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, 21 insertions, 25 deletions

diff --git a/testsuite/tests/partial-sigs/should_fail/TidyClash2.stderr b/testsuite/tests/partial-sigs/should_fail/TidyClash2.stderr
index f59ab4d6c9..00c3874a4b 100644
--- a/testsuite/tests/partial-sigs/should_fail/TidyClash2.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/TidyClash2.stderr
@@ -1,57 +1,53 @@
 

 TidyClash2.hs:13:20: error:

-    • Found type wildcard ‘_’ standing for ‘t2’

-      Where: ‘t2’ is a rigid type variable bound by

-               the inferred type of barry :: t2 -> t1 -> t at TidyClash2.hs:14:1

+    • Found type wildcard ‘_’ standing for ‘w1’

+      Where: ‘w1’ is a rigid type variable bound by

+               the inferred type of barry :: w1 -> w -> t at TidyClash2.hs:14:1-40

       To use the inferred type, enable PartialTypeSignatures

     • In the type signature: barry :: forall t. _ -> _ -> t

-    • Relevant bindings include

-        barry :: t2 -> t1 -> t (bound at TidyClash2.hs:14:1)

 

 TidyClash2.hs:13:25: error:

-    • Found type wildcard ‘_’ standing for ‘t1’

-      Where: ‘t1’ is a rigid type variable bound by

-               the inferred type of barry :: t2 -> t1 -> t at TidyClash2.hs:14:1

+    • Found type wildcard ‘_’ standing for ‘w’

+      Where: ‘w’ is a rigid type variable bound by

+               the inferred type of barry :: w1 -> w -> t at TidyClash2.hs:14:1-40

       To use the inferred type, enable PartialTypeSignatures

     • In the type signature: barry :: forall t. _ -> _ -> t

-    • Relevant bindings include

-        barry :: t2 -> t1 -> t (bound at TidyClash2.hs:14:1)

 

 TidyClash2.hs:14:13: error:

-    • Found type wildcard ‘_’ standing for ‘t2’

-      Where: ‘t2’ is a rigid type variable bound by

-               the inferred type of barry :: t2 -> t1 -> t at TidyClash2.hs:14:1

+    • Found type wildcard ‘_’ standing for ‘w1’

+      Where: ‘w1’ is a rigid type variable bound by

+               the inferred type of barry :: w1 -> w -> t at TidyClash2.hs:14:1-40

       To use the inferred type, enable PartialTypeSignatures

     • In a pattern type signature: _

       In the pattern: x :: _

       In an equation for ‘barry’:

           barry (x :: _) (y :: _) = undefined :: _

     • Relevant bindings include

-        barry :: t2 -> t1 -> t (bound at TidyClash2.hs:14:1)

+        barry :: w1 -> w -> t (bound at TidyClash2.hs:14:1)

 

 TidyClash2.hs:14:22: error:

-    • Found type wildcard ‘_’ standing for ‘t1’

-      Where: ‘t1’ is a rigid type variable bound by

-               the inferred type of barry :: t2 -> t1 -> t at TidyClash2.hs:14:1

+    • Found type wildcard ‘_’ standing for ‘w’

+      Where: ‘w’ is a rigid type variable bound by

+               the inferred type of barry :: w1 -> w -> t at TidyClash2.hs:14:1-40

       To use the inferred type, enable PartialTypeSignatures

     • In a pattern type signature: _

       In the pattern: y :: _

       In an equation for ‘barry’:

           barry (x :: _) (y :: _) = undefined :: _

     • Relevant bindings include

-        x :: t2 (bound at TidyClash2.hs:14:8)

-        barry :: t2 -> t1 -> t (bound at TidyClash2.hs:14:1)

+        x :: w1 (bound at TidyClash2.hs:14:8)

+        barry :: w1 -> w -> t (bound at TidyClash2.hs:14:1)

 

 TidyClash2.hs:14:40: error:

-    • Found type wildcard ‘_’ standing for ‘t3’

-      Where: ‘t3’ is a rigid type variable bound by

-               the inferred type of <expression> :: t3 at TidyClash2.hs:14:27

+    • Found type wildcard ‘_’ standing for ‘w2’

+      Where: ‘w2’ is a rigid type variable bound by

+               the inferred type of <expression> :: w2 at TidyClash2.hs:14:40

       To use the inferred type, enable PartialTypeSignatures

     • In an expression type signature: _

       In the expression: undefined :: _

       In an equation for ‘barry’:

           barry (x :: _) (y :: _) = undefined :: _

     • Relevant bindings include

-        y :: t1 (bound at TidyClash2.hs:14:17)

-        x :: t2 (bound at TidyClash2.hs:14:8)

-        barry :: t2 -> t1 -> t (bound at TidyClash2.hs:14:1)

+        y :: w (bound at TidyClash2.hs:14:17)

+        x :: w1 (bound at TidyClash2.hs:14:8)

+        barry :: w1 -> w -> t (bound at TidyClash2.hs:14:1)