c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]

Message ID 20200326212830.957583-1-ppalka@redhat.com
State New
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  • c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]
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Commit Message

Christophe Lyon via Gcc-patches March 26, 2020, 9:28 p.m.
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

-- 
2.26.0.rc1.11.g30e9940356

Comments

Christophe Lyon via Gcc-patches March 26, 2020, 9:46 p.m. | #1
On 3/26/20 5:28 PM, Patrick Palka 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)


Do we want to handle an open paren the same way?

> +    {

> +      /* 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");

> +      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 }

>
Christophe Lyon via Gcc-patches March 26, 2020, 10:05 p.m. | #2
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

> +      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 }
Christophe Lyon via Gcc-patches March 26, 2020, 10:36 p.m. | #3
On Thu, 26 Mar 2020, Jason Merrill wrote:

> On 3/26/20 5:28 PM, Patrick Palka 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)

> 

> Do we want to handle an open paren the same way?


Hmm, wouldn't the error recovery be significantly different for an open
paren here?  Just pretending that there's another "requires" token would
mean we proceed as if we got "requires requires (" but that is still
ill-formed I think.

> 

> > +    {

> > +      /* 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");

> > +      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 }

> > 

> 

>
Christophe Lyon via Gcc-patches March 26, 2020, 10:52 p.m. | #4
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
Christophe Lyon via Gcc-patches March 27, 2020, 2:04 a.m. | #5
On 3/26/20 6:36 PM, Patrick Palka wrote:
> On Thu, 26 Mar 2020, Jason Merrill wrote:

> 

>> On 3/26/20 5:28 PM, Patrick Palka 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)

>>

>> Do we want to handle an open paren the same way?

> 

> Hmm, wouldn't the error recovery be significantly different for an open

> paren here?  Just pretending that there's another "requires" token would

> mean we proceed as if we got "requires requires (" but that is still

> ill-formed I think.


Not ill-formed; a requires-expression can have a parameter-list.

>>

>>> +    {

>>> +      /* 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");

>>> +      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 }

>>>

>>

>>

>
Christophe Lyon via Gcc-patches March 27, 2020, 3:49 a.m. | #6
On Thu, 26 Mar 2020, Jason Merrill wrote:

> On 3/26/20 6:36 PM, Patrick Palka wrote:

> > On Thu, 26 Mar 2020, Jason Merrill wrote:

> > 

> > > On 3/26/20 5:28 PM, Patrick Palka 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)

> > > 

> > > Do we want to handle an open paren the same way?

> > 

> > Hmm, wouldn't the error recovery be significantly different for an open

> > paren here?  Just pretending that there's another "requires" token would

> > mean we proceed as if we got "requires requires (" but that is still

> > ill-formed I think.

> 

> Not ill-formed; a requires-expression can have a parameter-list.


Ah, of course..  But then again, wouldn't "requires (" be not
necessarily ill-formed, since the open paren could possibly be a part of
the constraint-expression e.g. "requires (a || b)"?

We could perhaps look ahead of the open paren and check if we match the
ill-formed pattern "requires (...) {...}" and if so, handle the open
paren the same way.  Oh, but even that pattern is not necessarily
ill-formed, at least when parsing a lambda: the following
    "[] <typename T> () -> void requires (true) { }"
is apparently a well-formed lambda-expression whose tokens starting from
its requires-clause would match that pattern.  So we would probably have
to condition this open-paren handling on !lambda_p.

> 

> > > 

> > > > +    {

> > > > +      /* 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");

> > > > +      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 }

> > > > 

> > > 

> > > 

> > 

> 

>
Christophe Lyon via Gcc-patches March 27, 2020, 5:02 a.m. | #7
On 3/26/20 11:49 PM, Patrick Palka wrote:
> On Thu, 26 Mar 2020, Jason Merrill wrote:

> 

>> On 3/26/20 6:36 PM, Patrick Palka wrote:

>>> On Thu, 26 Mar 2020, Jason Merrill wrote:

>>>

>>>> On 3/26/20 5:28 PM, Patrick Palka 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)

>>>>

>>>> Do we want to handle an open paren the same way?

>>>

>>> Hmm, wouldn't the error recovery be significantly different for an open

>>> paren here?  Just pretending that there's another "requires" token would

>>> mean we proceed as if we got "requires requires (" but that is still

>>> ill-formed I think.

>>

>> Not ill-formed; a requires-expression can have a parameter-list.

> 

> Ah, of course..  But then again, wouldn't "requires (" be not

> necessarily ill-formed, since the open paren could possibly be a part of

> the constraint-expression e.g. "requires (a || b)"?


Ah, yes.

> We could perhaps look ahead of the open paren and check if we match the

> ill-formed pattern "requires (...) {...}" and if so, handle the open

> paren the same way.  Oh, but even that pattern is not necessarily

> ill-formed, at least when parsing a lambda: the following

>      "[] <typename T> () -> void requires (true) { }"

> is apparently a well-formed lambda-expression whose tokens starting from

> its requires-clause would match that pattern.  So we would probably have

> to condition this open-paren handling on !lambda_p.


Yeah, this is too much trouble.  Your patch is OK as is.

>>

>>>>

>>>>> +    {

>>>>> +      /* 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");

>>>>> +      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 }

>>>>>

>>>>

>>>>

>>>

>>

>>

>

Patch

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");
+      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 }