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, 197 insertions, 198 deletions

diff --git a/testsuite/tests/ghci/scripts/Defer02.stderr b/testsuite/tests/ghci/scripts/Defer02.stderr
index 7318eb632a..bcc6c5eff5 100644
--- a/testsuite/tests/ghci/scripts/Defer02.stderr
+++ b/testsuite/tests/ghci/scripts/Defer02.stderr
@@ -1,198 +1,197 @@
-
-../../typecheck/should_run/Defer01.hs:11:40: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match type ‘Char’ with ‘[Char]’
-      Expected type: String
-        Actual type: Char
-    • In the first argument of ‘putStr’, namely ‘','’
-      In the second argument of ‘(>>)’, namely ‘putStr ','’
-      In the expression: putStr "Hello World" >> putStr ','
-
-../../typecheck/should_run/Defer01.hs:14:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match expected type ‘Int’ with actual type ‘Char’
-    • In the expression: 'p'
-      In an equation for ‘a’: a = 'p'
-
-../../typecheck/should_run/Defer01.hs:18:7: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (Eq B) arising from a use of ‘==’
-    • In the expression: x == x
-      In an equation for ‘b’: b x = x == x
-
-../../typecheck/should_run/Defer01.hs:25:1: warning: [-Woverlapping-patterns (in -Wdefault)]
-    Pattern match has inaccessible right hand side
-    In an equation for ‘c’: c (C2 x) = ...
-
-../../typecheck/should_run/Defer01.hs:25:4: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match type ‘Int’ with ‘Bool’
-      Inaccessible code in
-        a pattern with constructor: C2 :: Bool -> C Bool,
-        in an equation for ‘c’
-    • In the pattern: C2 x
-      In an equation for ‘c’: c (C2 x) = True
-
-../../typecheck/should_run/Defer01.hs:28:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (Num (a -> a)) arising from the literal ‘1’
-        (maybe you haven't applied a function to enough arguments?)
-    • In the expression: 1
-      In an equation for ‘d’: d = 1
-
-../../typecheck/should_run/Defer01.hs:31:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’
-    • The function ‘e’ is applied to one argument,
-      but its type ‘Char’ has none
-      In the expression: e 'q'
-      In an equation for ‘f’: f = e 'q'
-    • Relevant bindings include
-        f :: t (bound at ../../typecheck/should_run/Defer01.hs:31:1)
-
-../../typecheck/should_run/Defer01.hs:34:8: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match expected type ‘Char’ with actual type ‘a’
-      ‘a’ is a rigid type variable bound by
-        the type signature for:
-          h :: forall a. a -> (Char, Char)
-        at ../../typecheck/should_run/Defer01.hs:33:6
-    • In the expression: x
-      In the expression: (x, 'c')
-      In an equation for ‘h’: h x = (x, 'c')
-    • Relevant bindings include
-        x :: a (bound at ../../typecheck/should_run/Defer01.hs:34:3)
-        h :: a -> (Char, Char)
-          (bound at ../../typecheck/should_run/Defer01.hs:34:1)
-
-../../typecheck/should_run/Defer01.hs:39:17: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match expected type ‘Bool’ with actual type ‘T a’
-    • In the first argument of ‘not’, namely ‘(K a)’
-      In the expression: (not (K a))
-      In the expression: seq (not (K a)) ()
-    • Relevant bindings include
-        a :: a (bound at ../../typecheck/should_run/Defer01.hs:39:3)
-        i :: a -> () (bound at ../../typecheck/should_run/Defer01.hs:39:1)
-
-../../typecheck/should_run/Defer01.hs:43:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (MyClass a1) arising from a use of ‘myOp’
-    • In the expression: myOp 23
-      In an equation for ‘j’: j = myOp 23
-
-../../typecheck/should_run/Defer01.hs:43:10: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Ambiguous type variable ‘a1’ arising from the literal ‘23’
-      prevents the constraint ‘(Num a1)’ from being solved.
-      Probable fix: use a type annotation to specify what ‘a1’ should be.
-      These potential instances exist:
-        instance Num Integer -- Defined in ‘GHC.Num’
-        instance Num Double -- Defined in ‘GHC.Float’
-        instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
-        (use -fprint-potential-instances to see them all)
-    • In the first argument of ‘myOp’, namely ‘23’
-      In the expression: myOp 23
-      In an equation for ‘j’: j = myOp 23
-
-../../typecheck/should_run/Defer01.hs:45:1: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    Couldn't match type ‘Int’ with ‘Bool’
-    Inaccessible code in
-      the type signature for:
-        k :: Int ~ Bool => Int -> Bool
-
-../../typecheck/should_run/Defer01.hs:45:6: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match type ‘Int’ with ‘Bool’
