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

[David Malcolm <dmalcolm at redhat dot com>]

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