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, 37 insertions, 37 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/T3330a.stderr b/testsuite/tests/indexed-types/should_fail/T3330a.stderr
index 0950875229..ffda424227 100644
--- a/testsuite/tests/indexed-types/should_fail/T3330a.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T3330a.stderr
@@ -1,37 +1,37 @@
-
-T3330a.hs:19:34: error:
-    • Couldn't match type ‘ix’
-                     with ‘r ix1 -> Writer [AnyF s] (r'0 ix1)’
-      ‘ix’ is a rigid type variable bound by
-        the type signature for:
-          children :: forall (s :: * -> *) ix (r :: * -> *).
-                      s ix -> PF s r ix -> [AnyF s]
-        at T3330a.hs:18:13
-      Expected type: (s0 ix0 -> ix1)
-                     -> r ix1 -> Writer [AnyF s] (r'0 ix1)
-        Actual type: s ix
-    • In the first argument of ‘hmapM’, namely ‘p’
-      In the first argument of ‘execWriter’, namely ‘(hmapM p collect x)’
-      In the expression: execWriter (hmapM p collect x)
-    • Relevant bindings include
-        x :: PF s r ix (bound at T3330a.hs:19:12)
-        p :: s ix (bound at T3330a.hs:19:10)
-        children :: s ix -> PF s r ix -> [AnyF s] (bound at T3330a.hs:19:1)
-
-T3330a.hs:19:44: error:
-    • Couldn't match type ‘ix’
-                     with ‘r0 ix0 -> Writer [AnyF s0] (r0 ix0)’
-      ‘ix’ is a rigid type variable bound by
-        the type signature for:
-          children :: forall (s :: * -> *) ix (r :: * -> *).
-                      s ix -> PF s r ix -> [AnyF s]
-        at T3330a.hs:18:13
-      Expected type: PF s r (r0 ix0 -> Writer [AnyF s0] (r0 ix0))
-        Actual type: PF s r ix
-    • In the third argument of ‘hmapM’, namely ‘x’
-      In the first argument of ‘execWriter’, namely ‘(hmapM p collect x)’
-      In the expression: execWriter (hmapM p collect x)
-    • Relevant bindings include
-        x :: PF s r ix (bound at T3330a.hs:19:12)
-        p :: s ix (bound at T3330a.hs:19:10)
-        children :: s ix -> PF s r ix -> [AnyF s] (bound at T3330a.hs:19:1)
+

+T3330a.hs:19:34: error:

+    • Couldn't match type ‘ix’

+                     with ‘r ix1 -> Writer [AnyF s] (r'0 ix1)’

+      ‘ix’ is a rigid type variable bound by

+        the type signature for:

+          children :: forall (s :: * -> *) ix (r :: * -> *).

+                      s ix -> PF s r ix -> [AnyF s]

+        at T3330a.hs:18:1-43

+      Expected type: (s0 ix0 -> ix1)

+                     -> r ix1 -> Writer [AnyF s] (r'0 ix1)

+        Actual type: s ix

+    • In the first argument of ‘hmapM’, namely ‘p’

+      In the first argument of ‘execWriter’, namely ‘(hmapM p collect x)’

+      In the expression: execWriter (hmapM p collect x)

+    • Relevant bindings include

+        x :: PF s r ix (bound at T3330a.hs:19:12)

+        p :: s ix (bound at T3330a.hs:19:10)

+        children :: s ix -> PF s r ix -> [AnyF s] (bound at T3330a.hs:19:1)

+

+T3330a.hs:19:44: error:

+    • Couldn't match type ‘ix’

+                     with ‘r0 ix0 -> Writer [AnyF s0] (r0 ix0)’

+      ‘ix’ is a rigid type variable bound by

+        the type signature for:

+          children :: forall (s :: * -> *) ix (r :: * -> *).

+                      s ix -> PF s r ix -> [AnyF s]

+        at T3330a.hs:18:1-43

+      Expected type: PF s r (r0 ix0 -> Writer [AnyF s0] (r0 ix0))

+        Actual type: PF s r ix

+    • In the third argument of ‘hmapM’, namely ‘x’

+      In the first argument of ‘execWriter’, namely ‘(hmapM p collect x)’

+      In the expression: execWriter (hmapM p collect x)

+    • Relevant bindings include

+        x :: PF s r ix (bound at T3330a.hs:19:12)

+        p :: s ix (bound at T3330a.hs:19:10)

+        children :: s ix -> PF s r ix -> [AnyF s] (bound at T3330a.hs:19:1)