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, 32 deletions

diff --git a/testsuite/tests/arrows/should_fail/T5380.stderr b/testsuite/tests/arrows/should_fail/T5380.stderr
index 0e258a2c28..6c7caf789d 100644
--- a/testsuite/tests/arrows/should_fail/T5380.stderr
+++ b/testsuite/tests/arrows/should_fail/T5380.stderr
@@ -1,32 +1,32 @@
-
-T5380.hs:7:27: error:
-    • Couldn't match expected type ‘Bool’ with actual type ‘not_bool’
-      ‘not_bool’ is a rigid type variable bound by
-        the type signature for:
-          testB :: forall not_bool not_unit.
-                   not_bool -> (() -> ()) -> () -> not_unit
-        at T5380.hs:6:10
-    • In the expression: b
-      In the expression: proc () -> if b then f -< () else f -< ()
-      In an equation for ‘testB’:
-          testB b f = proc () -> if b then f -< () else f -< ()
-    • Relevant bindings include
-        b :: not_bool (bound at T5380.hs:7:7)
-        testB :: not_bool -> (() -> ()) -> () -> not_unit
-          (bound at T5380.hs:7:1)
-
-T5380.hs:7:34: error:
-    • Couldn't match type ‘not_unit’ with ‘()’
-      ‘not_unit’ is a rigid type variable bound by
-        the type signature for:
-          testB :: forall not_bool not_unit.
-                   not_bool -> (() -> ()) -> () -> not_unit
-        at T5380.hs:6:10
-      Expected type: () -> not_unit
-        Actual type: () -> ()
-    • In the expression: f
-      In the command: f -< ()
-      In the expression: proc () -> if b then f -< () else f -< ()
-    • Relevant bindings include
-        testB :: not_bool -> (() -> ()) -> () -> not_unit
-          (bound at T5380.hs:7:1)
+

+T5380.hs:7:27: error:

+    • Couldn't match expected type ‘Bool’ with actual type ‘not_bool’

+      ‘not_bool’ is a rigid type variable bound by

+        the type signature for:

+          testB :: forall not_bool not_unit.

+                   not_bool -> (() -> ()) -> () -> not_unit

+        at T5380.hs:6:1-49

+    • In the expression: b

+      In the expression: proc () -> if b then f -< () else f -< ()

+      In an equation for ‘testB’:

+          testB b f = proc () -> if b then f -< () else f -< ()

+    • Relevant bindings include

+        b :: not_bool (bound at T5380.hs:7:7)

+        testB :: not_bool -> (() -> ()) -> () -> not_unit

+          (bound at T5380.hs:7:1)

+

+T5380.hs:7:34: error:

+    • Couldn't match type ‘not_unit’ with ‘()’

+      ‘not_unit’ is a rigid type variable bound by

+        the type signature for:

+          testB :: forall not_bool not_unit.

+                   not_bool -> (() -> ()) -> () -> not_unit

+        at T5380.hs:6:1-49

+      Expected type: () -> not_unit

+        Actual type: () -> ()

+    • In the expression: f

+      In the command: f -< ()

+      In the expression: proc () -> if b then f -< () else f -< ()

+    • Relevant bindings include

+        testB :: not_bool -> (() -> ()) -> () -> not_unit

+          (bound at T5380.hs:7:1)