-      Inaccessible code in
-        the type signature for:
-          k :: Int ~ Bool => Int -> Bool
-    • In the ambiguity check for ‘k’
-      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
-      In the type signature:
-        k :: (Int ~ Bool) => Int -> Bool
-
-../../typecheck/should_run/Defer01.hs:46:1: warning: [-Woverlapping-patterns (in -Wdefault)]
-    Pattern match is redundant
-    In an equation for ‘k’: k x = ...
-
-../../typecheck/should_run/Defer01.hs:49:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Couldn't match expected type ‘IO a0’
-                  with actual type ‘Char -> IO ()’
-    • Probable cause: ‘putChar’ is applied to too few arguments
-      In the first argument of ‘(>>)’, namely ‘putChar’
-      In the expression: putChar >> putChar 'p'
-      In an equation for ‘l’: l = putChar >> putChar 'p'
-*** Exception: ../../typecheck/should_run/Defer01.hs:11:40: error:
-    • Couldn't match type ‘Char’ with ‘[Char]’
-      Expected type: String
-        Actual type: Char
-    • In the first argument of ‘putStr’, namely ‘','’
-      In the second argument of ‘(>>)’, namely ‘putStr ','’
-      In the expression: putStr "Hello World" >> putStr ','
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:14:5: error:
-    • Couldn't match expected type ‘Int’ with actual type ‘Char’
-    • In the expression: 'p'
-      In an equation for ‘a’: a = 'p'
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:18:7: error:
-    • No instance for (Eq B) arising from a use of ‘==’
-    • In the expression: x == x
-      In an equation for ‘b’: b x = x == x
-(deferred type error)
-
-<interactive>:7:11: error:
-    • Couldn't match type ‘Bool’ with ‘Int’
-      Expected type: C Int
-        Actual type: C Bool
-    • In the first argument of ‘c’, namely ‘(C2 True)’
-      In the first argument of ‘print’, namely ‘(c (C2 True))’
-      In the expression: print (c (C2 True))
-*** Exception: ../../typecheck/should_run/Defer01.hs:28:5: error:
-    • No instance for (Num (a -> a)) arising from the literal ‘1’
-        (maybe you haven't applied a function to enough arguments?)
-    • In the expression: 1
-      In an equation for ‘d’: d = 1
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:31:5: error:
-    • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’
-    • The function ‘e’ is applied to one argument,
-      but its type ‘Char’ has none
-      In the expression: e 'q'
-      In an equation for ‘f’: f = e 'q'
-    • Relevant bindings include
-        f :: t (bound at ../../typecheck/should_run/Defer01.hs:31:1)
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:34:8: error:
-    • Couldn't match expected type ‘Char’ with actual type ‘a’
-      ‘a’ is a rigid type variable bound by
-        the type signature for:
-          h :: forall a. a -> (Char, Char)
-        at ../../typecheck/should_run/Defer01.hs:33:6
-    • In the expression: x
-      In the expression: (x, 'c')
-      In an equation for ‘h’: h x = (x, 'c')
-    • Relevant bindings include
-        x :: a (bound at ../../typecheck/should_run/Defer01.hs:34:3)
-        h :: a -> (Char, Char)
-          (bound at ../../typecheck/should_run/Defer01.hs:34:1)
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:39:17: error:
-    • Couldn't match expected type ‘Bool’ with actual type ‘T a’
-    • In the first argument of ‘not’, namely ‘(K a)’
-      In the expression: (not (K a))
-      In the expression: seq (not (K a)) ()
-    • Relevant bindings include
-        a :: a (bound at ../../typecheck/should_run/Defer01.hs:39:3)
-        i :: a -> () (bound at ../../typecheck/should_run/Defer01.hs:39:1)
-(deferred type error)
-*** Exception: ../../typecheck/should_run/Defer01.hs:43:5: error:
-    • No instance for (MyClass a1) arising from a use of ‘myOp’
-    • In the expression: myOp 23
-      In an equation for ‘j’: j = myOp 23
-(deferred type error)
-
-<interactive>:13:8: error:
-    • Couldn't match type ‘Int’ with ‘Bool’ arising from a use of ‘k’
-    • In the first argument of ‘print’, namely ‘(k 2)’
-      In the expression: print (k 2)
-      In an equation for ‘it’: it = print (k 2)
-*** Exception: ../../typecheck/should_run/Defer01.hs:49:5: error:
-    • Couldn't match expected type ‘IO a0’
-                  with actual type ‘Char -> IO ()’
-    • Probable cause: ‘putChar’ is applied to too few arguments
-      In the first argument of ‘(>>)’, namely ‘putChar’
-      In the expression: putChar >> putChar 'p'
-      In an equation for ‘l’: l = putChar >> putChar 'p'
-(deferred type error)
+

