On 1/11/21 4:40 PM, Jason Merrill wrote:
> On 1/7/21 4:06 PM, Patrick Palka wrote:
>> This is essentially a followup to r11-3714 -- we ICEing from another
>> "unguarded" call to build_concept_check, this time in do_auto_deduction,
>> due to the presence of templated trees when !processing_template_decl.
>>
>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>> trunk and perhaps the 10 branch?
>>
>> gcc/cp/ChangeLog:
>>
>>     PR c++/98346
>>     * pt.c (do_auto_deduction): Temporarily increment
>>     processing_template_decl before calling build_concept_check.
>>
>> gcc/testsuite/ChangeLog:
>>
>>     PR c++/98346
>>     * g++.dg/cpp2a/concepts-placeholder3.C: New test.
>> ---
>>   gcc/cp/pt.c                                       |  2 ++
>>   .../g++.dg/cpp2a/concepts-placeholder3.C          | 15 +++++++++++++++
>>   2 files changed, 17 insertions(+)
>>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder3.C
>>
>> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
>> index beabcc4b027..111a694e0c5 100644
>> --- a/gcc/cp/pt.c
>> +++ b/gcc/cp/pt.c
>> @@ -29464,7 +29464,9 @@ do_auto_deduction (tree type, tree init, tree 
>> auto_node,
>>             cargs = targs;
>>       /* Rebuild the check using the deduced arguments.  */
>> +    ++processing_template_decl;
>>       check = build_concept_check (cdecl, cargs, tf_none);
>> +    --processing_template_decl;
> 
> This shouldn't be necessary; if processing_template_decl is 0, we should 
> have non-dependent args.
> 
> I think your patch only works for this testcase because the concept is 
> trivial and doesn't actually try to to do anything with the arguments.
> 
> Handling of PLACEHOLDER_TYPE_CONSTRAINTS is overly complex, partly 
> because the 'auto' is represented as an argument in its own constraints.
> 
> A constrained auto variable declaration has the same problem.

Appling the patch below turns up similar problems in a couple of 
existing testcases.