Do eager instantation in terms

This patch implements eager instantiation, a small but critical change
to the type inference engine, #17173.  The main change is this:

  When inferring types, always return an instantiated type
  (for now, deeply instantiated; in future shallowly instantiated)

There is more discussion in
https://www.tweag.io/posts/2020-04-02-lazy-eager-instantiation.html

There is quite a bit of refactoring in this patch:

* The ir_inst field of GHC.Tc.Utils.TcType.InferResultk
  has entirely gone.  So tcInferInst and tcInferNoInst have collapsed
  into tcInfer.

* Type inference of applications, via tcInferApp and
  tcInferAppHead, are substantially refactored, preparing
  the way for Quick Look impredicativity.

* New pure function GHC.Tc.Gen.Expr.collectHsArgs and applyHsArgs
  are beatifully dual.  We can see the zipper!

* GHC.Tc.Gen.Expr.tcArgs is now much nicer; no longer needs to return
  a wrapper

* In HsExpr, HsTypeApp now contains the the actual type argument,
  and is used in desugaring, rather than putting it in a mysterious
  wrapper.

* I struggled a bit with good error reporting in
  Unify.matchActualFunTysPart. It's a little bit simpler than before,
  but still not great.

Some smaller things

* Rename tcPolyExpr --> tcCheckExpr
         tcMonoExpr --> tcLExpr
* tcPatSig moves from GHC.Tc.Gen.HsType to GHC.Tc.Gen.Pat

Metric Decrease:
    T9961

Reduction of 1.6% in comiler allocation on T9961, I think.

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/annotations/should_fail/annfail08.stderr b/testsuite/tests/annotations/should_fail/annfail08.stderr
index 47a54243fe..6fafaf919e 100644
--- a/testsuite/tests/annotations/should_fail/annfail08.stderr
+++ b/testsuite/tests/annotations/should_fail/annfail08.stderr
@@ -5,7 +5,7 @@ annfail08.hs:9:1: error:
         (maybe you haven't applied a function to enough arguments?)
     • In the annotation: {-# ANN f (id + 1) #-}
 
-annfail08.hs:9:12: error:
+annfail08.hs:9:15: error:
     • No instance for (Num (a0 -> a0)) arising from a use of ‘+’
         (maybe you haven't applied a function to enough arguments?)
     • In the annotation: {-# ANN f (id + 1) #-}