+..\..\typecheck\should_run\Defer01.hs:11:40: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Couldn't match type ‘Char’ with ‘[Char]’

+      Expected type: String

+        Actual type: Char

+    • In the first argument of ‘putStr’, namely ‘','’

+      In the second argument of ‘(>>)’, namely ‘putStr ','’

+      In the expression: putStr "Hello World" >> putStr ','

+

+..\..\typecheck\should_run\Defer01.hs:14:5: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Couldn't match expected type ‘Int’ with actual type ‘Char’

+    • In the expression: 'p'

+      In an equation for ‘a’: a = 'p'

+

+..\..\typecheck\should_run\Defer01.hs:18:7: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • No instance for (Eq B) arising from a use of ‘==’

+    • In the expression: x == x

+      In an equation for ‘b’: b x = x == x

+

+..\..\typecheck\should_run\Defer01.hs:25:1: warning: [-Woverlapping-patterns (in -Wdefault)]

+    Pattern match has inaccessible right hand side

+    In an equation for ‘c’: c (C2 x) = ...

+

+..\..\typecheck\should_run\Defer01.hs:25:4: warning: [-Wdeferred-type-errors (in -Wdefault)]

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

+      Inaccessible code in

+        a pattern with constructor: C2 :: Bool -> C Bool,

+        in an equation for ‘c’

+    • In the pattern: C2 x

+      In an equation for ‘c’: c (C2 x) = True

+

+..\..\typecheck\should_run\Defer01.hs:28:5: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • No instance for (Num (a -> a)) arising from the literal ‘1’

+        (maybe you haven't applied a function to enough arguments?)

+    • In the expression: 1

+      In an equation for ‘d’: d = 1

+

+..\..\typecheck\should_run\Defer01.hs:31:5: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’

+    • The function ‘e’ is applied to one argument,

+      but its type ‘Char’ has none

+      In the expression: e 'q'

+      In an equation for ‘f’: f = e 'q'

+    • Relevant bindings include

+        f :: t (bound at ..\..\typecheck\should_run\Defer01.hs:31:1)

+

+..\..\typecheck\should_run\Defer01.hs:34:8: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Couldn't match expected type ‘Char’ with actual type ‘a’

+      ‘a’ is a rigid type variable bound by

+        the type signature for:

+          h :: forall a. a -> (Char, Char)

+        at ..\..\typecheck\should_run\Defer01.hs:33:1-21

+    • In the expression: x

+      In the expression: (x, 'c')

+      In an equation for ‘h’: h x = (x, 'c')

+    • Relevant bindings include

+        x :: a (bound at ..\..\typecheck\should_run\Defer01.hs:34:3)

+        h :: a -> (Char, Char)

+          (bound at ..\..\typecheck\should_run\Defer01.hs:34:1)

+

+..\..\typecheck\should_run\Defer01.hs:39:17: warning: [-Wdeferred-type-errors (in -Wdefault)]

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

+    • In the first argument of ‘not’, namely ‘(K a)’

+      In the expression: (not (K a))

+      In the expression: seq (not (K a)) ()

+    • Relevant bindings include

+        a :: a (bound at ..\..\typecheck\should_run\Defer01.hs:39:3)

+        i :: a -> () (bound at ..\..\typecheck\should_run\Defer01.hs:39:1)

+

+..\..\typecheck\should_run\Defer01.hs:43:5: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • No instance for (MyClass a1) arising from a use of ‘myOp’

+    • In the expression: myOp 23

+      In an equation for ‘j’: j = myOp 23

+

+..\..\typecheck\should_run\Defer01.hs:43:10: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Ambiguous type variable ‘a1’ arising from the literal ‘23’

+      prevents the constraint ‘(Num a1)’ from being solved.

+      Probable fix: use a type annotation to specify what ‘a1’ should be.

+      These potential instances exist:

+        instance Num Integer -- Defined in ‘GHC.Num’

+        instance Num Double -- Defined in ‘GHC.Float’

+        instance Num Float -- Defined in ‘GHC.Float’

+        ...plus two others

+        (use -fprint-potential-instances to see them all)

+    • In the first argument of ‘myOp’, namely ‘23’

+      In the expression: myOp 23

+      In an equation for ‘j’: j = myOp 23

+

+..\..\typecheck\should_run\Defer01.hs:45:1: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    Couldn't match type ‘Int’ with ‘Bool’

+    Inaccessible code in

+      the type signature for:

+        k :: Int ~ Bool => Int -> Bool

+

+..\..\typecheck\should_run\Defer01.hs:45:6: warning: [-Wdeferred-type-errors (in -Wdefault)]

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

+      Inaccessible code in

+        the type signature for:

+          k :: Int ~ Bool => Int -> Bool

+    • In the ambiguity check for ‘k’

+      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes

+      In the type signature: k :: (Int ~ Bool) => Int -> Bool

+

+..\..\typecheck\should_run\Defer01.hs:46:1: warning: [-Woverlapping-patterns (in -Wdefault)]

+    Pattern match is redundant

+    In an equation for ‘k’: k x = ...

