[gcc r14-6368] c++: Accept the use of built-in trait identifiers

[Jason Merrill]

https://gcc.gnu.org/g:df3559d951ba6572e254a1bd1ef9a34b6e543325

commit r14-6368-gdf3559d951ba6572e254a1bd1ef9a34b6e543325
Author: Ken Matsui <kmatsui@gcc.gnu.org>
Date:   Wed Dec 6 21:32:58 2023 -0800

    c++: Accept the use of built-in trait identifiers
    
    This patch accepts the use of built-in trait identifiers when they are
    actually not used as traits.  Specifically, we check if the subsequent
    token is '(' for ordinary built-in traits or is '<' only for the special
    __type_pack_element built-in trait.  If those identifiers are used
    differently, the parser treats them as normal identifiers.  This allows
    us to accept code like: struct __is_pointer {};.
    
    gcc/cp/ChangeLog:
    
            * parser.cc (cp_lexer_lookup_trait): Rename to ...
            (cp_lexer_peek_trait): ... this.  Handle a subsequent token for
            the corresponding built-in trait.
            (cp_lexer_lookup_trait_expr): Rename to ...
            (cp_lexer_peek_trait_expr): ... this.
            (cp_lexer_lookup_trait_type): Rename to ...
            (cp_lexer_peek_trait_type): ... this.
            (cp_lexer_next_token_is_decl_specifier_keyword): Call
            cp_lexer_peek_trait_type.
            (cp_parser_simple_type_specifier): Likewise.
            (cp_parser_primary_expression): Call cp_lexer_peek_trait_expr.
    
    Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>

Diff:
---
 gcc/cp/parser.cc | 53 ++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 34 insertions(+), 19 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index f23ef6be2db..9e76426566b 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -246,12 +246,12 @@ static void cp_lexer_start_debugging
   (cp_lexer *) ATTRIBUTE_UNUSED;
 static void cp_lexer_stop_debugging
   (cp_lexer *) ATTRIBUTE_UNUSED;
-static const cp_trait *cp_lexer_lookup_trait
-  (const cp_token *);
-static const cp_trait *cp_lexer_lookup_trait_expr
-  (const cp_token *);
-static const cp_trait *cp_lexer_lookup_trait_type
-  (const cp_token *);
+static const cp_trait *cp_lexer_peek_trait
+  (cp_lexer *);
+static const cp_trait *cp_lexer_peek_trait_expr
+  (cp_lexer *);
+static const cp_trait *cp_lexer_peek_trait_type
+  (cp_lexer *);
 
 static cp_token_cache *cp_token_cache_new
   (cp_token *, cp_token *);
@@ -1188,15 +1188,29 @@ cp_keyword_starts_decl_specifier_p (enum rid keyword)
     }
 }
 
-/* Look ups the corresponding built-in trait if a given token is
-   a built-in trait.  Otherwise, returns nullptr.  */
+/* Peeks the corresponding built-in trait if the first token is
+   a built-in trait and the second token is either `(' or `<' depending
+   on the trait.  Otherwise, returns nullptr.  */
 
 static const cp_trait *
-cp_lexer_lookup_trait (const cp_token *token)
+cp_lexer_peek_trait (cp_lexer *lexer)
 {
-  if (token->type == CPP_NAME && IDENTIFIER_TRAIT_P (token->u.value))
-    return &cp_traits[IDENTIFIER_CP_INDEX (token->u.value)];
+  const cp_token *token1 = cp_lexer_peek_token (lexer);
+  if (token1->type == CPP_NAME && IDENTIFIER_TRAIT_P (token1->u.value))
+    {
+      const cp_trait &trait = cp_traits[IDENTIFIER_CP_INDEX (token1->u.value)];
+      const bool is_pack_element = (trait.kind == CPTK_TYPE_PACK_ELEMENT);
 
+      /* Check if the subsequent token is a `<' token to
+	 __type_pack_element or is a `(' token to everything else.  */
+      const cp_token *token2 = cp_lexer_peek_nth_token (lexer, 2);
+      if (is_pack_element && token2->type != CPP_LESS)
+	return nullptr;
+      if (!is_pack_element && token2->type != CPP_OPEN_PAREN)
+	return nullptr;
+
+      return &trait;
+    }
   return nullptr;
 }
 
@@ -1204,9 +1218,9 @@ cp_lexer_lookup_trait (const cp_token *token)
    built-in trait.  */
 
 static const cp_trait *
-cp_lexer_lookup_trait_expr (const cp_token *token)
+cp_lexer_peek_trait_expr (cp_lexer *lexer)
 {
-  const cp_trait *trait = cp_lexer_lookup_trait (token);
+  const cp_trait *trait = cp_lexer_peek_trait (lexer);
   if (trait && !trait->type)
     return trait;
 
@@ -1217,9 +1231,9 @@ cp_lexer_lookup_trait_expr (const cp_token *token)
    built-in trait.  */
 
 static const cp_trait *
-cp_lexer_lookup_trait_type (const cp_token *token)
+cp_lexer_peek_trait_type (cp_lexer *lexer)
 {
-  const cp_trait *trait = cp_lexer_lookup_trait (token);
+  const cp_trait *trait = cp_lexer_peek_trait (lexer);
   if (trait && trait->type)
     return trait;
 
@@ -1233,9 +1247,10 @@ cp_lexer_next_token_is_decl_specifier_keyword (cp_lexer *lexer)
 {
   cp_token *token;
 
-  token = cp_lexer_peek_token (lexer);
-  if (cp_lexer_lookup_trait_type (token))
+  if (cp_lexer_peek_trait_type (lexer))
     return true;
+
+  token = cp_lexer_peek_token (lexer);
   return cp_keyword_starts_decl_specifier_p (token->keyword);
 }
 
@@ -6133,7 +6148,7 @@ cp_parser_primary_expression (cp_parser *parser,
 	 `::' as the beginning of a qualified-id, or the "operator"
 	 keyword.  */
     case CPP_NAME:
-      if (const cp_trait* trait = cp_lexer_lookup_trait_expr (token))
+      if (const cp_trait* trait = cp_lexer_peek_trait_expr (parser->lexer))
 	return cp_parser_trait (parser, trait);
       /* FALLTHRU */
     case CPP_SCOPE:
@@ -20153,7 +20168,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
     }
 
   /* If token is a type-yielding built-in traits, parse it.  */
-  const cp_trait* trait = cp_lexer_lookup_trait_type (token);
+  const cp_trait* trait = cp_lexer_peek_trait_type (parser->lexer);
   if (trait)
     {
       type = cp_parser_trait (parser, trait);