PR fortran/93253
	* check.c (gfc_invalid_boz): With -std=legacy, suggest the new
	-fallow-invalid-boz flag. With -std=legacy and that flag, no
	longer warn.
	* gfortran.texi (BOZ literal constants): List another missing
	extension and refer to -fallow-invalid-boz.
	* lang.opt (fallow-invalid-boz): Also mention 'X' in the help text
	as it is not covered by the previous wording.
	* primary.c (match_boz_constant): Tweak wording such that it is
	clear how to fix the nonstandard use.

	PR fortran/93253
	* fortran.dg/boz_7.f90: Updated dg-error.


diff --git a/gcc/fortran/check.c b/gcc/fortran/check.c
index c7f0187b377..f03764abaef 100644
--- a/gcc/fortran/check.c
+++ b/gcc/fortran/check.c
@@ -56,18 +56,30 @@ reset_boz (gfc_expr *x)
    gfc_invalid_boz() is a helper function to simplify error/warning
    generation.  gfortran accepts the nonstandard 'X' for 'Z', and gfortran
    allows the BOZ indicator to appear as a suffix.  If -fallow-invalid-boz
-   is used, then issue a warning; otherwise issue an error.  */
+   is used, then issue a warning (unless -std=legacy); otherwise issue an
+   error.  */
 
 bool
 gfc_invalid_boz (const char *msg, locus *loc)
 {
   if (flag_allow_invalid_boz)
     {
-      gfc_warning (0, msg, loc);
+      if (gfc_option.warn_std & GFC_STD_LEGACY)
+	gfc_warning (0, msg, loc);
       return false;
     }
 
-  gfc_error (msg, loc);
+  if (!(gfc_option.warn_std & GFC_STD_LEGACY))
+    {
+      const char hint[] = ". Use %<-fallow-invalid-boz%> if you cannot fix it";
+      size_t len = strlen (msg) + strlen (hint) + 1;
+      char *msg2 = (char *) alloca (len);
+      strcpy (msg2, msg);
+      strcat (msg2, hint);
+      gfc_error (msg2, loc);
+    }
+  else
+    gfc_error (msg, loc);
   return true;
 }
 
diff --git a/gcc/fortran/gfortran.texi b/gcc/fortran/gfortran.texi
index 4cf8b3a5c24..98fc74f3e67 100644
--- a/gcc/fortran/gfortran.texi
+++ b/gcc/fortran/gfortran.texi
@@ -1863,9 +1863,11 @@ Fortran standard states that the treatment of the sign bit is processor
 dependent.  Gfortran interprets the sign bit as a user would expect.
 
 As a deprecated extension, GNU Fortran allows hexadecimal BOZ literal
-constants to be specified using the @code{X} prefix.  The BOZ literal
+constants to be specified using the @code{X} prefix.  That the BOZ literal
 constant can also be specified by adding a suffix to the string, for
-example, @code{Z'ABC'} and @code{'ABC'X} are equivalent.
+example, @code{Z'ABC'} and @code{'ABC'X} are equivalent.  And BOZ literals
+outside @code{DATA} and the intrinsic functions listed in the Fortran
+standard.  Use @option{-fallow-invalid-boz} to enable the extension.
 
 @node Real array indices
 @subsection Real array indices
diff --git a/gcc/fortran/lang.opt b/gcc/fortran/lang.opt
index 3858331bcc0..59523f74acf 100644
--- a/gcc/fortran/lang.opt
+++ b/gcc/fortran/lang.opt
@@ -387,7 +387,7 @@ All intrinsics procedures are available regardless of selected standard.
 
 fallow-invalid-boz
 Fortran RejectNegative Var(flag_allow_invalid_boz)
-Allow a BOZ literal constant to appear in an invalid context.
+Allow a BOZ literal constant to appear in an invalid context and with X instead of Z.
 
 fallow-leading-underscore
 Fortran Undocumented Var(flag_allow_leading_underscore)
diff --git a/gcc/fortran/primary.c b/gcc/fortran/primary.c
index e2b6fcb2106..07b8ac08ba2 100644
--- a/gcc/fortran/primary.c
+++ b/gcc/fortran/primary.c
@@ -433,7 +433,7 @@ match_boz_constant (gfc_expr **result)
 
   if (x_hex
       && gfc_invalid_boz ("Hexadecimal constant at %L uses "
-			  "nonstandard syntax", &gfc_current_locus))
+			  "nonstandard X instead of Z", &gfc_current_locus))
     return MATCH_ERROR;
 
   old_loc = gfc_current_locus;
diff --git a/gcc/testsuite/gfortran.dg/boz_7.f90 b/gcc/testsuite/gfortran.dg/boz_7.f90
index 45fa7a7df19..d2a51ac03e2 100644
--- a/gcc/testsuite/gfortran.dg/boz_7.f90
+++ b/gcc/testsuite/gfortran.dg/boz_7.f90
@@ -7,6 +7,6 @@
 !
 integer :: k, m
 integer :: j = z'000abc' ! { dg-error "BOZ used outside a DATA statement" }
-data k/x'0003'/ ! { dg-error "nonstandard syntax" }
+data k/x'0003'/ ! { dg-error "nonstandard X instead of Z" }
 data m/'0003'z/ ! { dg-error "nonstandard postfix" }
 end