+	  return error_mark_node;
+	}
+
       if (require_constant
 	  && TREE_CODE (inside_init) == COMPOUND_LITERAL_EXPR)
 	{
@@ -10486,7 +10510,7 @@ initialize_elementwise_p (tree type, tree value)
     return !VECTOR_TYPE_P (value_type);
 
   if (AGGREGATE_TYPE_P (type))
-    return type != TYPE_MAIN_VARIANT (value_type);
+    return !comptypes (type, TYPE_MAIN_VARIANT (value_type));
 
   return false;
 }
diff --git a/gcc/testsuite/gcc.dg/c23-enum-7.c b/gcc/testsuite/gcc.dg/c23-enum-7.c
index c9ef0882b41..ff8e145c2a8 100644
--- a/gcc/testsuite/gcc.dg/c23-enum-7.c
+++ b/gcc/testsuite/gcc.dg/c23-enum-7.c
@@ -26,17 +26,15 @@ enum e13 : short x13; /* { dg-error "'enum' underlying type may not be specified
 enum e14 : short f14 (); /* { dg-error "'enum' underlying type may not be specified here" } */
 typeof (enum e15 : long) x15; /* { dg-error "'enum' underlying type may not be specified here" } */
 int f16 (enum e16 : char p); /* { dg-error "'enum' underlying type may not be specified here" } */
-/* { dg-warning "will not be visible outside of this definition or declaration" "warning" { target *-*-* } .-1 } */
 int f17 (enum e17 : char); /* { dg-error "'enum' underlying type may not be specified here" } */
-/* { dg-warning "will not be visible outside of this definition or declaration" "warning" { target *-*-* } .-1 } */
 struct s18 { enum e18 : int x; }; /* { dg-error "'enum' underlying type may not be specified here" } */
 
 /* But those are OK if the enum content is defined.  */
 enum e19 : short { E19 } x19;
 enum e20 : long { E20 } f20 ();
 typeof (enum e21 : long { E21 }) x21;
-int f22 (enum e22 : long long { E22 } p); /* { dg-warning "will not be visible outside of this definition or declaration" } */
-int f23 (enum e23 : long long { E23 } p); /* { dg-warning "will not be visible outside of this definition or declaration" } */
+int f22 (enum e22 : long long { E22 } p);
+int f23 (enum e23 : long long { E23 } p);
 struct s24 { enum e24 : int { E24 } x; };
 
 /* Incompatible kinds of tags in the same scope are errors.  */
diff --git a/gcc/testsuite/gcc.dg/c23-tag-1.c b/gcc/testsuite/gcc.dg/c23-tag-1.c
new file mode 100644
index 00000000000..4a6207ec626
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-1.c
@@ -0,0 +1,67 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=c23" }
+ */
+
+// allowed and forbidden redefinitions of the same struct/union in the same scope
+
+typedef struct p { int a; } pd_t;
+typedef struct p { int a; } pd_t;
+
+typedef struct bar { int x; } X;
+typedef struct bar { float x; } Y; /* { dg-error "redefinition of struct or union" } */
+
+void test(void)
+{
+	struct foo { int x; };
+	struct foo { float x; }; /* { dg-error "redefinition of struct or union" } */
+}
+
+struct aa { int a; };
+
+void f(void)
+{
+	typedef struct aa A;
+	struct bb { struct aa a; } x;
+	struct aa { int a; };
+	typedef struct aa A;		/* { dg-error "redefinition" } */
+	struct bb { struct aa a; } y; /* { dg-error "redefinition of struct or union" } */
+	(void)x; (void)y;
+}
+
+
+
+void h(void)
+{
+	struct a2 { int a; };
+	{
+		typedef struct a2 A;
+		struct b2 { struct a2 a; } x;
+		struct a2 { int a; };
+		typedef struct a2 A;		/* { dg-error "redefinition" } */
+		struct b2 { struct a2 a; } y; /* { dg-error "redefinition of struct or union" } */
+		(void)x; (void)y;
+	}
+}
+
+
+union cc { int x; float y; } z;
+union cc { int x; float y; } z1;
+union cc { float y; int x; } z2;	/* { dg-error "redefinition of struct or union" } */
+
+void g(void)
+{
+	struct s { int a; };
+	struct s { int a; } x0;
+	struct p { struct s c; } y1 = { x0 };
+	struct p { struct s { int a; } c; } y = { x0 };
+}
+
+struct q { struct { int a; }; }; 
+struct q { struct { int a; }; }; 
+struct q { int a; };  /* { dg-error "redefinition of struct or union" } */
+
+struct r { int a; char b[]; };
+struct r { int a; char b[]; };
+struct r { int a; char b[1]; }; /* { dg-error "redefinition of struct or union" } */
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-2.c b/gcc/testsuite/gcc.dg/c23-tag-2.c
new file mode 100644
index 00000000000..5dd4a21e9df
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-2.c
@@ -0,0 +1,43 @@
