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, 42 insertions, 42 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/T4093b.stderr b/testsuite/tests/indexed-types/should_fail/T4093b.stderr
index 0950de8c66..92530d58bf 100644
--- a/testsuite/tests/indexed-types/should_fail/T4093b.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T4093b.stderr
@@ -1,42 +1,42 @@
-
-T4093b.hs:31:13: error:
-    • Could not deduce: e ~ C
-      from the context: (EitherCO e (A C O n) (A O O n) ~ A e O n,
-                         EitherCO x (A C C n) (A C O n) ~ A C x n)
-        bound by the type signature for:
-                   blockToNodeList :: (EitherCO e (A C O n) (A O O n) ~ A e O n,
-                                       EitherCO x (A C C n) (A C O n) ~ A C x n) =>
-                                      Block n e x -> A e x n
-        at T4093b.hs:(19,1)-(22,26)
-      ‘e’ is a rigid type variable bound by
-        the type signature for:
-          blockToNodeList :: forall (n :: * -> * -> *) e x.
-                             (EitherCO e (A C O n) (A O O n) ~ A e O n,
-                              EitherCO x (A C C n) (A C O n) ~ A C x n) =>
-                             Block n e x -> A e x n
-        at T4093b.hs:20:12
-      Expected type: EitherCO e (A C O n) (A O O n)
-        Actual type: (MaybeC C (n C O), MaybeC O (n O C))
-    • In the expression: (JustC n, NothingC)
-      In an equation for ‘f’: f n _ = (JustC n, NothingC)
-      In an equation for ‘blockToNodeList’:
-          blockToNodeList b
-            = foldBlockNodesF (f, l) b z
-            where
-                z ::
-                  EitherCO e (EitherCO e (A C O n) (A O O n)) (EitherCO e (A C O n) (A O O n))
-                z = undefined
-                f ::
-                  n C O
-                  -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)
-                f n _ = (JustC n, NothingC)
-                ....
-    • Relevant bindings include
-        f :: n C O
-             -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)
-          (bound at T4093b.hs:31:5)
-        l :: n O C
-             -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C C n) (A O C n)
-          (bound at T4093b.hs:34:5)
-        b :: Block n e x (bound at T4093b.hs:25:17)
-        blockToNodeList :: Block n e x -> A e x n (bound at T4093b.hs:25:1)
+

+T4093b.hs:31:13: error:

+    • Could not deduce: e ~ C

+      from the context: (EitherCO e (A C O n) (A O O n) ~ A e O n,

+                         EitherCO x (A C C n) (A C O n) ~ A C x n)

+        bound by the type signature for:

+                   blockToNodeList :: (EitherCO e (A C O n) (A O O n) ~ A e O n,

+                                       EitherCO x (A C C n) (A C O n) ~ A C x n) =>

+                                      Block n e x -> A e x n

+        at T4093b.hs:(19,1)-(22,26)

+      ‘e’ is a rigid type variable bound by

+        the type signature for:

+          blockToNodeList :: forall (n :: * -> * -> *) e x.

+                             (EitherCO e (A C O n) (A O O n) ~ A e O n,

+                              EitherCO x (A C C n) (A C O n) ~ A C x n) =>

+                             Block n e x -> A e x n

+        at T4093b.hs:(19,1)-(22,26)

+      Expected type: EitherCO e (A C O n) (A O O n)

+        Actual type: (MaybeC C (n C O), MaybeC O (n O C))

+    • In the expression: (JustC n, NothingC)

+      In an equation for ‘f’: f n _ = (JustC n, NothingC)

+      In an equation for ‘blockToNodeList’:

+          blockToNodeList b

+            = foldBlockNodesF (f, l) b z

+            where

+                z ::

+                  EitherCO e (EitherCO e (A C O n) (A O O n)) (EitherCO e (A C O n) (A O O n))

+                z = undefined

+                f ::

+                  n C O

+                  -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)

+                f n _ = (JustC n, NothingC)

+                ....

+    • Relevant bindings include

+        f :: n C O

+             -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)

+          (bound at T4093b.hs:31:5)

+        l :: n O C

+             -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C C n) (A O C n)

+          (bound at T4093b.hs:34:5)

+        b :: Block n e x (bound at T4093b.hs:25:17)

+        blockToNodeList :: Block n e x -> A e x n (bound at T4093b.hs:25:1)