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

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

[David Malcolm]

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 }
+
-- 
1.8.5.3

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

[David Malcolm]

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 }
> +

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

[Jason Merrill]

On Tue, Sep 26, 2017 at 3:27 PM, David Malcolm <dmalcolm@redhat.com> wrote:
>         * cp-tree.h (struct saved_scope): Add "location" field.

saved_scope seems like the wrong place for this; it's only interesting
at parse time, so having it saved during template instantiation
doesn't seem useful.  I'd prefer to put it in cp_parser and have
maybe_show_extern_c_location look in the_parser.

Jason

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

[David Malcolm]

On Wed, 2017-10-11 at 15:51 -0400, Jason Merrill wrote:
> On Tue, Sep 26, 2017 at 3:27 PM, David Malcolm <dmalcolm@redhat.com>
> wrote:
> >         * cp-tree.h (struct saved_scope): Add "location" field.
> 
> saved_scope seems like the wrong place for this; it's only
> interesting
> at parse time, so having it saved during template instantiation
> doesn't seem useful.  I'd prefer to put it in cp_parser and have
> maybe_show_extern_c_location look in the_parser.

Thanks.

I have a new version of the patch *mostly* working that way, but one of
the uses of maybe_show_extern_c_location is within decl.c:grokfndecl
(when complaining about user-defined literal operators within C
linkage), and there doesn't seem to be access to the cp_parser * from
there.

I could fix this by adding a cp_parser * for this to grokfndecl, which
would mean adding it to grokdeclarator, but I get the impression that
the code is structured so that the decl-handling isn't meant to know
about the parser.

Thoughts?
Dave

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

[Jason Merrill]

On Wed, Oct 11, 2017 at 4:50 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> On Wed, 2017-10-11 at 15:51 -0400, Jason Merrill wrote:
>> On Tue, Sep 26, 2017 at 3:27 PM, David Malcolm <dmalcolm@redhat.com>
>> wrote:
>> >         * cp-tree.h (struct saved_scope): Add "location" field.
>>
>> saved_scope seems like the wrong place for this; it's only
>> interesting
>> at parse time, so having it saved during template instantiation
>> doesn't seem useful.  I'd prefer to put it in cp_parser and have
>> maybe_show_extern_c_location look in the_parser.
>
> Thanks.
>
> I have a new version of the patch *mostly* working that way, but one of
> the uses of maybe_show_extern_c_location is within decl.c:grokfndecl
> (when complaining about user-defined literal operators within C
> linkage), and there doesn't seem to be access to the cp_parser * from
> there.
>
> I could fix this by adding a cp_parser * for this to grokfndecl, which
> would mean adding it to grokdeclarator, but I get the impression that
> the code is structured so that the decl-handling isn't meant to know
> about the parser.

That's why I was thinking to look at "the_parser", which functions in
parser.c can get at directly.

Jason

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

[David Malcolm]

On Wed, 2017-10-11 at 17:20 -0400, Jason Merrill wrote:
> On Wed, Oct 11, 2017 at 4:50 PM, David Malcolm <dmalcolm@redhat.com>
> wrote:
> > On Wed, 2017-10-11 at 15:51 -0400, Jason Merrill wrote:
> > > On Tue, Sep 26, 2017 at 3:27 PM, David Malcolm <dmalcolm@redhat.c
> > > om>
> > > wrote:
> > > >         * cp-tree.h (struct saved_scope): Add "location" field.
> > > 
> > > saved_scope seems like the wrong place for this; it's only
> > > interesting
> > > at parse time, so having it saved during template instantiation
> > > doesn't seem useful.  I'd prefer to put it in cp_parser and have
> > > maybe_show_extern_c_location look in the_parser.
> > 
> > Thanks.
> > 
> > I have a new version of the patch *mostly* working that way, but
> > one of
> > the uses of maybe_show_extern_c_location is within
> > decl.c:grokfndecl
> > (when complaining about user-defined literal operators within C
> > linkage), and there doesn't seem to be access to the cp_parser *
> > from
> > there.
> > 
> > I could fix this by adding a cp_parser * for this to grokfndecl,
> > which
> > would mean adding it to grokdeclarator, but I get the impression
> > that
> > the code is structured so that the decl-handling isn't meant to
> > know
> > about the parser.
> 
> That's why I was thinking to look at "the_parser", which functions in
> parser.c can get at directly.
> 
> Jason

Aha - thanks: I missed the underscore and read it as "the parser". 
Sorry.

