[PATCH v6] c++: Fix constrained auto deduction in templ spec scopes [PR114915]

[Jason Merrill]

On 7/18/24 12:03 PM, Seyed Sajad Kahani wrote:
> When deducing auto for `adc_return_type`, `adc_variable_type`, and
> `adc_decomp_type` contexts (at the usage time), we try to resolve the outermost
> template arguments to be used for satisfaction. This is done by one of the
> following, depending on the scope:
> 
> 1. Checking the `DECL_TEMPLATE_INFO` of the current function scope and
> extracting DECL_TI_ARGS from it for function scope deductions (pt.cc:31236).
> 2. Checking the `DECL_TEMPLATE_INFO` of the declaration (alongside with other
> conditions) for non-function scope variable declaration deductions
> (decl.cc:8527).
> 
> Then, we do not retrieve the deeper layers of the template arguments; instead,
> we fill the missing levels with dummy levels (pt.cc:31260).
> 
> The problem (that is shown in PR114915) is that we do not consider the case
> where the deduction happens in a template specialization scope. In this case,
> the type is not dependent on the outermost template arguments (which are
> the specialization arguments). Yet, we still resolve the outermost template
> arguments, and then the number of layers in the template arguments exceeds the
> number of levels in the type. This causes the missing levels to be negative.
> This leads to the rejection of valid code and ICEs (like segfault) in the
> release mode. In the debug mode, it is possible to show as an assertion failure
> (when creating a tree_vec with a negative size).
> 
> This patch resolves PR114915 by replacing the logic that fills in the
> missing levels in do_auto_deduction in cp/pt.cc.
> The new approach now trims targs if the depth of targs is deeper than desired
> (this will only happen in specific contexts), and still fills targs with empty
> layers if it has fewer depths than expected.

I would prefer to set outer_targs correctly in the first place, where 
it's currently set a few lines above.  And to factor that out so other 
callers can use it as well instead of DECL_TI_ARGS.

It seems like we want something close to outer_template_args, but it 
doesn't currently handle function scope decls or full specializations.

Jason