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

Message ID 1507833929-25385-1-git-send-email-dmalcolm@redhat.com
State New
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  • C++: show location of unclosed extern "C" specifications (v2)
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Commit Message

David Malcolm Oct. 12, 2017, 6:45 p.m.
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

Comments

Jason Merrill Oct. 12, 2017, 9:07 p.m. | #1
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

Patch

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 }	
+      }
+    }
+  }
+}