Am working on an updated patch.
Dave

[PATCH] C++: show location of unclosed extern "C" specifications (v2)

[David Malcolm]

On Wed, 2017-10-11 at 15:51 -0400, Jason Merrill wrote:
> On Tue, Sep 26, 2017 at 3:27 PM, David Malcolm <dmalcolm@redhat.com>
> wrote:
> >         * cp-tree.h (struct saved_scope): Add "location" field.
> 
> saved_scope seems like the wrong place for this; it's only
> interesting
> at parse time, so having it saved during template instantiation
> doesn't seem useful.  I'd prefer to put it in cp_parser and have
> maybe_show_extern_c_location look in the_parser.
> 
> Jason

Here's an updated version that moves it there.

Other changes in v2:
- handle nested linkage specifications
- put the note on the string-literal, rather than the extern:
    note: 'extern "C"' linkage started here
     extern "C" {
            ^~~

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
OK for trunk?

Thanks
Dave


Blurb from v1:

If the user fails to close an extern "C" linkage specifier, and then
uses templates, they will run into "template with C linkage" errors.

From personal experience, it can be hard to tell where the
extern "C" began.  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.

gcc/cp/ChangeLog:
	* cp-tree.h (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_new): Initialize new field
	"innermost_linkage_specification_location".
	(cp_parser_linkage_specification): Store the location
	of the string-literal token within the cp_parser.
	(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.
	(maybe_show_extern_c_location): New function.
	* parser.h (struct cp_parser): New field
	"innermost_linkage_specification_location".

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                                   |  1 +
 gcc/cp/decl.c                                      |  1 +
 gcc/cp/parser.c                                    | 29 ++++++++++
 gcc/cp/parser.h                                    |  4 ++
 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           | 66 ++++++++++++++++++++++
 7 files changed, 118 insertions(+), 1 deletion(-)
 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 82ebc28..df3d198 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6362,6 +6362,7 @@ 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 99f0af2..51a5305 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 28bc8e4..a556a21 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -3828,6 +3828,9 @@ cp_parser_new (void)
   /* Allow constrained-type-specifiers. */
   parser->prevent_constrained_type_specifiers = 0;
 
+  /* We haven't yet seen an 'extern "C"'.  */
+  parser->innermost_linkage_specification_location = UNKNOWN_LOCATION;
+
   return parser;
 }
 
@@ -13740,6 +13743,7 @@ cp_parser_linkage_specification (cp_parser* parser)
   cp_parser_require_keyword (parser, RID_EXTERN, RT_EXTERN);
 
   /* Look for the string-literal.  */
+  cp_token *string_token = cp_lexer_peek_token (parser->lexer);
   linkage = cp_parser_string_literal (parser, false, false);
 
   /* Transform the literal into an identifier.  If the literal is a
@@ -13758,6 +13762,13 @@ cp_parser_linkage_specification (cp_parser* parser)
   /* We're now using the new linkage.  */
   push_lang_context (linkage);
 
+  /* Preserve the location of the string-literal for the innermost linkage
+     specification, tracking the locations of nested specifications
+     via a local.  */
+  location_t saved_location
+    = parser->innermost_linkage_specification_location;
+  parser->innermost_linkage_specification_location = string_token->location;
+
   /* If the next token is a `{', then we're using the first
      production.  */
   if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
@@ -13788,6 +13799,9 @@ cp_parser_linkage_specification (cp_parser* parser)
 
   /* We're done with the linkage-specification.  */
   pop_lang_context ();
+
+  /* Restore location of parent linkage specification, if any.  */
+  parser->innermost_linkage_specification_location = saved_location;
 }
 
 /* Parse a static_assert-declaration.
@@ -16537,6 +16551,7 @@ 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);
@@ -26861,6 +26876,7 @@ 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);
@@ -39479,4 +39495,17 @@ finish_fully_implicit_template (cp_parser *parser, tree member_decl_opt)
   return member_decl_opt;
 }
 
