[gcc r14-8669] c: Fix ICE for nested enum redefinitions with/without fixed underlying type [PR112571]

[Joseph Myers <jsm28@gcc.gnu.org>]

https://gcc.gnu.org/g:d22d1a9346f27db41459738c6eb404f8f0956e6f

commit r14-8669-gd22d1a9346f27db41459738c6eb404f8f0956e6f
Author: Joseph Myers <josmyers@redhat.com>
Date:   Wed Jan 31 21:39:53 2024 +0000

    c: Fix ICE for nested enum redefinitions with/without fixed underlying type [PR112571]
    
    Bug 112571 reports an ICE-on-invalid for cases where an enum is
    defined, without a fixed underlying type, inside the enum type
    specifier for a definition of that same enum with a fixed underlying
    type.
    
    The ultimate cause is attempting to access ENUM_UNDERLYING_TYPE in a
    case where it is NULL.  Avoid this by clearing
    ENUM_FIXED_UNDERLYING_TYPE_P in thie case of inconsistent definitions.
    
    Bootstrapped wth no regressions for x86_64-pc-linux-gnu.
    
            PR c/112571
    
    gcc/c/
            * c-decl.cc (start_enum): Clear ENUM_FIXED_UNDERLYING_TYPE_P when
            defining without a fixed underlying type an enumeration previously
            declared with a fixed underlying type.
    
    gcc/testsuite/
            * gcc.dg/c23-enum-9.c, gcc.dg/c23-enum-10.c: New tests.

Diff:
---
 gcc/c/c-decl.cc                    | 7 +++++--
 gcc/testsuite/gcc.dg/c23-enum-10.c | 6 ++++++
 gcc/testsuite/gcc.dg/c23-enum-9.c  | 8 ++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 8d18a3e11f46..934e557dc3bb 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9905,8 +9905,11 @@ start_enum (location_t loc, struct c_enum_contents *the_enum, tree name,
 
   if (ENUM_FIXED_UNDERLYING_TYPE_P (enumtype)
       && fixed_underlying_type == NULL_TREE)
-    error_at (loc, "%<enum%> declared with but defined without "
-	      "fixed underlying type");
+    {
+      error_at (loc, "%<enum%> declared with but defined without "
+		"fixed underlying type");
+      ENUM_FIXED_UNDERLYING_TYPE_P (enumtype) = false;
+    }
 
   the_enum->enum_next_value = integer_zero_node;
   the_enum->enum_type = enumtype;
diff --git a/gcc/testsuite/gcc.dg/c23-enum-10.c b/gcc/testsuite/gcc.dg/c23-enum-10.c
new file mode 100644
index 000000000000..dd5f3453b1f3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-enum-10.c
@@ -0,0 +1,6 @@
+/* PR c/112571.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+enum X : typeof (enum X { A }); /* { dg-error "declared with but defined without fixed underlying type" } */
+/* { dg-error "invalid 'enum' underlying type" "invalid" { target *-*-* } .-1 } */
diff --git a/gcc/testsuite/gcc.dg/c23-enum-9.c b/gcc/testsuite/gcc.dg/c23-enum-9.c
new file mode 100644
index 000000000000..10bb493ca3ce
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-enum-9.c
@@ -0,0 +1,8 @@
+/* PR c/112571.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c23" } */
+
+enum h : typeof (enum h { D }) { D }; /* { dg-error "declared with but defined without fixed underlying type" } */
+/* { dg-error "invalid 'enum' underlying type" "invalid" { target *-*-* } .-1 } */
+/* { dg-error "nested redefinition" "nested" { target *-*-* } .-2 } */
+/* { dg-error "conflicting redefinition" "conflicting" { target *-*-* } .-3 } */