c: Fix up RAW_DATA_CST handling in check_constexpr_init [PR120057]

The pr120057-1.c testcase is incorrectly rejected since
 r15-4377 (and for a while it also ICEd after the error), i.e.
the optimization of large C initializers using RAW_DATA_CST.
Similarly, the embed-18.c testcase is incorrectly rejected since
the embed support has been introduced and RAW_DATA_CST used for that.

The callers of check_constexpr_init (store_init_value and
output_init_element) compute int_const_expr as
      int_const_expr = (TREE_CODE (init) == INTEGER_CST
                        && !TREE_OVERFLOW (init)
                        && INTEGRAL_TYPE_P (TREE_TYPE (init)));
but that is only passed through down to check_constexpr_init.
I think tweaking those 2 callers to also allow RAW_DATA_CST for
int_const_expr when check_constexpr_init needs top special case it
no matter what would be larger, so the patch just changes
check_constexpr_init to deal with RAW_DATA_CST in the initializers.
For TYPE_UNSIGNED char precision integral types RAW_DATA_CST is
always valid, for !TYPE_UNSIGNED we need to check for 128-255 values
being turned into negative ones.

2025-05-02  Jakub Jelinek  <jakub@redhat.com>

	PR c/120057
	* c-typeck.cc (check_constexpr_init): Handle RAW_DATA_CST.

	* gcc.dg/cpp/embed-18.c: New test.
	* gcc.dg/pr120057-1.c: New test.
	* gcc.dg/pr120057-2.c: New test.

4 files changed, 54 insertions, 2 deletions

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 05fb129..0e1f842 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -9143,8 +9143,24 @@ check_constexpr_init (location_t loc, tree type, tree init,
       /* The initializer must be an integer constant expression,
 	 representable in the target type.  */
       if (!int_const_expr)
-	error_at (loc, "%<constexpr%> integer initializer is not an "
-		  "integer constant expression");
+	{
+	  if (TREE_CODE (init) == RAW_DATA_CST
+	      && TYPE_PRECISION (type) == CHAR_BIT)
+	    {
+	      if (!TYPE_UNSIGNED (type))
+		for (unsigned int i = 0;
+		     i < (unsigned) RAW_DATA_LENGTH (init); ++i)
+		  if (RAW_DATA_SCHAR_ELT (init, i) < 0)
+		    {
+		      error_at (loc, "%<constexpr%> initializer not "
+				"representable in type of object");
+		      break;
+		    }
+	    }
+	  else
+	    error_at (loc, "%<constexpr%> integer initializer is not an "
+		      "integer constant expression");
+	}
       else if (!int_fits_type_p (init, type))
 	error_at (loc, "%<constexpr%> initializer not representable in "
 		  "type of object");
diff --git a/gcc/testsuite/gcc.dg/cpp/embed-18.c b/gcc/testsuite/gcc.dg/cpp/embed-18.c
new file mode 100644
index 0000000..198a6db
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/embed-18.c
@@ -0,0 +1,7 @@
+/* PR c/120057 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 --embed-dir=${srcdir}/c-c++-common/cpp/embed-dir" } */
+
+constexpr unsigned char magna_carta[] = {
+#embed <magna-carta.txt>
+};
diff --git a/gcc/testsuite/gcc.dg/pr120057-1.c b/gcc/testsuite/gcc.dg/pr120057-1.c
new file mode 100644
index 0000000..8832de6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr120057-1.c
@@ -0,0 +1,18 @@
+/* PR c/120057 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+constexpr char foo[65] = {
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1
+};
+constexpr char bar[66] = {
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2
+};
diff --git a/gcc/testsuite/gcc.dg/pr120057-2.c b/gcc/testsuite/gcc.dg/pr120057-2.c
new file mode 100644
index 0000000..1372baf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr120057-2.c
@@ -0,0 +1,11 @@
+/* PR c/120057 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+constexpr signed char foo[65] = {
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 129, 11, 12, 13, 14, 15, 16,	/* { dg-error "'constexpr' initializer not representable in type of object" } */
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
+  1
+};