+/* 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 (the_parser->innermost_linkage_specification_location != UNKNOWN_LOCATION)
+    inform (the_parser->innermost_linkage_specification_location,
+	    "%<extern \"C\"%> linkage started here");
+}
+
 #include "gt-cp-parser.h"
diff --git a/gcc/cp/parser.h b/gcc/cp/parser.h
index 0994e1e..f4f4a01 100644
--- a/gcc/cp/parser.h
+++ b/gcc/cp/parser.h
@@ -412,6 +412,10 @@ struct GTY(()) cp_parser {
      context e.g., because they could never be deduced.  */
   int prevent_constrained_type_specifiers;
 
+  /* Location of the string-literal token within the current linkage
+     specification, if any, or UNKNOWN_LOCATION otherwise.  */
+  location_t innermost_linkage_specification_location;
+
 };
 
 /* In parser.c  */
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..ea54cba
--- /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 "8: '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..d85a049 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 "8: '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..6cbf69f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/extern-c.C
@@ -0,0 +1,66 @@
+template <typename T> void specializable (T);
+
+/* Invalid template: within "extern C".  */
+
+extern "C" { // { dg-message "8: '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 "8: '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 "8: 'extern .C.' linkage started here" "" { target *-*-* } open_extern_c_no_braces }
+
+
+/* Nested extern "C" specifications.
+   We should report within the innermost extern "C" that's still open.  */
+
+extern "C" {
+  extern "C" { // { dg-line middle_open_extern_c }
+    extern "C" {
+    }
+
+    template <typename T>  // { dg-error "template with C linkage" }
+    void within_nested_extern_c (void);
+    // { dg-message "10: 'extern .C.' linkage started here" "" { target *-*-* } middle_open_extern_c }
+
+    extern "C++" {
+      /* Valid template: within extern "C++".  */
+      template <typename T>
+      void within_nested_extern_cpp (void);
+
+      extern "C" {  // { dg-line last_open_extern_c }
+	/* Invalid template: within "extern C".  */
+	template <typename T> // { dg-error "template with C linkage" }
+	void within_extern_c_within_extern_cpp (void);
+	// { dg-message "14: 'extern .C.' linkage started here" "" { target *-*-* } last_open_extern_c }	
+      }
+    }
+  }
+}
-- 
1.8.5.3

Re: [PATCH] C++: show location of unclosed extern "C" specifications (v2)

[Jason Merrill]

On Thu, Oct 12, 2017 at 2:45 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> - put the note on the string-literal, rather than the extern:
>     note: 'extern "C"' linkage started here
>      extern "C" {
>             ^~~

Maybe a range spanning both tokens?

OK with or without that change.

Jason

[committed] C++: show location of unclosed extern "C" specifications (v3)

[David Malcolm]

On Thu, 2017-10-12 at 17:07 -0400, Jason Merrill wrote:
> On Thu, Oct 12, 2017 at 2:45 PM, David Malcolm <dmalcolm@redhat.com>
> wrote:
> > - put the note on the string-literal, rather than the extern:
> >     note: 'extern "C"' linkage started here
> >      extern "C" {
> >             ^~~
> 
> Maybe a range spanning both tokens?
> 
> OK with or without that change.
> 
> Jason

Good idea; I've implemented it.

Committed to trunk as r253726 (after usual testing).

For reference, here's what I committed:

gcc/cp/ChangeLog:
	* cp-tree.h (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_new): Initialize new field
	"innermost_linkage_specification_location".
	(cp_parser_linkage_specification): Store the location
	of the linkage specification within the cp_parser.
	(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.
	(maybe_show_extern_c_location): New function.
	* parser.h (struct cp_parser): New field
	"innermost_linkage_specification_location".

