PING Re: [PATCH] C++: show location of problematic extern "C" specifications

[David Malcolm <dmalcolm at redhat dot com>]

Ping

On Tue, 2017-09-26 at 15:27 -0400, David Malcolm wrote:
> There are a few places where the C++ FE will complain when attempting
> to do things within an extern "C" linkage specifier.
> 
> I've run into problems where it wasn't clear where the pertinent
> extern "C" was; for example, when failing to close an extern "C"
> linkage
> specifier in a header, leading to "template with C linkage" errors in
> a different source file.
> 
> As of r251026 there will be a message highlighting the unclosed '{',
> but
> this may be hard to spot at the very end of the errors.
> 
> This patch adds a note to the various diagnostics that complain
> about C linkage, showing the user where the extern "C" specification
> began.
> 
> Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
> 
> OK for trunk?
> 
> gcc/cp/ChangeLog:
> 	* cp-tree.h (struct saved_scope): Add "location" field.
> 	(maybe_show_extern_c_location): New decl.
> 	* decl.c (grokfndecl): When complaining about literal operators
> 	with C linkage, issue a note giving the location of the
> 	extern "C".
> 	* parser.c (cp_parser_linkage_specification): Store the
> location
> 	of the "extern" token within the scope_chain.
> 	(maybe_show_extern_c_location): New function.
> 	(cp_parser_explicit_specialization): When complaining about
> 	template specializations with C linkage, issue a note giving
> the
> 	location of the extern "C".
> 	(cp_parser_explicit_template_declaration): Likewise for
> templates.
> 
> gcc/testsuite/ChangeLog:
> 	* g++.dg/cpp0x/udlit-extern-c.C: New test case.
> 	* g++.dg/diagnostic/unclosed-extern-c.C: Add example of a
> template
> 	erroneously covered by an unclosed extern "C".
> 	* g++.dg/template/extern-c.C: New test case.
> ---
>  gcc/cp/cp-tree.h                                   |  3 ++
>  gcc/cp/decl.c                                      |  1 +
>  gcc/cp/parser.c                                    | 19 ++++++++++-
>  gcc/testsuite/g++.dg/cpp0x/udlit-extern-c.C        |  7 ++++
>  .../g++.dg/diagnostic/unclosed-extern-c.C          | 11 +++++-
>  gcc/testsuite/g++.dg/template/extern-c.C           | 39
> ++++++++++++++++++++++
>  6 files changed, 78 insertions(+), 2 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp0x/udlit-extern-c.C
>  create mode 100644 gcc/testsuite/g++.dg/template/extern-c.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index e508598..762cc7b 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -1568,6 +1568,8 @@ struct GTY(()) saved_scope {
>    hash_map<tree, tree> *GTY((skip)) x_local_specializations;
>  
>    struct saved_scope *prev;
> +
> +  location_t location;
>  };
>  
>  extern GTY(()) struct saved_scope *scope_chain;
> @@ -6352,6 +6354,7 @@ extern bool parsing_nsdmi (void);
>  extern bool parsing_default_capturing_generic_lambda_in_template
> (void);
>  extern void inject_this_parameter (tree, cp_cv_quals);
>  extern location_t defarg_location (tree);
> +extern void maybe_show_extern_c_location (void);
>  
>  /* in pt.c */
>  extern bool check_template_shadow		(tree);
> diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
> index 50fa1ba..d08ac9a 100644
> --- a/gcc/cp/decl.c
> +++ b/gcc/cp/decl.c
> @@ -8724,6 +8724,7 @@ grokfndecl (tree ctype,
>        if (DECL_LANGUAGE (decl) == lang_c)
>  	{
>  	  error ("literal operator with C linkage");
> +	  maybe_show_extern_c_location ();
>  	  return NULL_TREE;
>  	}
>  
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index d831d66..b90f40d 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -13823,7 +13823,8 @@ cp_parser_linkage_specification (cp_parser*
> parser)
>    tree linkage;
>  
>    /* Look for the `extern' keyword.  */
> -  cp_parser_require_keyword (parser, RID_EXTERN, RT_EXTERN);
> +  cp_token *extern_token
> +    = cp_parser_require_keyword (parser, RID_EXTERN, RT_EXTERN);
>  
>    /* Look for the string-literal.  */
>    linkage = cp_parser_string_literal (parser, false, false);
> @@ -13843,6 +13844,7 @@ cp_parser_linkage_specification (cp_parser*
> parser)
>  
>    /* We're now using the new linkage.  */
>    push_lang_context (linkage);