+/* { dg-do compile { target { ! "*-*-*" } } }
+ * { dg-options "-std=c23" }
+ */
+
+// compatibility of structs in assignment
+
+typedef struct p { int a; } pd_t;
+
+void test1(void)
+{
+  pd_t y0;
+  struct p { int a; } x;
+  y0 = x;
+}
+
+void test2(void)
+{
+  struct p { int a; } x;
+  struct p y0 = x;
+}
+
+void test3(void)
+{
+  struct p { int a; } x;
+  pd_t y0 = x;
+}
+
+typedef struct p { int a; } p2_t;
+
+void test4(void)
+{
+  p2_t x;
+  pd_t y0 = x;
+}
+
+void test5(void)
+{
+  struct q { int a; } a;
+  struct q { int a; } b;
+  a = b;
+}
+
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-3.c b/gcc/testsuite/gcc.dg/c23-tag-3.c
new file mode 100644
index 00000000000..4847e783c82
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-3.c
@@ -0,0 +1,16 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=c23" }
+ */
+
+// conflicting types via linkage
+
+extern struct foo { int x; } x;
+extern struct bar { float x; } y;
+
+void test(void)
+{
+  extern struct foo { int x; } x;
+  extern struct bar { int x; } y;	/* { dg-error "conflicting types" } */
+}
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-4.c b/gcc/testsuite/gcc.dg/c23-tag-4.c
new file mode 100644
index 00000000000..8083c43f607
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-4.c
@@ -0,0 +1,26 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=c23" }
+ */
+
+// conflicting types for anonymous structs / unions
+
+extern struct { int x; } a;
+extern struct { int x; } a;	/* { dg-error "conflicting types" } */
+
+extern union { int x; } b;
+extern union { int x; } b;	/* { dg-error "conflicting types" } */
+
+typedef struct { int x; } u;
+typedef struct { int x; } v;
+
+u c;
+v c;				/* { dg-error "conflicting types" } */
+
+typedef union { int x; } q;
+typedef union { int x; } r;
+
+q d;
+r d;				/* { dg-error "conflicting types" } */
+
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-5.c b/gcc/testsuite/gcc.dg/c23-tag-5.c
new file mode 100644
index 00000000000..ff40d07aef1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-5.c
@@ -0,0 +1,33 @@
+/* { dg-do run { target { ! "*-*-*" } } }
+ * { dg-options "-std=c23" }
+ */
+
+// nesting and parameters
+
+#define product_type(T, A, B) \
+struct product_ ## T { A a ; B b ; }
+#define sum_type(T, A, B) \
+struct sum_ ## T { _Bool flag ; union { A a ; B b ; }; }
+
+float foo1(product_type(iSfd_, int, sum_type(fd, float, double)) x)
+{
+	return x.b.a;
+}
+
+static void test1(void)
+{
+	product_type(iSfd_, int, sum_type(fd, float, double)) y = { 3, { 1, { .a = 1. } } };
+	product_type(iSfd_, int, sum_type(fd, float, double)) z = y;
+	product_type(iSfd_, int, sum_type(fd, float, double)) *zp = &y;
+	float a = foo1(y);
+	product_type(iSid_, int, sum_type(id, int, double)) *wp = &y; /* { dg-warning "incompatible pointer type" } */
+	float b = foo1(y);
+	product_type(iSid_, int, sum_type(id, int, double)) w = *wp;
+	(void)a; (void)b; (void)z; (void)zp; (void)w; (void)wp;
+}
+
+int main()
+{
+	test1();
+}
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-6.c b/gcc/testsuite/gcc.dg/c23-tag-6.c
new file mode 100644
index 00000000000..f8eb5698af5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-6.c
@@ -0,0 +1,25 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=c23" }
+ */
+
+// (in-)completeness
+
+struct foo {
+	char x[10];
+} x;
+
+struct foo {
+	_Static_assert(_Generic(&x, struct foo*: 0, default: 1));
+	char x[_Generic(&x, struct foo*: 1, default: 10)];
+};
+
+void f(void)
+{
+	struct foo { char x[_Generic(&x, struct foo*: 1, default: 10)]; };
+
+	struct foo z;
+	_Static_assert(10 == sizeof(z.x), "");
+}
+
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-7.c b/gcc/testsuite/gcc.dg/c23-tag-7.c
new file mode 100644
index 00000000000..dd3b5988e24
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-7.c
@@ -0,0 +1,12 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=c23" }
+ */
+
+// recursive declarations
+
+extern struct bar { struct bar* p; int x; } b;
+extern struct bar { struct bar* p; int x; } b;
+
+struct foo { struct foo { struct foo* p; int x; }* p; int x; } a;	/* { dg-error "nested" } */