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                                   |  1 +
 gcc/cp/decl.c                                      |  1 +
 gcc/cp/parser.c                                    | 39 ++++++++++++-
 gcc/cp/parser.h                                    |  4 ++
 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           | 66 ++++++++++++++++++++++
 7 files changed, 127 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 dc98dd8..b74b6d9 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6356,6 +6356,7 @@ 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 f251a90..a3cc80c 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -8729,6 +8729,7 @@ 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 810e2b7..2337be5 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -3937,6 +3937,9 @@ cp_parser_new (void)
   /* Allow constrained-type-specifiers. */
   parser->prevent_constrained_type_specifiers = 0;
 
+  /* We haven't yet seen an 'extern "C"'.  */
+  parser->innermost_linkage_specification_location = UNKNOWN_LOCATION;
+
   return parser;
 }
 
@@ -13848,9 +13851,11 @@ 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.  */
+  cp_token *string_token = cp_lexer_peek_token (parser->lexer);
   linkage = cp_parser_string_literal (parser, false, false);
 
   /* Transform the literal into an identifier.  If the literal is a
@@ -13869,6 +13874,20 @@ cp_parser_linkage_specification (cp_parser* parser)
   /* We're now using the new linkage.  */
   push_lang_context (linkage);
 
+  /* Preserve the location of the the innermost linkage specification,
+     tracking the locations of nested specifications via a local.  */
+  location_t saved_location
+    = parser->innermost_linkage_specification_location;
+  /* Construct a location ranging from the start of the "extern" to
+     the end of the string-literal, with the caret at the start, e.g.:
+       extern "C" {
+       ^~~~~~~~~~
+  */
+  parser->innermost_linkage_specification_location
+    = make_location (extern_token->location,
+		     extern_token->location,
+		     get_finish (string_token->location));
+
   /* If the next token is a `{', then we're using the first
      production.  */
   if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
@@ -13899,6 +13918,9 @@ cp_parser_linkage_specification (cp_parser* parser)
 
   /* We're done with the linkage-specification.  */
   pop_lang_context ();
+
+  /* Restore location of parent linkage specification, if any.  */
+  parser->innermost_linkage_specification_location = saved_location;
 }
 
 /* Parse a static_assert-declaration.
@@ -16643,6 +16665,7 @@ 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);
@@ -26979,6 +27002,7 @@ 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);
@@ -39552,4 +39576,17 @@ finish_fully_implicit_template (cp_parser *parser, tree member_decl_opt)
   return member_decl_opt;
 }
 
+/* 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 (the_parser->innermost_linkage_specification_location != UNKNOWN_LOCATION)
+    inform (the_parser->innermost_linkage_specification_location,
+	    "%<extern \"C\"%> linkage started here");
+}
+
 #include "gt-cp-parser.h"
diff --git a/gcc/cp/parser.h b/gcc/cp/parser.h
index 0994e1e..f4f4a01 100644
--- a/gcc/cp/parser.h
+++ b/gcc/cp/parser.h
@@ -412,6 +412,10 @@ struct GTY(()) cp_parser {
      context e.g., because they could never be deduced.  */
   int prevent_constrained_type_specifiers;
 
+  /* Location of the string-literal token within the current linkage
+     specification, if any, or UNKNOWN_LOCATION otherwise.  */
+  location_t innermost_linkage_specification_location;
+
 };
 
 /* In parser.c  */
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..c0dd7cb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/extern-c.C
@@ -0,0 +1,66 @@
+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 }
+
+
+/* Nested extern "C" specifications.
+   We should report within the innermost extern "C" that's still open.  */
+
+extern "C" {
+  extern "C" { // { dg-line middle_open_extern_c }
+    extern "C" {
+    }
+
+    template <typename T>  // { dg-error "template with C linkage" }
+    void within_nested_extern_c (void);
+    // { dg-message "3: 'extern .C.' linkage started here" "" { target *-*-* } middle_open_extern_c }
+
+    extern "C++" {
+      /* Valid template: within extern "C++".  */
+      template <typename T>
+      void within_nested_extern_cpp (void);
+
+      extern "C" {  // { dg-line last_open_extern_c }
+	/* Invalid template: within "extern C".  */
+	template <typename T> // { dg-error "template with C linkage" }
+	void within_extern_c_within_extern_cpp (void);
+	// { dg-message "7: 'extern .C.' linkage started here" "" { target *-*-* } last_open_extern_c }	
+      }
+    }
+  }
+}
-- 
1.8.5.3