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, 19 insertions, 19 deletions

diff --git a/testsuite/tests/typecheck/should_fail/tcfail201.stderr b/testsuite/tests/typecheck/should_fail/tcfail201.stderr
index b142cb18bd..9df11cafff 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail201.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail201.stderr
@@ -1,19 +1,19 @@
-
-tcfail201.hs:17:56: error:
-    • Couldn't match type ‘a’ with ‘HsDoc t0’
-      ‘a’ is a rigid type variable bound by
-        the type signature for:
-          gfoldl' :: forall (c :: * -> *) a.
-                     (forall a1 b. c (a1 -> b) -> a1 -> c b)
-                     -> (forall g. g -> c g) -> a -> c a
-        at tcfail201.hs:15:12
-      Expected type: c a
-        Actual type: c (HsDoc t0)
-    • In the expression: z DocEmpty
-      In a case alternative: DocEmpty -> z DocEmpty
-      In the expression: case hsDoc of { DocEmpty -> z DocEmpty }
-    • Relevant bindings include
-        hsDoc :: a (bound at tcfail201.hs:16:13)
-        gfoldl' :: (forall a1 b. c (a1 -> b) -> a1 -> c b)
-                   -> (forall g. g -> c g) -> a -> c a
-          (bound at tcfail201.hs:16:1)
+

+tcfail201.hs:17:56: error:

+    • Couldn't match type ‘a’ with ‘HsDoc t0’

+      ‘a’ is a rigid type variable bound by

+        the type signature for:

+          gfoldl' :: forall (c :: * -> *) a.

+                     (forall a1 b. c (a1 -> b) -> a1 -> c b)

+                     -> (forall g. g -> c g) -> a -> c a

+        at tcfail201.hs:15:1-85

+      Expected type: c a

+        Actual type: c (HsDoc t0)

+    • In the expression: z DocEmpty

+      In a case alternative: DocEmpty -> z DocEmpty

+      In the expression: case hsDoc of { DocEmpty -> z DocEmpty }

+    • Relevant bindings include

+        hsDoc :: a (bound at tcfail201.hs:16:13)

+        gfoldl' :: (forall a1 b. c (a1 -> b) -> a1 -> c b)

+                   -> (forall g. g -> c g) -> a -> c a

+          (bound at tcfail201.hs:16:1)