Improve typechecking of let-bindings

This major commit was initially triggered by #11339, but it spiraled
into a major review of the way in which type signatures for bindings
are handled, especially partial type signatures.  On the way I fixed a
number of other bugs, namely
   #12069
   #12033
   #11700
   #11339
   #11670

The main change is that I completely reorganised the way in which type
signatures in bindings are handled. The new story is in TcSigs
Note [Overview of type signatures].  Some specific:

* Changes in the data types for signatures in TcRnTypes:
  TcIdSigInfo and new TcIdSigInst

* New module TcSigs deals with typechecking type signatures
  and pragmas. It contains code mostly moved from TcBinds,
  which is already too big

* HsTypes: I swapped the nesting of HsWildCardBndrs
  and HsImplicitBndsrs, so that the wildcards are on the
  oustide not the insidde in a LHsSigWcType.  This is just
  a matter of convenient, nothing deep.

There are a host of other changes as knock-on effects, and
it all took FAR longer than I anticipated :-).  But it is
a significant improvement, I think.

Lots of error messages changed slightly, some just variants but
some modest improvements.

New tests

* typecheck/should_compile
    * SigTyVars: a scoped-tyvar test
    * ExPat, ExPatFail: existential pattern bindings
    * T12069
    * T11700
    * T11339

* partial-sigs/should_compile
    * T12033
    * T11339a
    * T11670

One thing to check:

* Small change to output from ghc-api/landmines.
  Need to check with Alan Zimmerman

1 files changed, 32 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T11339b.hs b/testsuite/tests/typecheck/should_compile/T11339b.hs
new file mode 100644
index 0000000000..5401364e77
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T11339b.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE NoMonomorphismRestriction, RankNTypes, ScopedTypeVariables #-}
+
+module T11339b where
+
+import Control.Applicative ( Const(Const, getConst) )
+import Data.Functor.Identity ( Identity(Identity) )
+
+type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t
+
+failing :: forall s t a b . Traversal s t a b -> Traversal s t a b -> Traversal s t a b
+failing left right afb s = case pins t of
+  [] -> right afb s
+  _  -> t afb
+  where
+    t :: Applicative f => (a -> f b) -> f t
+    -- Works because of NoMonomorphismRestriction
+    Bazaar { getBazaar = t } = left sell s
+
+    sell :: a -> Bazaar a b b
+    sell w   = Bazaar ($ w)
+
+    pins :: ((a -> Const [Identity a] b) -> Const [Identity a] t) -> [Identity a]
+    pins f   = getConst (f (\ra -> Const [Identity ra]))
+
+newtype Bazaar a b t = Bazaar { getBazaar :: (forall f. Applicative f => (a -> f b) -> f t) }
+
+instance Functor (Bazaar a b) where
+  fmap f (Bazaar k) = Bazaar (fmap f . k)
+
+instance Applicative (Bazaar a b) where
+  pure a = Bazaar $ \_ -> pure a
+  Bazaar mf <*> Bazaar ma = Bazaar $ \afb -> mf afb <*> ma afb