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

3 files changed, 64 insertions, 64 deletions

diff --git a/testsuite/tests/patsyn/should_fail/T11010.stderr b/testsuite/tests/patsyn/should_fail/T11010.stderr
index 47492cde3a..1bd83c83b4 100644
--- a/testsuite/tests/patsyn/should_fail/T11010.stderr
+++ b/testsuite/tests/patsyn/should_fail/T11010.stderr
@@ -1,14 +1,14 @@
-
-T11010.hs:9:36: error:
-    • Couldn't match type ‘a1’ with ‘Int’
-      ‘a1’ is a rigid type variable bound by
-        a pattern with constructor:
-          Fun :: forall b a. String -> (a -> b) -> Expr a -> Expr b,
-        in a pattern synonym declaration
-        at T11010.hs:9:26
-      Expected type: a -> b
-        Actual type: a1 -> b
-    • In the declaration for pattern synonym ‘IntFun’
-    • Relevant bindings include
-        x :: Expr a1 (bound at T11010.hs:9:36)
-        f :: a1 -> b (bound at T11010.hs:9:34)
+

+T11010.hs:9:36: error:

+    • Couldn't match type ‘a1’ with ‘Int’

+      ‘a1’ is a rigid type variable bound by

+        a pattern with constructor:

+          Fun :: forall b a. String -> (a -> b) -> Expr a -> Expr b,

+        in a pattern synonym declaration

+        at T11010.hs:9:26-36

+      Expected type: a -> b

+        Actual type: a1 -> b

+    • In the declaration for pattern synonym ‘IntFun’

+    • Relevant bindings include

+        x :: Expr a1 (bound at T11010.hs:9:36)

+        f :: a1 -> b (bound at T11010.hs:9:34)

diff --git a/testsuite/tests/patsyn/should_fail/T11039.stderr b/testsuite/tests/patsyn/should_fail/T11039.stderr
index 9b749d1758..15a56e01ac 100644
--- a/testsuite/tests/patsyn/should_fail/T11039.stderr
+++ b/testsuite/tests/patsyn/should_fail/T11039.stderr
@@ -1,9 +1,9 @@
-
-T11039.hs:8:15: error:
-    • Couldn't match type ‘f’ with ‘A’
-      ‘f’ is a rigid type variable bound by
-        the signature for pattern synonym ‘Q’ at T11039.hs:7:14
-      Expected type: f a
-        Actual type: A a
-    • In the pattern: A a
-      In the declaration for pattern synonym ‘Q’
+

+T11039.hs:8:15: error:

+    • Couldn't match type ‘f’ with ‘A’

+      ‘f’ is a rigid type variable bound by

+        the signature for pattern synonym ‘Q’ at T11039.hs:7:14-38

+      Expected type: f a

+        Actual type: A a

+    • In the pattern: A a

+      In the declaration for pattern synonym ‘Q’

diff --git a/testsuite/tests/patsyn/should_fail/T11667.stderr b/testsuite/tests/patsyn/should_fail/T11667.stderr
index 44bf88ced9..0407d00d09 100644
--- a/testsuite/tests/patsyn/should_fail/T11667.stderr
+++ b/testsuite/tests/patsyn/should_fail/T11667.stderr
@@ -1,41 +1,41 @@
-
-T11667.hs:12:22: error:
-    • Could not deduce (Num a) arising from the literal ‘42’
-      from the context: Eq a
-        bound by the signature for pattern synonym ‘Pat1’
-        at T11667.hs:12:9-12
-      Possible fix:
-        add (Num a) to the "required" context of
-          the signature for pattern synonym ‘Pat1’
-    • In the pattern: 42
-      In the pattern: Just 42
-      In the declaration for pattern synonym ‘Pat1’
-
-T11667.hs:18:28: error:
-    • Couldn't match type ‘b’ with ‘Bool’
-        arising from the "provided" constraints claimed by
-          the signature of ‘Pat2’
-      ‘b’ is a rigid type variable bound by
-        the signature for pattern synonym ‘Pat2’ at T11667.hs:17:17
-    • In the declaration for pattern synonym ‘Pat2’
-    • Relevant bindings include y :: b (bound at T11667.hs:18:21)
-
-T11667.hs:24:24: error:
-    • No instance for (Show a)
-        arising from the "provided" constraints claimed by
-          the signature of ‘Pat3’
-      In other words, a successful match on the pattern
-        Just x
-      does not provide the constraint (Show a)
-    • In the declaration for pattern synonym ‘Pat3’
-
-T11667.hs:31:16: error:
-    • Could not deduce (Num a) arising from a use of ‘MkS’
-      from the context: (Eq a, Show a)
-        bound by the signature for pattern synonym ‘Pat4’
-        at T11667.hs:31:1-21
-      Possible fix:
-        add (Num a) to the "required" context of
-          the signature for pattern synonym ‘Pat4’
-    • In the expression: MkS 42
-      In an equation for ‘Pat4’: Pat4 = MkS 42
+

+T11667.hs:12:22: error:

+    • Could not deduce (Num a) arising from the literal ‘42’

+      from the context: Eq a

+        bound by the signature for pattern synonym ‘Pat1’

+        at T11667.hs:12:9-12

+      Possible fix:

+        add (Num a) to the "required" context of

+          the signature for pattern synonym ‘Pat1’

+    • In the pattern: 42

+      In the pattern: Just 42

+      In the declaration for pattern synonym ‘Pat1’

+

+T11667.hs:18:28: error:

+    • Couldn't match type ‘b’ with ‘Bool’

+        arising from the "provided" constraints claimed by

+          the signature of ‘Pat2’

+      ‘b’ is a rigid type variable bound by

+        the signature for pattern synonym ‘Pat2’ at T11667.hs:17:17-50

+    • In the declaration for pattern synonym ‘Pat2’

+    • Relevant bindings include y :: b (bound at T11667.hs:18:21)

+

+T11667.hs:24:24: error:

+    • No instance for (Show a)

+        arising from the "provided" constraints claimed by

+          the signature of ‘Pat3’

+      In other words, a successful match on the pattern

+        Just x

+      does not provide the constraint (Show a)

+    • In the declaration for pattern synonym ‘Pat3’

+

+T11667.hs:31:16: error:

+    • Could not deduce (Num a) arising from a use of ‘MkS’

+      from the context: (Eq a, Show a)

+        bound by the signature for pattern synonym ‘Pat4’

+        at T11667.hs:31:1-21

+      Possible fix:

+        add (Num a) to the "required" context of

+          the signature for pattern synonym ‘Pat4’

+    • In the expression: MkS 42

+      In an equation for ‘Pat4’: Pat4 = MkS 42