From 6186708312780bb2139da01946abdde39667e985 Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Wed, 11 Aug 2021 15:58:30 -0400 Subject: [PATCH] c++: most vexing parse and braced CTAD [PR89062] MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Here grokdeclarator is emitting the error error: class template placeholder ‘Foo’ not permitted in this context during the tentative (and ultimately futile) parse of 'x' as a function declaration. This happens because when parsing 'Foo{1}', cp_parser_parameter_declaration yields a parameter declaration with no declarator and whose type is a CTAD placeholder, and stops short of consuming the '{'. The caller cp_parser_parameter_declaration_list then calls grokdeclarator on this declarator, hence the error, and soon thereafter we abort this tentative parse since the next token '{' doesn't make sense in the context of a parameter list. Note that we don't have this issue with parenthesized CTAD Foo x(Foo(1)); because in this case cp_parser_direct_declarator (called indirectly from c_p_p_d) consumes the '(' and returns cp_error_declarator instead of a NULL declarator (and also simulates a parse error), and grokdeclarator exits early for this declarator without emitting any error. Since grokdeclarator doesn't take a 'complain' parameter, to fix this we need to avoid calling grokdeclarator in this situation. To that end this patch makes c_p_p_d simulate an error when a construct is a CTAD expression and definitely not a parameter declaration, so that c_p_p_d_l can avoid calling grokdeclarator by checking for this simulated error. Alternatively we could keep all this logic inside c_p_p_d_l and not touch c_p_p_d at all, but this approach seems slightly less adhoc. PR c++/89062 gcc/cp/ChangeLog: * parser.c (cp_parser_parameter_declaration_list): Don't call grokdeclarator if cp_parser_error_occurred. (cp_parser_parameter_declaration): Simulate an error if we see the beginning of a CTAD form, i.e. if we see an opening brace after the decl-specifier-seq and the type is a CTAD placeholder. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/class-deduction97.C: New test. --- gcc/cp/parser.c | 17 +++++++++++++---- gcc/testsuite/g++.dg/cpp1z/class-deduction97.C | 6 ++++++ 2 files changed, 19 insertions(+), 4 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction97.C diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c index 1be42a1..87e8d37 100644 --- a/gcc/cp/parser.c +++ b/gcc/cp/parser.c @@ -24284,7 +24284,7 @@ cp_parser_parameter_declaration_list (cp_parser* parser, cp_parser_flags flags) and warn in grokparms if appropriate. */ deprecated_state = DEPRECATED_SUPPRESS; - if (parameter) + if (parameter && !cp_parser_error_occurred (parser)) { decl = grokdeclarator (parameter->declarator, ¶meter->decl_specifiers, @@ -24499,7 +24499,7 @@ cp_parser_parameter_declaration (cp_parser *parser, parser->default_arg_ok_p = false; /* After seeing a decl-specifier-seq, if the next token is not a - "(", there is no possibility that the code is a valid + "(" or "{", there is no possibility that the code is a valid expression. Therefore, if parsing tentatively, we commit at this point. */ if (!parser->in_template_argument_list_p @@ -24512,9 +24512,18 @@ cp_parser_parameter_declaration (cp_parser *parser, of some object of type "char" to "int". */ && !parser->in_type_id_in_expr_p && cp_parser_uncommitted_to_tentative_parse_p (parser) - && cp_lexer_next_token_is_not (parser->lexer, CPP_OPEN_BRACE) && cp_lexer_next_token_is_not (parser->lexer, CPP_OPEN_PAREN)) - cp_parser_commit_to_tentative_parse (parser); + { + if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE)) + { + if (decl_specifiers.type + && template_placeholder_p (decl_specifiers.type)) + /* This is a CTAD expression, not a parameter declaration. */ + cp_parser_simulate_error (parser); + } + else + cp_parser_commit_to_tentative_parse (parser); + } /* Parse the declarator. */ declarator_token_start = token; declarator = cp_parser_declarator (parser, diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction97.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction97.C new file mode 100644 index 0000000..3281868 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction97.C @@ -0,0 +1,6 @@ +// PR c++/89062 +// { dg-do compile { target c++17 } } + +template struct Foo { Foo(T); }; + +Foo x(Foo{1}); -- 2.7.4