> +  scope_chain->location = extern_token->location;
>  
>    /* If the next token is a `{', then we're using the first
>       production.  */
> @@ -16589,6 +16591,19 @@ cp_parser_explicit_instantiation (cp_parser*
> parser)
>    timevar_pop (TV_TEMPLATE_INST);
>  }
>  
> +/* Helper function for diagnostics that have complained about things
> +   being used with 'extern "C"' linkage.
> +
> +   Attempt to issue a note showing where the 'extern "C"' linkage
> began.  */
> +
> +void
> +maybe_show_extern_c_location (void)
> +{
> +  if (scope_chain->location != UNKNOWN_LOCATION)
> +    inform (scope_chain->location, "%<extern \"C\"%> linkage started
> here");
> +}
> +
> +
>  /* Parse an explicit-specialization.
>  
>     explicit-specialization:
> @@ -16623,6 +16638,7 @@ cp_parser_explicit_specialization (cp_parser*
> parser)
>    if (current_lang_name == lang_name_c)
>      {
>        error_at (token->location, "template specialization with C
> linkage");
> +      maybe_show_extern_c_location ();
>        /* Give it C++ linkage to avoid confusing other parts of the
>  	 front end.  */
>        push_lang_context (lang_name_cplusplus);
> @@ -26858,6 +26874,7 @@ cp_parser_explicit_template_declaration
> (cp_parser* parser, bool member_p)
>    if (current_lang_name == lang_name_c)
>      {
>        error_at (location, "template with C linkage");
> +      maybe_show_extern_c_location ();
>        /* Give it C++ linkage to avoid confusing other parts of the
>           front end.  */
>        push_lang_context (lang_name_cplusplus);
> diff --git a/gcc/testsuite/g++.dg/cpp0x/udlit-extern-c.C
> b/gcc/testsuite/g++.dg/cpp0x/udlit-extern-c.C
> new file mode 100644
> index 0000000..d47a49c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/udlit-extern-c.C
> @@ -0,0 +1,7 @@
> +// { dg-do compile { target c++11 } }
> +
> +extern "C" { // { dg-message "1: 'extern .C.' linkage started here"
> }
> +
> +constexpr double operator"" _deg ( double degrees ); // { dg-error
> "literal operator with C linkage" }
> +
> +}
> diff --git a/gcc/testsuite/g++.dg/diagnostic/unclosed-extern-c.C
> b/gcc/testsuite/g++.dg/diagnostic/unclosed-extern-c.C
> index fda3532..44f538e 100644
> --- a/gcc/testsuite/g++.dg/diagnostic/unclosed-extern-c.C
> +++ b/gcc/testsuite/g++.dg/diagnostic/unclosed-extern-c.C
> @@ -1,3 +1,12 @@
> -extern "C" { /* { dg-message "12: to match this '.'" } */
> +extern "C" { // { dg-line open_extern_c }
> +
> +  int foo (void);
> +
> +/* Missing close-brace for the extern "C" here.  */
> +
> +template <typename T> // { dg-error "template with C linkage" }
> +void bar (void);
> +// { dg-message "1: 'extern .C.' linkage started here" "" { target
> *-*-* } open_extern_c }
>  
>  void test (void); /* { dg-error "17: expected '.' at end of input" }
> */
> +// { message "12: to match this '.'" "" { target *-*-* }
> open_extern_c }
> diff --git a/gcc/testsuite/g++.dg/template/extern-c.C
> b/gcc/testsuite/g++.dg/template/extern-c.C
> new file mode 100644
> index 0000000..8a3a77a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/extern-c.C
> @@ -0,0 +1,39 @@
> +template <typename T> void specializable (T);
> +
> +/* Invalid template: within "extern C".  */
> +
> +extern "C" { // { dg-message "1: 'extern .C.' linkage started here"
> }
> +
> +template <typename T> // { dg-error "template with C linkage" }
> +void within_extern_c_braces (void);
> +
> +}
> +
> +/* Valid template: not within "extern C".  */
> +
> +template <typename T>
> +void not_within_extern_c (void);
> +
> +
> +/* Invalid specialization: within "extern C".  */
> +
> +extern "C" { // { dg-message "1: 'extern .C.' linkage started here"
> }
> +
> +template <>  // { dg-error "template specialization with C linkage"
> }
> +void specializable (int);
> +
> +}
> +
> +
> +/* Valid specialization: not within "extern C".  */
> +template <>
> +void specializable (char);
> +
> +
> +/* Example of extern C without braces.  */
> +
> +extern "C" template <typename T> // { dg-line
> open_extern_c_no_braces }
> +void within_extern_c_no_braces (void);
> +// { dg-error "12: template with C linkage" "" { target *-*-* }
> open_extern_c_no_braces }
> +// { dg-message "1: 'extern .C.' linkage started here" "" { target
> *-*-* } open_extern_c_no_braces }
> +