[C,v4] Fix for redeclared enumerator initialized with different type [PR115109]

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 6c09eb73128..01326570e2b 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -10277,6 +10277,7 @@  build_enumerator (location_t decl_loc, location_t loc,
 		  struct c_enum_contents *the_enum, tree name, tree value)
 {
   tree decl;
+  tree old_decl;
 
   /* Validate and default VALUE.  */
 
@@ -10336,6 +10337,23 @@  build_enumerator (location_t decl_loc, location_t loc,
 	 definition.  */
       value = convert (the_enum->enum_type, value);
     }
+  else if (flag_isoc23
+	   && (old_decl = lookup_name_in_scope (name, current_scope))
+	   && old_decl != error_mark_node
+	   && TREE_TYPE (old_decl)
+	   && TREE_TYPE (TREE_TYPE (old_decl))
+	   && TREE_CODE (old_decl) == CONST_DECL)
+    {
+      /* Enumeration constants in a redeclaration have the previous type.  */
+      tree previous_type = TREE_TYPE (DECL_INITIAL (old_decl));
+      if (!int_fits_type_p (value, previous_type))
+	{
+	  error_at (loc, "value of redeclared enumerator outside the range "
+			 "of %qT", previous_type);
+	  locate_old_decl (old_decl);
+	}
+      value = convert (previous_type, value);
+    }
   else
     {
       /* Even though the underlying type of an enum is unspecified, the
@@ -10402,9 +10420,14 @@  build_enumerator (location_t decl_loc, location_t loc,
 			     false);
     }
   else
-    the_enum->enum_next_value
-      = build_binary_op (EXPR_LOC_OR_LOC (value, input_location),
-			 PLUS_EXPR, value, integer_one_node, false);
+    {
+      /* In a redeclaration the type can already be the enumeral type.  */
+      if (TREE_CODE (TREE_TYPE (value)) == ENUMERAL_TYPE)
+	value = convert (ENUM_UNDERLYING_TYPE (TREE_TYPE (value)), value);
+      the_enum->enum_next_value
+	= build_binary_op (EXPR_LOC_OR_LOC (value, input_location),
+			   PLUS_EXPR, value, integer_one_node, false);
+    }
   the_enum->enum_overflow = tree_int_cst_lt (the_enum->enum_next_value, value);
   if (the_enum->enum_overflow
       && !ENUM_FIXED_UNDERLYING_TYPE_P (the_enum->enum_type))
diff --git a/gcc/testsuite/gcc.dg/c23-tag-enum-6.c b/gcc/testsuite/gcc.dg/c23-tag-enum-6.c
new file mode 100644
index 00000000000..29aef7ee3fd
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-enum-6.c
@@ -0,0 +1,20 @@ 
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -fno-short-enums" } */
+
+#include <limits.h>
+
+enum E : int { a = 1, b = 2 };
+enum E : int { b = _Generic(a, enum E: 2), a = 1 };
+
+enum H { x = 1 };
+enum H { x = 2UL + UINT_MAX };		/* { dg-error "outside the range" } */
+
+enum K : int { z = 1 };
+enum K : int { z = 2UL + UINT_MAX };	/* { dg-error "outside the range" } */
+
+enum F { A = 0, B = UINT_MAX };
+enum F { B = UINT_MAX, A };		/* { dg-error "outside the range" } */
+
+enum G : unsigned int { C = 0, D = UINT_MAX };
+enum G : unsigned int { D = UINT_MAX, C };		/* { dg-error "overflow" } */
+
diff --git a/gcc/testsuite/gcc.dg/c23-tag-enum-7.c b/gcc/testsuite/gcc.dg/c23-tag-enum-7.c
new file mode 100644
index 00000000000..d4c787c8f71
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-tag-enum-7.c
@@ -0,0 +1,41 @@ 
+/* { dg-do compile }
+ * { dg-options "-std=c23 -fno-short-enums" } */
+
+#include <limits.h>
+
+// enumerators are all representable in int
+enum E { a = 1UL, b = _Generic(a, int: 2) };
+static_assert(_Generic(a, int: 1));
+static_assert(_Generic(b, int: 1));
+enum E { a = 1UL, b = _Generic(a, int: 2) };
+static_assert(_Generic(a, int: 1));
+static_assert(_Generic(b, int: 1));
+
+// enumerators are not representable in int
+enum H { c = 1UL << (UINT_WIDTH + 1), d = 2 };
+static_assert(_Generic(c, enum H: 1));
+static_assert(_Generic(d, enum H: 1));
+enum H { c = 1UL << (UINT_WIDTH + 1), d = _Generic(c, enum H: 2) };
+static_assert(_Generic(c, enum H: 1));
+static_assert(_Generic(d, enum H: 1));
+
+// there is an overflow in the first declaration
+enum K { e = UINT_MAX, f, g = _Generic(e, unsigned int: 0) + _Generic(f, unsigned long: 1) };
+static_assert(_Generic(e, enum K: 1));
+static_assert(_Generic(f, enum K: 1));
+static_assert(_Generic(g, enum K: 1));
+enum K { e = UINT_MAX, f, g = _Generic(e, enum K: 0) + _Generic(f, enum K: 1) };
+static_assert(_Generic(e, enum K: 1));
+static_assert(_Generic(f, enum K: 1));
+static_assert(_Generic(g, enum K: 1));
+
+// there is an overflow in the first declaration
+enum U { k = INT_MAX, l, m = _Generic(k, int: 0) + _Generic(l, long: 1) };
+static_assert(_Generic(k, enum U: 1));
+static_assert(_Generic(l, enum U: 1));
+static_assert(_Generic(m, enum U: 1));
+enum U { k = INT_MAX, l, m = _Generic(k, enum U: 0) + _Generic(l, enum U: 1) };
+static_assert(_Generic(k, enum U: 1));
+static_assert(_Generic(l, enum U: 1));
+static_assert(_Generic(m, enum U: 1));
+
diff --git a/gcc/testsuite/gcc.dg/pr115109.c b/gcc/testsuite/gcc.dg/pr115109.c
new file mode 100644
index 00000000000..4baee0f3445
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr115109.c
@@ -0,0 +1,8 @@ 
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -fno-short-enums" } */
+
+#include <limits.h>
+
+enum E { a = 1UL << (ULONG_WIDTH - 5), b = 2 };
+enum E { a = 1ULL << (ULONG_WIDTH - 5), b = _Generic(a, enum E: 2) };
+