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[lambda] Latest experimental polymorphic lambda patches

Attached are my latest experimental polymorphic lambda patches against the latest lambda branch.  Template parameters
implied by auto in the lambda's call parameter list no longer lose qualifiers.  The following examples produce
equivalent functions:

   1.   [] (auto x, auto& y, auto const& z) { return x + y + z; }
   2.   [] <typename X, typename Y, typename Z> (X x, Y& y, Z const& z) { return x + y + z; }
   3.   [] <typename Y> (auto x, Y& y, auto const& z) { return x + y + z; }

Note that using an explicit template parameter is only really useful if you wish to either a) ensure consistency among
a number of call parameters, b) use a non-type template parameter or c) specify a call parameter type constraint (or
other complex parameter type).

I have flattened the latest auto-template-inference changes with the previous one to remove complexity due to
re-arrangement.  In particular it no longer abuses decl.c -- those changes have currently moved into parser.c but it
is pretty clear that they have more in common with pt.c owing to a need to pull in some local static functions from
that file.  I intend to split or move parts (or most) into pt.c on my next attempt (if I get time to make one!)

There are currently many issues:
  - efficiency of template parameter list growing.
  - implicit return type deduction doesn't work from
    inside a template context if the lambda's return
    expression involves a dependent type.  Specifying
    decltype explicitly in these contexts is a
  - dependent inferred return type needs to be
    deferred and decayed.  This may go some way (all
    the way?) to solving the previous point.
  - location of implementation.
  - only a few use-cases have been considered.



Subject: [PATCH] First pass polymorphic lambda support.

This commit adds experimental support for an optional template-parameter-list in angle-brackets at the start of a
Subject: [PATCH] Restored decltype in lambda return type deduction for when expr is dependent.
Subject: [PATCH] Second version of typename inference from auto parameters in lambda call operator.

Still quite hacky -- though better than previous.  No longer loses
qualifiers on replaced auto parameters so is functionally closer to
what's really needed.

 - This is just a behavioural proof to find out how things work.
 - Need to shuffle some stuff into pt.c and do away with code dup.
 - Not sure how to free tree_vec's and tidy up the counts and sizes
   (looks like they're only intended to grow.)
 - Added `type_decays_to (non_reference (finish_decltype_type' as
   suggested by Jason.  Currently doesn't remove cv-quals from
   non-class types though.  Need to treat implicit return type
   differently for dependent types -- should defer and mark that it
   needs to be computed later.

Attachment: 0001-First-pass-polymorphic-lambda-support.patch
Description: Binary data

Attachment: 0002-Restored-decltype-in-lambda-return-type-deduction-fo.patch
Description: Binary data

Attachment: 0003-Second-version-of-typename-inference-from-auto-param.patch
Description: Binary data

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