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Consider
   Work item:   k ~ *
   Inert item:  (a::k) ~ Int

Then we must kick out the inert item!  We weren't doing that,
something I discovered when fixing Trac #7384.

Discussed with Dimitrios, and we wrote a long comment
Note [Delicate equality kick-out] to explain.

I think I accidentally introduced this bug a month ago when
refactoring. It's a bit non-obvious, but since Derived constraints
have no evidence, we mustn't be strict in it.  Now there's a big
comment to prevent this bug happening again.

This fixes Trac #7384.

This bug caused Trac #7386, because in the (rather tricky) "type
inference" (aka run time type reconstruction) done by the GHCi
debugger, we were failing to instantiate a data type family
correctly.  When that code was written we didn't *have* data
families.

I wrote Note [Constructor arg types] to explain the new scheme.

There's no need to have the uniq in the rule, but its presence can
cause spurious ABI changes.

There are no longer any GLOBAL_VARs in main/DriverMkDepend.hs.

We now do the initial dependency generation for the vanilla way
regardless of what way flags and hisuf/osuf flags are given. This
makes it easier to generate the right dependency info in the end.

These names end up in the ABI, and hence part of the ABI hash.
We don't want uniques in them so that we don't get spurious ABI
hash changes.

The wrapper functions can end up in interface files, and thus are
part of the ABI hash. But uniqs easily change for no good reason
when recompiling, which can lead to an ABI hash needlessly changing.

We don't want to narrow the argument size before making the foreign
call: Word8 still gets passed as a Word-sized argument

Without -XImpredicativeTypes, the typing rules say that a function
should be instantiated only at a *monotype*.  In implementation terms,
that means that a unification variable should not unify with a type
involving foralls.  But we were not enforcing that rule, and that
gave rise to some confusing error messages, such as
  Trac #7264, #6069

This patch adds the test for foralls.  There are consequences

 * I put the test in occurCheckExpand, since that is where we see if a
   type can unify with a given unification variable.  So
   occurCheckExpand has to get DynFlags, so it can test for
   -XImpredicativeTypes

 * We want this to work
      foo :: (forall a. a -> a) -> Int
      foo = error "foo"
   But that means instantiating error at a polytype!  But error is special
   already because you can instantiate it at an unboxed type like Int#.
   So I extended the specialness to allow type variables of openTypeKind
   to unify with polytypes, regardless of -XImpredicativeTypes.

   Conveniently, this works in TcUnify.matchExpectedFunTys, which generates
   unification variable for the function arguments, which can be polymorphic.

 * GHC has a special typing rule for ($) (see Note [Typing rule
   for ($)] in TcExpr).  It unifies the argument and result with a
   unification variable to exclude unboxed types -- but that means I
   now need a kind of unificatdion variable that *can* unify with a
   polytype.

   So for this sole case I added PolyTv to the data type TcType.MetaInfo.
   I suspect we'll find mor uses for this, and the changes are tiny,
   but it still feel a bit of a hack.  Well the special rule for ($)
   is a hack!

There were some consequential changes in error reporting (TcErrors).

With the new register allocation scheme, the LLVM back end must always use
liveness information.

We now have accurate global register liveness information attached to all Cmm
procedures and jumps. With this patch, the LLVM back end uses this information
to pass only the live floating point (F and D) registers on tail calls. This
makes the LLVM back end compatible with the new register allocation strategy.

Ideally the GHC LLVM calling convention would put all registers that are always
live first in the parameter sequence. Unfortunately the specification is written
so that on x86-64 SpLim (always live) is passed after the R registers. Therefore
we must always pass *something* in the R registers, so we pass the LLVM value
undef.

On x86-64 F and D registers are both drawn from SSE registers, so there is no
reason not to draw them from the same pool of available SSE registers. This
means that whereas previously a function could only receive two Double arguments
in registers even if it did not have any Float arguments, now it can receive up
to 6 arguments that are any mix of Float and Double in registers.

This patch breaks the LLVM back end. The next patch will fix this breakage.

All Cmm procedures now include the set of global registers that are live on
procedure entry, i.e., the global registers used to pass arguments to the
procedure. Only global registers that are use to pass arguments are included in
this list.

We would like to calculate register liveness for global registers as well as
local registers, so this patch generalizes the existing infrastructure to set
the stage.

Jumps now always have live register information attached, so drop Maybes.

Fixes this, when building unregisterised:

rts/dist/build/AutoApply.hc:87:1:
     error: ‘stg_ap_v_entry’ undeclared (first use in this function)

Occurrences in terms are uses, in patterns they are not.
In this way we get unused-constructor warnings from modules like this

   module M( f, g, T ) where
     data T = T1 | T2 Bool

     f x = T2 x

     g T1 = True
     g (T2 x) = x

Here a T1 value cannot be constructed, so we can warn. The use
in a pattern doesn't count.  See Note [Patterns are not uses]
in RnPat.

Interestingly this change exposed three module in GHC itself
that had unused constructors, which I duly removed:
  * ghc/Main.hs
  * compiler/ghci/ByteCodeAsm
  * compiler/nativeGen/PPC/RegInfo
Their changes are in this patch.

There was a long thread on cvs-ghc which concluded that although it's
the Right Thing to deprecate Rank2Types and PolymorphicComponents, in
favour of RankNTypes, it would cause lots of busy-work for library
authors and the pain isn't worth the gain.

So this patch removes the deprecation, and documents the synonym-ity.