c++: Fix parsing of invalid enum specifiers [PR90995]

The testcase shows some accepts-invalid (the ones without alignas) and
ice-on-invalid-code (the ones with alignas) cases.
If the enum doesn't have an underlying type and is not a definition,
the caller retries to parse it as elaborated type specifier.
E.g. for enum struct S s it will then pedwarn that elaborated type specifier
shouldn't have the struct/class keywords.
The problem is if the enum specifier is not followed by { when it has
underlying type.  In that case we have already called
cp_parser_parse_definitely to end the tentative parsing started at the
beginning of cp_parser_enum_specifier.  But the
cp_parser_error (parser, "expected %<;%> or %<{%>");
doesn't emit any error because the whole function is called from yet another
tentative parse and the caller starts parsing the elaborated type
specifier where the cp_parser_enum_specifier stopped (i.e. after the
underlying type token(s)).  The ultimate caller than commits the tentative
parsing (and even if it wouldn't, it wouldn't know what kind of error
to report).  I think after seeing enum {,struct,class} : type not being
followed by { or ;, there is no reason not to report it right away, as it
can't be valid C++, which is what the patch does.  Not sure if we shouldn't
also return error_mark_node instead of NULL_TREE, so that the caller doesn't
try to parse it as elaborated type specifier (the patch doesn't do that
right now).

Furthermore, while reading the code, I've noticed that
parser->colon_corrects_to_scope_p is saved and set to false at the start
of the function, but not restored back in some cases.  Don't have a testcase
where this would be a problem, but it just seems wrong.  Either we can in
the two spots replace return NULL_TREE; with { type = NULL_TREE; goto out; }
or we could perhaps abuse warning_sentinel or create a special class with
dtor to clean the flag up.

And lastly, I've fixed some formatting issues in the function while reading
it.

2020-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/90995
	* parser.c (cp_parser_enum_specifier): Use temp_override for
	parser->colon_corrects_to_scope_p, replace goto out with return.
	If scoped enum or enum with underlying type is not followed by
	{ or ;, call cp_parser_commit_to_tentative_parse before calling
	cp_parser_error and make sure to return error_mark_node instead of
	NULL_TREE.  Formatting fixes.

	* g++.dg/cpp0x/enum40.C: New test.

2 files changed, 31 insertions, 31 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 0cdbd02..1db1e09 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,13 @@
+2020-03-17  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/90995
+	* parser.c (cp_parser_enum_specifier): Use temp_override for
+	parser->colon_corrects_to_scope_p, replace goto out with return.
+	If scoped enum or enum with underlying type is not followed by
+	{ or ;, call cp_parser_commit_to_tentative_parse before calling
+	cp_parser_error and make sure to return error_mark_node instead of
+	NULL_TREE.  Formatting fixes.
+
 2020-03-17  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
 	PR c++/94197
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 58a1bea..26e0236 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -19001,9 +19001,7 @@ cp_parser_enum_specifier (cp_parser* parser)
   bool is_unnamed = false;
   tree underlying_type = NULL_TREE;
   cp_token *type_start_token = NULL;
-  bool saved_colon_corrects_to_scope_p = parser->colon_corrects_to_scope_p;
-
-  parser->colon_corrects_to_scope_p = false;
+  temp_override<bool> cleanup (parser->colon_corrects_to_scope_p, false);
 
   /* Parse tentatively so that we can back up if we don't find a
      enum-specifier.  */
@@ -19043,24 +19041,24 @@ cp_parser_enum_specifier (cp_parser* parser)
 
   push_deferring_access_checks (dk_no_check);
   nested_name_specifier
-      = cp_parser_nested_name_specifier_opt (parser,
-					     /*typename_keyword_p=*/true,
-					     /*check_dependency_p=*/false,
-					     /*type_p=*/false,
-					     /*is_declaration=*/false);
+    = cp_parser_nested_name_specifier_opt (parser,
+					   /*typename_keyword_p=*/true,
+					   /*check_dependency_p=*/false,
+					   /*type_p=*/false,
+					   /*is_declaration=*/false);
 
   if (nested_name_specifier)
     {
       tree name;
 
       identifier = cp_parser_identifier (parser);
-      name =  cp_parser_lookup_name (parser, identifier,
-				     enum_type,
-				     /*is_template=*/false,
-				     /*is_namespace=*/false,
-				     /*check_dependency=*/true,
-				     /*ambiguous_decls=*/NULL,
-				     input_location);
+      name = cp_parser_lookup_name (parser, identifier,
+				    enum_type,
+				    /*is_template=*/false,
+				    /*is_namespace=*/false,
+				    /*check_dependency=*/true,
+				    /*ambiguous_decls=*/NULL,
+				    input_location);
       if (name && name != error_mark_node)
 	{
 	  type = TREE_TYPE (name);
@@ -19140,23 +19138,21 @@ cp_parser_enum_specifier (cp_parser* parser)
     {
       if (cxx_dialect < cxx11 || (!scoped_enum_p && !underlying_type))
 	{
+	  if (has_underlying_type)
+	    cp_parser_commit_to_tentative_parse (parser);
 	  cp_parser_error (parser, "expected %<{%>");
 	  if (has_underlying_type)
-	    {
-	      type = NULL_TREE;
-	      goto out;
-	    }
+	    return error_mark_node;
 	}
       /* An opaque-enum-specifier must have a ';' here.  */
       if ((scoped_enum_p || underlying_type)
 	  && cp_lexer_next_token_is_not (parser->lexer, CPP_SEMICOLON))
 	{
+	  if (has_underlying_type)
+	    cp_parser_commit_to_tentative_parse (parser);
 	  cp_parser_error (parser, "expected %<;%> or %<{%>");
 	  if (has_underlying_type)
-	    {
-	      type = NULL_TREE;
-	      goto out;
-	    }
+	    return error_mark_node;
 	}
     }
 
@@ -19172,9 +19168,7 @@ cp_parser_enum_specifier (cp_parser* parser)
 	  push_scope (nested_name_specifier);
 	}
       else if (TREE_CODE (nested_name_specifier) == NAMESPACE_DECL)
-	{
-	  push_nested_namespace (nested_name_specifier);
-	}
+	push_nested_namespace (nested_name_specifier);
     }
 
   /* Issue an error message if type-definitions are forbidden here.  */
@@ -19334,12 +19328,8 @@ cp_parser_enum_specifier (cp_parser* parser)
 	  pop_scope (nested_name_specifier);
 	}
       else if (TREE_CODE (nested_name_specifier) == NAMESPACE_DECL)
-	{
-	  pop_nested_namespace (nested_name_specifier);
-	}
+	pop_nested_namespace (nested_name_specifier);
     }
- out:
-  parser->colon_corrects_to_scope_p = saved_colon_corrects_to_scope_p;
   return type;
 }