Remove flattening variables

This patch redesigns the flattener to simplify type family applications
directly instead of using flattening meta-variables and skolems. The key new
innovation is the CanEqLHS type and the new CEqCan constraint (Ct). A CanEqLHS
is either a type variable or exactly-saturated type family application; either
can now be rewritten using a CEqCan constraint in the inert set.

Because the flattener no longer reduces all type family applications to
variables, there was some performance degradation if a lengthy type family
application is now flattened over and over (not making progress). To
compensate, this patch contains some extra optimizations in the flattener,
leading to a number of performance improvements.

Close #18875.
Close #18910.

There are many extra parts of the compiler that had to be affected in writing
this patch:

* The family-application cache (formerly the flat-cache) sometimes stores
  coercions built from Given inerts. When these inerts get kicked out, we must
  kick out from the cache as well. (This was, I believe, true previously, but
  somehow never caused trouble.) Kicking out from the cache requires adding a
  filterTM function to TrieMap.

* This patch obviates the need to distinguish "blocking" coercion holes from
  non-blocking ones (which, previously, arose from CFunEqCans). There is thus
  some simplification around coercion holes.

* Extra commentary throughout parts of the code I read through, to preserve
  the knowledge I gained while working.

* A change in the pure unifier around unifying skolems with other types.
  Unifying a skolem now leads to SurelyApart, not MaybeApart, as documented
  in Note [Binding when looking up instances] in GHC.Core.InstEnv.

* Some more use of MCoercion where appropriate.

* Previously, class-instance lookup automatically noticed that e.g. C Int was
  a "unifier" to a target [W] C (F Bool), because the F Bool was flattened to
  a variable. Now, a little more care must be taken around checking for
  unifying instances.

* Previously, tcSplitTyConApp_maybe would split (Eq a => a). This is silly,
  because (=>) is not a tycon in Haskell. Fixed now, but there are some
  knock-on changes in e.g. TrieMap code and in the canonicaliser.

* New function anyFreeVarsOf{Type,Co} to check whether a free variable
  satisfies a certain predicate.

* Type synonyms now remember whether or not they are "forgetful"; a forgetful
  synonym drops at least one argument. This is useful when flattening; see
  flattenView.

* The pattern-match completeness checker invokes the solver. This invocation
  might need to look through newtypes when checking representational equality.
  Thus, the desugarer needs to keep track of the in-scope variables to know
  what newtype constructors are in scope. I bet this bug was around before but
  never noticed.

* Extra-constraints wildcards are no longer simplified before printing.
  See Note [Do not simplify ConstraintHoles] in GHC.Tc.Solver.

* Whether or not there are Given equalities has become slightly subtler.
  See the new HasGivenEqs datatype.

* Note [Type variable cycles in Givens] in GHC.Tc.Solver.Canonical
  explains a significant new wrinkle in the new approach.

* See Note [What might match later?] in GHC.Tc.Solver.Interact, which
  explains the fix to #18910.

* The inert_count field of InertCans wasn't actually used, so I removed
  it.

Though I (Richard) did the implementation, Simon PJ was very involved
in design and review.

This updates the Haddock submodule to avoid #18932 by adding
a type signature.

-------------------------
Metric Decrease:
    T12227
    T5030
    T9872a
    T9872b
    T9872c
Metric Increase:
    T9872d
-------------------------

1 files changed, 5 insertions, 5 deletions

diff --git a/testsuite/tests/typecheck/should_fail/ContextStack2.hs b/testsuite/tests/typecheck/should_fail/ContextStack2.hs
index 53634a5cd5..0e01ab6956 100644
--- a/testsuite/tests/typecheck/should_fail/ContextStack2.hs
+++ b/testsuite/tests/typecheck/should_fail/ContextStack2.hs
@@ -12,11 +12,11 @@ type instance TF (a,b) = (TF a, TF b)
 t :: (a ~ TF (a,Int)) => Int
 t = undefined
 
-{- a ~ TF (a,Int)  
+{- a ~ TF (a,Int)
      ~ (TF a, TF Int)
      ~ (TF (TF (a,Int)), TF Int)
      ~ (TF (TF a, TF Int), TF Int)
-     ~ ((TF (TF a), TF (TF Int)), TF Int) 
+     ~ ((TF (TF a), TF (TF Int)), TF Int)
 
 
       fsk ~ a
@@ -28,7 +28,7 @@ t = undefined
          a ~ (TF a, TF Int)
         (flatten rhs)
         a ~ (fsk1, TF Int)
-(wk)  TF a ~ fsk1   
+(wk)  TF a ~ fsk1
 
 --> (rewrite inert)
 
@@ -43,7 +43,7 @@ t = undefined
 *     TF (fsk1, fsk2) ~ fsk1
 (wk)  TF Tnt ~ fsk2
 
--->   
+-->
       fsk ~ (fsk1, TF Int)
       a   ~ (fsk1, TF Int)
 
@@ -51,7 +51,7 @@ t = undefined
         (flatten rhs)
         fsk1 ~ (fsk3, TF fsk2)
 
-   
+
 (wk)  TF Int ~ fsk2
       TF fsk1 ~ fsk3
 -}