c++: lambdas inside constraints [PR92652]

When parsing a constraint-expression, a requires-clause or a
requires-expression, we temporarily increment processing_template_decl
so that we always obtain template trees which we later reduce via
substitution even when not inside a template.

But incrementing processing_template_decl when we're already inside a
template has the unintended side effect of shifting up the template
parameter levels of a lambda defined inside one of these constructs,
which leads to confusion later during substitution into the lambda.

This patch fixes this issue by incrementing processing_template_decl
during parsing of these constructs only if it is 0.

Passes 'make check-c++', and also tested by building cmcstl2, does this
look OK to commit after a full bootstrap/regtest?

gcc/cp/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* parser.c (cp_parser_requires_clause_expression): Temporarily
	increment processing_template_decl only if it is 0.
	(cp_parser_constraint_expression): Likewise.
	(cp_parser_requires_expression): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* g++.dg/cpp2a/concepts-lambda8.C: New test.
	* g++.dg/cpp2a/concepts-lambda9.C: New test.
	* g++.dg/cpp2a/concepts-lambda10.C: New test.
---
 gcc/cp/parser.c                                | 15 +++++++++------
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C |  7 +++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C  | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C  | 11 +++++++++++
 4 files changed, 38 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C

On 5/28/20 11:03 AM, Patrick Palka wrote:
> When parsing a constraint-expression, a requires-clause or a
> requires-expression, we temporarily increment processing_template_decl
> so that we always obtain template trees which we later reduce via
> substitution even when not inside a template.
> 
> But incrementing processing_template_decl when we're already inside a
> template has the unintended side effect of shifting up the template
> parameter levels of a lambda defined inside one of these constructs,
> which leads to confusion later during substitution into the lambda.
> 
> This patch fixes this issue by incrementing processing_template_decl
> during parsing of these constructs only if it is 0.
> 
> Passes 'make check-c++', and also tested by building cmcstl2, does this
> look OK to commit after a full bootstrap/regtest?
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/92652
> 	PR c++/93698
> 	PR c++/94128
> 	* parser.c (cp_parser_requires_clause_expression): Temporarily
> 	increment processing_template_decl only if it is 0.
> 	(cp_parser_constraint_expression): Likewise.
> 	(cp_parser_requires_expression): Likewise.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/92652
> 	PR c++/93698
> 	PR c++/94128
> 	* g++.dg/cpp2a/concepts-lambda8.C: New test.
> 	* g++.dg/cpp2a/concepts-lambda9.C: New test.
> 	* g++.dg/cpp2a/concepts-lambda10.C: New test.
> ---
>   gcc/cp/parser.c                                | 15 +++++++++------
>   gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C |  7 +++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C  | 11 +++++++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C  | 11 +++++++++++
>   4 files changed, 38 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C
> 
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index 54ca875ce54..3bca1f3770a 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -27663,11 +27663,12 @@ static tree
>   cp_parser_requires_clause_expression (cp_parser *parser, bool lambda_p)
>   {
>     processing_constraint_expression_sentinel parsing_constraint;
> -  ++processing_template_decl;
> +  temp_override<int> ovr (processing_template_decl);
> +  if (!processing_template_decl)
> +    processing_template_decl = 1;

This needs a comment about why you're doing it this way instead of the 
usual ++.  OK with that change.

>     cp_expr expr = cp_parser_constraint_logical_or_expression (parser, lambda_p);
>     if (check_for_bare_parameter_packs (expr))
>       expr = error_mark_node;
> -  --processing_template_decl;
>     return expr;
>   }
>   
> @@ -27684,12 +27685,13 @@ static tree
>   cp_parser_constraint_expression (cp_parser *parser)
>   {
>     processing_constraint_expression_sentinel parsing_constraint;
> -  ++processing_template_decl;
> +  temp_override<int> ovr (processing_template_decl);
> +  if (!processing_template_decl)
> +    processing_template_decl = 1;
>     cp_expr expr = cp_parser_binary_expression (parser, false, true,
>   					      PREC_NOT_OPERATOR, NULL);
>     if (check_for_bare_parameter_packs (expr))
>       expr = error_mark_node;
> -  --processing_template_decl;
>     expr.maybe_add_location_wrapper ();
>     return expr;
>   }
> @@ -27798,9 +27800,10 @@ cp_parser_requires_expression (cp_parser *parser)
>         parms = NULL_TREE;
>   
>       /* Parse the requirement body. */
> -    ++processing_template_decl;
> +    temp_override<int> ovr (processing_template_decl);
> +    if (!processing_template_decl)
> +      processing_template_decl = 1;
>       reqs = cp_parser_requirement_body (parser);
> -    --processing_template_decl;
>       if (reqs == error_mark_node)
>         return error_mark_node;
>     }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C
> new file mode 100644
> index 00000000000..392da312b28
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C
> @@ -0,0 +1,7 @@
> +// PR c++/94128
> +// { dg-do compile { target c++20 } }
> +
> +void test(auto param)
> +requires requires{ { [](auto p){return p;}(param) }; };
> +
> +void test2() { test(1); }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C
> new file mode 100644
> index 00000000000..c1c9be682d0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C
> @@ -0,0 +1,11 @@
> +// PR c++/92652
> +// { dg-do compile { target concepts } }
> +
> +template < typename T >
> +    requires ([]{return true ;}())
> +void h() { }
> +
> +int main()
> +{
> +    h<int>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C
> new file mode 100644
> index 00000000000..6b81ba0adac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C
> @@ -0,0 +1,11 @@
> +// PR c++/93698
> +// { dg-do compile { target concepts } }
> +
> +#include <utility>
> +
> +template <int N>
> +concept foo = []<std::size_t... Is>(std::index_sequence<Is...>) constexpr {
> +  return (Is + ...) > 10;
> +}(std::make_index_sequence<N>());
> +
> +bool a = foo<7>;
>