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-8.c b/gcc/testsuite/gcc.dg/c23-tag-8.c
new file mode 100644
index 00000000000..8b3b5ef5dfe
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-8.c
@@ -0,0 +1,10 @@
+/* { dg-do compile }
+   { dg-options "-std=c23" } */
+
+void foo(void)
+{
+	struct bar { struct bar* next; };
+	struct bar { struct bar* next; };
+	struct bar { struct bar { struct bar* next; }* next; };	/* { dg-error "nested" } */
+}
+
diff --git a/gcc/testsuite/gcc.dg/gnu23-tag-1.c b/gcc/testsuite/gcc.dg/gnu23-tag-1.c
new file mode 100644
index 00000000000..3c0303d4c3f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/gnu23-tag-1.c
@@ -0,0 +1,10 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=gnu23" }
+ */
+
+struct r { int a; char b[]; };
+struct r { int a; char b[0]; };	/* allowed GNU extension */
+struct r { int a; char b[1]; }; /* { dg-error "redefinition of struct or union" } */
+
+
diff --git a/gcc/testsuite/gcc.dg/gnu23-tag-2.c b/gcc/testsuite/gcc.dg/gnu23-tag-2.c
new file mode 100644
index 00000000000..73725c79ebf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/gnu23-tag-2.c
@@ -0,0 +1,19 @@
+/*
+ * { dg-do compile }
+ * { dg-options "-std=gnu23" }
+ */
+
+// conflicting attributes
+
+extern struct __attribute__(( transaction_safe )) foo { int x; } x;
+extern struct __attribute__(( unused )) foo2 { int x; } x2;
+extern struct __attribute__(( may_alias )) foo3 { int x; } x3;
+
+void test(void)
+{
+  extern struct foo { int x; } x;		/* { dg-error "conflicting types" } */
+  extern struct  foo2 { int x; } x2;
+  extern struct  foo3 { int x; } x3;
+}
+
+
diff --git a/gcc/testsuite/gcc.dg/gnu23-tag-3.c b/gcc/testsuite/gcc.dg/gnu23-tag-3.c
new file mode 100644
index 00000000000..7ec121fe80e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/gnu23-tag-3.c
@@ -0,0 +1,28 @@
+/* 
+ * { dg-do compile } 
+ * { dg-options "-Wno-vla -std=gnu2x" } 
+ */
+
+// arrays in structs
+
+void foo(int n, int m)
+{
+	struct f { int b; int a[n]; };
+	struct f { int b; int a[n]; };	/* { dg-error "redefinition of struct or union" } */
+	struct f { int b; int a[m]; };	/* { dg-error "redefinition of struct or union" } */
+	struct f { int b; int a[5]; };	/* { dg-error "redefinition of struct or union" } */
+	struct f { int b; int a[]; };	/* { dg-error "redefinition of struct or union" } */
+
+	struct g { int a[n]; int b; };
+	struct g { int a[n]; int b; };	/* { dg-error "redefinition of struct or union" } */
+	struct g { int a[m]; int b; };	/* { dg-error "redefinition of struct or union" } */
+	struct g { int a[4]; int b; };	/* { dg-error "redefinition of struct or union" } */
+
+	struct h { int (*a)[n]; int b; };
+	struct h { int (*a)[n]; int b; };	/* { dg-error "redefinition of struct or union" } */
+	struct h { int (*a)[m]; int b; };	/* { dg-error "redefinition of struct or union" } */
+	struct h { int (*a)[4]; int b; };	/* { dg-error "redefinition of struct or union" } */
+	struct h { int (*a)[]; int b; };	/* { dg-error "redefinition of struct or union" } */
+}
+
+
diff --git a/gcc/testsuite/gcc.dg/gnu23-tag-4.c b/gcc/testsuite/gcc.dg/gnu23-tag-4.c
new file mode 100644
index 00000000000..8db81c7b87d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/gnu23-tag-4.c
@@ -0,0 +1,31 @@
+/* { dg-do compile }
+ * { dg-options "-std=gnu23" } */
+
+// structs with variably modified types
+
+void bar(int n, int m)
+{
+	struct f { int b; int a[n]; } *x;
+	{ struct f { int b; int a[n]; } *x2 = x; }
+	{ struct f { int b; int a[m]; } *x2 = x; }
+	{ struct f { int b; int a[5]; } *x2 = x; }
+	{ struct f { int b; int a[0]; }  *x2 = x; }
+	{ struct f { int b; int a[]; }  *x2 = x; }
+
+	struct g { int a[n]; int b; } *y;
+	{ struct g { int a[n]; int b; } *y2 = y; }
+	{ struct g { int a[m]; int b; } *y2 = y; }
+	{ struct g { int a[4]; int b; } *y2 = y; }
+
+	struct h { int b; int a[5]; } *w;
+	{ struct h { int b; int a[5]; } *w2 = w; }
+	{ struct h { int b; int a[n]; } *w2 = w; }
+	{ struct h { int b; int a[m]; } *w2 = w; }
+
+	struct i { int b; int (*a)(int c[n]); } *u;
+	{ struct i { int b; int (*a)(int c[4]); } *u2 = u; }
+	{ struct i { int b; int (*a)(int c[]); } *u2 = u; }
+	{ struct i { int b; int (*a)(int c[*]); } *u2 = u; }
+}
+
+