+

+..\..\typecheck\should_run\Defer01.hs:49:5: warning: [-Wdeferred-type-errors (in -Wdefault)]

+    • Couldn't match expected type ‘IO a0’

+                  with actual type ‘Char -> IO ()’

+    • Probable cause: ‘putChar’ is applied to too few arguments

+      In the first argument of ‘(>>)’, namely ‘putChar’

+      In the expression: putChar >> putChar 'p'

+      In an equation for ‘l’: l = putChar >> putChar 'p'

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:11:40: error:

+    • Couldn't match type ‘Char’ with ‘[Char]’

+      Expected type: String

+        Actual type: Char

+    • In the first argument of ‘putStr’, namely ‘','’

+      In the second argument of ‘(>>)’, namely ‘putStr ','’

+      In the expression: putStr "Hello World" >> putStr ','

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:14:5: error:

+    • Couldn't match expected type ‘Int’ with actual type ‘Char’

+    • In the expression: 'p'

+      In an equation for ‘a’: a = 'p'

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:18:7: error:

+    • No instance for (Eq B) arising from a use of ‘==’

+    • In the expression: x == x

+      In an equation for ‘b’: b x = x == x

+(deferred type error)

+

+<interactive>:7:11: error:

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

+      Expected type: C Int

+        Actual type: C Bool

+    • In the first argument of ‘c’, namely ‘(C2 True)’

+      In the first argument of ‘print’, namely ‘(c (C2 True))’

+      In the expression: print (c (C2 True))

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:28:5: error:

+    • No instance for (Num (a -> a)) arising from the literal ‘1’

+        (maybe you haven't applied a function to enough arguments?)

+    • In the expression: 1

+      In an equation for ‘d’: d = 1

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:31:5: error:

+    • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’

+    • The function ‘e’ is applied to one argument,

+      but its type ‘Char’ has none

+      In the expression: e 'q'

+      In an equation for ‘f’: f = e 'q'

+    • Relevant bindings include

+        f :: t (bound at ..\..\typecheck\should_run\Defer01.hs:31:1)

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:34:8: error:

+    • Couldn't match expected type ‘Char’ with actual type ‘a’

+      ‘a’ is a rigid type variable bound by

+        the type signature for:

+          h :: forall a. a -> (Char, Char)

+        at ..\..\typecheck\should_run\Defer01.hs:33:1-21

+    • In the expression: x

+      In the expression: (x, 'c')

+      In an equation for ‘h’: h x = (x, 'c')

+    • Relevant bindings include

+        x :: a (bound at ..\..\typecheck\should_run\Defer01.hs:34:3)

+        h :: a -> (Char, Char)

+          (bound at ..\..\typecheck\should_run\Defer01.hs:34:1)

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:39:17: error:

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

+    • In the first argument of ‘not’, namely ‘(K a)’

+      In the expression: (not (K a))

+      In the expression: seq (not (K a)) ()

+    • Relevant bindings include

+        a :: a (bound at ..\..\typecheck\should_run\Defer01.hs:39:3)

+        i :: a -> () (bound at ..\..\typecheck\should_run\Defer01.hs:39:1)

+(deferred type error)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:43:5: error:

+    • No instance for (MyClass a1) arising from a use of ‘myOp’

+    • In the expression: myOp 23

+      In an equation for ‘j’: j = myOp 23

+(deferred type error)

+

+<interactive>:13:8: error:

+    • Couldn't match type ‘Int’ with ‘Bool’ arising from a use of ‘k’

+    • In the first argument of ‘print’, namely ‘(k 2)’

+      In the expression: print (k 2)

+      In an equation for ‘it’: it = print (k 2)

+*** Exception: ..\..\typecheck\should_run\Defer01.hs:49:5: error:

+    • Couldn't match expected type ‘IO a0’

+                  with actual type ‘Char -> IO ()’

+    • Probable cause: ‘putChar’ is applied to too few arguments

+      In the first argument of ‘(>>)’, namely ‘putChar’

+      In the expression: putChar >> putChar 'p'

+      In an equation for ‘l’: l = putChar >> putChar 'p'

+(deferred type error)