[PATCH] c++: ICE in dependent_type_p with constrained auto [PR97052]

[Jason Merrill]

On 10/5/20 4:30 PM, Patrick Palka wrote:
> On Wed, 30 Sep 2020, Jason Merrill wrote:
> 
>> On 9/29/20 5:01 PM, Patrick Palka wrote:
>>> This patch fixes an "unguarded" call to coerce_template_parms in
>>> build_standard_check: processing_template_decl could be zero if we
>>> we get here during processing of the first 'auto' parameter of an
>>> abbreviated function template.  In the testcase below, this leads to an
>>> ICE when coerce_template_parms substitutes into C's dependent default
>>> template argument.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu and tested by building
>>> cmcstl2 and range-v3.  Does this look OK for trunk?
>>
>> This looks OK, but is there a place higher in the call stack where we should
>> have already set processing_template_decl?
> 
> The call stack at that point is:
> 
>    build_variable_check
>    build_concept_check
>    build_type_constraint
>    finish_type_constraints
>    cp_parser_placeholder_type_specifier
>    cp_parser_simple_type_specifier
>    ...
> 
> So it seems the most natural place to set processing_template_decl would
> be in build_type_constraint, around the call to build_concept_check,
> since that's where we create the WILDCARD_DECL that eventually reaches
> coerce_template_parms.
> 
> And in order to additionally avoid a similar ICE when processing the
> type constraint of a non-templated variable, we also need to guard the
> call to build_concept check in make_constrained_placeholder_type.  The
> testcase below now contains such an example.

Setting the flag in cp_parser_placeholder_type_specifier would cover 
both of those, right?

> So something like this perhaps:
> 
> -- >8 --
> 
> Subject: [PATCH] c++: ICE in dependent_type_p with constrained auto [PR97052]
> 
> This patch fixes an "unguarded" call to coerce_template_parms in
> build_standard_check: processing_template_decl could be zero if we
> get here during processing of the first 'auto' parameter of an
> abbreviated function template, or if we're processing the type
> constraint of a non-templated variable.  In the testcase below, this
> leads to an ICE when coerce_template_parms instantiates C's dependent
> default template argument.
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/97052
> 	* constraint.cc (build_type_constraint): Temporarily increment
> 	processing_template_decl before calling build_concept_check.
> 	* pt.c (make_constrained_placeholder_type): Likewise.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/97052
> 	* g++.dg/cpp2a/concepts-defarg2: New test.
> ---
>   gcc/cp/constraint.cc                          |  2 ++
>   gcc/cp/pt.c                                   |  2 ++
>   gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C | 13 +++++++++++++
>   3 files changed, 17 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C
> 
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index d49957a6c4a..050b55ce092 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -1427,7 +1427,9 @@ tree
>   build_type_constraint (tree decl, tree args, tsubst_flags_t complain)
>   {
>     tree wildcard = build_nt (WILDCARD_DECL);
> +  ++processing_template_decl;
>     tree check = build_concept_check (decl, wildcard, args, complain);
> +  --processing_template_decl;
>     if (check == error_mark_node)
>       return error_mark_node;
>     return unpack_concept_check (check);
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 72efecff37f..efdd017a4d5 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -27914,7 +27914,9 @@ make_constrained_placeholder_type (tree type, tree con, tree args)
>     tree expr = tmpl;
>     if (TREE_CODE (con) == FUNCTION_DECL)
>       expr = ovl_make (tmpl);
> +  ++processing_template_decl;
>     expr = build_concept_check (expr, type, args, tf_warning_or_error);
> +  --processing_template_decl;
>   
>     PLACEHOLDER_TYPE_CONSTRAINTS (type) = expr;
>   
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C
> new file mode 100644
> index 00000000000..a63ca4e133d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg2.C
> @@ -0,0 +1,13 @@
> +// PR c++/97052
> +// { dg-do compile { target c++20 } }
> +
> +template<typename T, typename U = typename T::type>
> +concept C = true;
> +
> +constexpr bool f(C auto) {
> +  return true;
> +}
> +
> +static_assert(f(0));
> +
> +C auto x = 0;
>