[gcc r12-7574] PR c++/95999: Improved error recovery in enumeration lists.

[gcc r12-7574] PR c++/95999: Improved error recovery in enumeration lists.

[Roger Sayle]

https://gcc.gnu.org/g:bae10419f6e07dbde8e66ff4ff70d050f82ad451

commit r12-7574-gbae10419f6e07dbde8e66ff4ff70d050f82ad451
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Thu Mar 10 07:41:12 2022 +0000

    PR c++/95999: Improved error recovery in enumeration lists.
    
    This patch resolves PR c++/95999 which is an ICE-after-error regression
    in the g++ front-end.  When parsing an enumerator list, the C++ parser
    assumes that cp_parser_constant_expression always returns either an
    INTEGER_CST or error_mark_node, but in the testcase reported in the
    PR, it actually returns a VAR_DECL.
    
    The usual (but perhaps controversial) design philosophy is that the
    routine that reports the error normally has a duty to indicate this to
    the rest of the compiler (via error_mark_node), but here the return
    value from calling require_rvalue_constant_expression (parser.cc:10666)
    is ignored.  I initially experimented with setting EXPRESSION to
    error_mark_node here in cp_parser_constant_expression but (perhaps
    conveniently) that's insufficient to resolve the problem.  The simple
    fix in this patch is to tweak the two places that require INTEGER_CST
    to treat all other tree types as though they are error_mark_node.
    
    2022-03-10  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/cp/ChangeLog
            PR c++/95999
            * decl.cc (finish_enum_value_list): If VALUE isn't an INTEGER_CST
            consider it to be zero (i.e. treat it like error_mark_node).
            (build_enumerator): Likewise, if PREV_VALUE isn't an INTEGER_CST,
            set VALUE to error_mark_node.
    
    gcc/testsuite/ChangeLog
            PR c++/95999
            * g++.dg/parse/pr95999.C: New test case.

Diff:
---
 gcc/cp/decl.cc                       | 4 ++--
 gcc/testsuite/g++.dg/parse/pr95999.C | 8 ++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 992e38385c2..58210905ee9 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -16263,7 +16263,7 @@ finish_enum_value_list (tree enumtype)
 
 	  /* Update the minimum and maximum values, if appropriate.  */
 	  value = DECL_INITIAL (decl);
-	  if (value == error_mark_node)
+	  if (TREE_CODE (value) != INTEGER_CST)
 	    value = integer_zero_node;
 	  /* Figure out what the minimum and maximum values of the
 	     enumerators are.  */
@@ -16552,7 +16552,7 @@ build_enumerator (tree name, tree value, tree enumtype, tree attributes,
 		 which case the type is an unspecified integral type
 		 sufficient to contain the incremented value.  */
 	      prev_value = DECL_INITIAL (TREE_VALUE (TYPE_VALUES (enumtype)));
-	      if (error_operand_p (prev_value))
+	      if (TREE_CODE (prev_value) != INTEGER_CST)
 		value = error_mark_node;
 	      else
 		{
diff --git a/gcc/testsuite/g++.dg/parse/pr95999.C b/gcc/testsuite/g++.dg/parse/pr95999.C
new file mode 100644
index 00000000000..cf8437ef262
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr95999.C
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+int a;
+enum struct b;
+template <typename = enum struct b { c = a d } 
+template <> enum struct b { e };  // { dg-error "explicit specialization" }
+// { dg-excess-errors "note" }
+// { dg-excess-errors "5:" }
+