Re: C++ PATCH for c++/81429 - wrong parsing of constructor with C++11 attribute

[Jason Merrill <jason at redhat dot com>]

On Tue, Aug 6, 2019, 11:28 PM Marek Polacek <polacek@redhat.com> wrote:

> In this PR, we are wrongly parsing a constructor if its first parameter
> begins
> with a C++11 attribute, e.g.:
>
>   struct S {
>     S([[maybe_unused]] int a) { }
>   };
>
> If the GNU attribute format is used instead, there's no problem.
> C++11-style
> attribute on a later parameter is fine also.
>
> The problem is in cp_parser_constructor_declarator_p, in particular the
> code
> that checks whether we're dealing with something like "S (f) (int);", which
> is not a constructor.  We're checking if what comes after the first '(' is
> either ')', "...", of a parameter decl.  A parameter decl can start with
> the
> "attribute" keyword, which cp_lexer_next_token_is_decl_specifier_keyword
> recognizes, but a parameter decl can also start with a C++11-style
> attribute,
> which we forgot to realize.
>
> The first test uses -Wunused-parameter in order to check that the attribute
> is in effect.
>
> And I also noticed that we don't issue a pedwarn about maybe_unused (C++17
> attribute) in C++14: PR 91382.
>

Since unsupported attributes are supposed to be ignored, I don't think we
need to complain.

The patch is ok.

Bootstrapped/regtested on x86_64-linux, ok for trunk?  I think this should
> also go into 9.3.
>
> 2019-08-06  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/81429 - wrong parsing of constructor with C++11 attribute.
>         * parser.c (cp_parser_constructor_declarator_p): Handle the
> scenario
>         when a parameter declaration begins with [[attribute]].
>
>         * g++.dg/cpp0x/gen-attrs-68.C: New test.
>         * g++.dg/cpp0x/gen-attrs-69.C: New test.
>
> diff --git gcc/cp/parser.c gcc/cp/parser.c
> index 79da7b52eb9..b8996c707b6 100644
> --- gcc/cp/parser.c
> +++ gcc/cp/parser.c
> @@ -27855,7 +27855,9 @@ cp_parser_constructor_declarator_p (cp_parser
> *parser, cp_parser_flags flags,
>           /* A parameter declaration begins with a decl-specifier,
>              which is either the "attribute" keyword, a storage class
>              specifier, or (usually) a type-specifier.  */
> -         && !cp_lexer_next_token_is_decl_specifier_keyword
> (parser->lexer))
> +         && !cp_lexer_next_token_is_decl_specifier_keyword (parser->lexer)
> +         /* A parameter declaration can also begin with [[attribute]].  */
> +         && !cp_next_tokens_can_be_std_attribute_p (parser))
>         {
>           tree type;
>           tree pushed_scope = NULL_TREE;
> diff --git gcc/testsuite/g++.dg/cpp0x/gen-attrs-68.C
> gcc/testsuite/g++.dg/cpp0x/gen-attrs-68.C
> new file mode 100644
> index 00000000000..6bede0659db
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/gen-attrs-68.C
> @@ -0,0 +1,40 @@
> +// PR c++/81429 - wrong parsing of constructor with C++11 attribute.
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-Wunused-parameter -Wno-pedantic" }
> +
> +void fn1([[maybe_unused]] int a) { }
> +void fn2(int a [[maybe_unused]]) { }
> +void fn3(__attribute__((unused)) int a) { }
> +void fn4(int a __attribute__((unused))) { }
> +
> +struct S1 {
> +  S1([[maybe_unused]] int a) { }
> +};
> +
> +struct S2 {
> +  S2([[maybe_unused]] int f, [[maybe_unused]] int a) { }
> +};
> +
> +struct S3 {
> +  S3(int a [[maybe_unused]]) { }
> +};
> +
> +struct S4 {
> +  S4(int f [[maybe_unused]], int a [[maybe_unused]]) { }
> +};
> +
> +struct S5 {
> +  S5(__attribute__((unused)) int a) { }
> +};
> +
> +struct S6 {
> +  S6(__attribute__((unused)) int f, __attribute__((unused)) int a) { }
> +};
> +
> +struct S7 {
> +  S7(int a __attribute__((unused))) { }
> +};
> +
> +struct S8 {
> +  S8(int f __attribute__((unused)), int a __attribute__((unused))) { }
> +};
> diff --git gcc/testsuite/g++.dg/cpp0x/gen-attrs-69.C
> gcc/testsuite/g++.dg/cpp0x/gen-attrs-69.C
> new file mode 100644
> index 00000000000..43173dbbdf4
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/gen-attrs-69.C
> @@ -0,0 +1,40 @@
> +// PR c++/81429 - wrong parsing of constructor with C++11 attribute.
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-Wno-pedantic" }
> +
> +void fn1([[maybe_unused]] int);
> +void fn2(int a [[maybe_unused]]);
> +void fn3(__attribute__((unused)) int);
> +void fn4(int __attribute__((unused)));
> +
> +struct S1 {
> +  S1([[maybe_unused]] int);
> +};
> +
> +struct S2 {
> +  S2([[maybe_unused]] int, [[maybe_unused]] int);
> +};
> +
> +struct S3 {
> +  S3(int a [[maybe_unused]]);
> +};
> +
> +struct S4 {
> +  S4(int a [[maybe_unused]], int b [[maybe_unused]]);
> +};
> +
> +struct S5 {
> +  S5(__attribute__((unused)) int);
> +};
> +
> +struct S6 {
> +  S6(__attribute__((unused)) int, __attribute__((unused)) int);
> +};
> +
> +struct S7 {
> +  S7(int __attribute__((unused)));
> +};
> +
> +struct S8 {
> +  S8(int __attribute__((unused)), int __attribute__((unused)));
> +};
>