On Thu, 26 Mar 2020, David Malcolm wrote:
> On Thu, 2020-03-26 at 17:28 -0400, Patrick Palka via Gcc-patches wrote:
> > This adds support to detect and recover from the case where an
> > opening brace
> > immediately follows the start of a requires-clause. So rather than
> > emitting the
> > error
> >
> > error: expected primary-expression before '{' token
> >
> > followed by a slew of irrevelant errors, we now assume the user had
> > intended to
> > write "requires requires {" and diagnose and recover accordingly.
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK?
> >
> > gcc/cp/ChangeLog:
> >
> > PR c++/94306
> > * parser.c (cp_parser_requires_clause_opt): Diagnose and
> > recover from
> > "requires {" when "requires requires {" was probably intended.
> >
> > gcc/testsuite/ChangeLog:
> >
> > PR c++/94306
> > * g++.dg/concepts/diagnostic8.C: New test.
> > ---
> > gcc/cp/parser.c | 17 ++++++++++++++++-
> > gcc/testsuite/g++.dg/concepts/diagnostic8.C | 6 ++++++
> > 2 files changed, 22 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic8.C
> >
> > diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> > index 05363653691..73c2c2cb010 100644
> > --- a/gcc/cp/parser.c
> > +++ b/gcc/cp/parser.c
> > @@ -27639,7 +27639,22 @@ cp_parser_requires_clause_opt (cp_parser
> > *parser, bool lambda_p)
> > }
> > return NULL_TREE;
> > }
> > - cp_lexer_consume_token (parser->lexer);
> > +
> > + cp_token *tok2 = cp_lexer_peek_nth_token (parser->lexer, 2);
> > + if (tok2->type == CPP_OPEN_BRACE)
> > + {
> > + /* An opening brace following the start of a requires-clause
> > is
> > + ill-formed; the user likely forgot the second `requires' that
> > + would start a requires-expression. */
> > + gcc_rich_location richloc (tok2->location);
> > + richloc.add_fixit_insert_before (" requires");
>
> Thanks for adding a fix-it hint. That said, is this spacing here
> correct? If I'm reading it right, this adds " requires" immediately
> before the open brace, leading to a suggestion of:
> "requires {"
> becoming:
> "requires requires{"
>
> Perhaps adding it immediately after the first requires via:
> richloc.add_fixit_insert_after (first_requires_location,
> " requires");
> would be better, which ought to lead to a suggestion of:
> "requires {"
> becoming:
> "requires requires {"
>
> (assuming I'm reading the patch right)
>
> Might be an idea to test the effect of the fix-it hint e.g. via
> -fdiagnostics-generate-patch.
>
> Hope this is constructive
> Dave
That makes a lot of sense, thanks. Luckily we have the location of the
first "requires" already handy. Here's the updated patch which
incorporates the improved fix-it hint:
-- >8 --
gcc/cp/ChangeLog:
PR c++/94306
* parser.c (cp_parser_requires_clause_opt): Diagnose and recover from
"requires {" when "requires requires {" was probably intended.
gcc/testsuite/ChangeLog:
PR c++/94306
* g++.dg/concepts/diagnostic8.C: New test.
---
gcc/cp/parser.c | 17 ++++++++++++++++-
gcc/testsuite/g++.dg/concepts/diagnostic8.C | 6 ++++++
2 files changed, 22 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic8.C
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 05363653691..5bc1ecd102e 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -27639,7 +27639,22 @@ cp_parser_requires_clause_opt (cp_parser *parser, bool
lambda_p)
}
return NULL_TREE;
}
- cp_lexer_consume_token (parser->lexer);
+
+ cp_token *tok2 = cp_lexer_peek_nth_token (parser->lexer, 2);
+ if (tok2->type == CPP_OPEN_BRACE)
+ {
+ /* An opening brace following the start of a requires-clause is
+ ill-formed; the user likely forgot the second `requires' that
+ would start a requires-expression. */
+ gcc_rich_location richloc (tok2->location);
+ richloc.add_fixit_insert_after (tok->location, " requires");
+ error_at (&richloc, "missing additional %<requires%> to start "
+ "a requires-expression");
+ /* Don't consume the `requires', so that it's reused as the start of a
+ requires-expression. */
+ }
+ else
+ cp_lexer_consume_token (parser->lexer);
if (!flag_concepts_ts)
return cp_parser_requires_clause_expression (parser, lambda_p);
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic8.C
b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
new file mode 100644
index 00000000000..70d7e4a9cc1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
@@ -0,0 +1,6 @@
+// PR c++/94306
+// { dg-do compile { target c++2a } }
+
+template<typename T> struct S { };
+template<typename T> requires { typename T::type; } struct S<T> { };
+// { dg-error "missing additional .requires." "" { target *-*-* } .-1 }
--
2.26.0.rc1.11.g30e9940356
