Better error reporting for inaccessible code

This patch fixes Trac #15558.  There turned out to be
two distinct problems

* In TcExpr.tc_poly_expr_nc we had

    tc_poly_expr_nc (L loc expr) res_ty
      = do { traceTc "tcPolyExprNC" (ppr res_ty)
           ; (wrap, expr')
               <- tcSkolemiseET GenSigCtxt res_ty $ \ res_ty ->
                  setSrcSpan loc $
                    -- NB: setSrcSpan *after* skolemising,
                    -- so we get better skolem locations
                  tcExpr expr res_ty

  Putting the setSrcSpan inside the tcSkolemise means that
  the location on the Implication constraint is the /call/
  to the function rather than the /argument/ to the call,
  and that is really quite wrong.

  I don't know what Richard's comment NB means -- I moved the
  setSrcSpan outside, and the "binding site" info in error
  messages actually improved.

  The reason I found this is that it affects the span reported
  for Trac #15558.

* In TcErrors.mkGivenErrorReporter we carefully munge the location
  for an insoluble Given constraint (Note [Inaccessible code]).
  But the 'implic' passed in wasn't necesarily the immediately-
  enclosing implication -- but for location-munging purposes
  it jolly well should be.

  Solution: use the innermost implication. This actually
  simplifies the code -- no need to pass an implication in to
  mkGivenErrorReporter.

7 files changed, 32 insertions, 8 deletions

diff --git a/testsuite/tests/dependent/should_fail/T14066d.stderr b/testsuite/tests/dependent/should_fail/T14066d.stderr
index 3f5eb9825a..8ece51029b 100644
--- a/testsuite/tests/dependent/should_fail/T14066d.stderr
+++ b/testsuite/tests/dependent/should_fail/T14066d.stderr
@@ -4,7 +4,7 @@ T14066d.hs:11:35: error:
       ‘b1’ is a rigid type variable bound by
         a type expected by the context:
           forall c b1 (a :: c). (Proxy a, Proxy c, b1)
-        at T14066d.hs:11:33-35
+        at T14066d.hs:11:35
       ‘b’ is a rigid type variable bound by
         the type signature for:
           f :: forall b. b -> (Proxy Maybe, ())
diff --git a/testsuite/tests/gadt/T15558.hs b/testsuite/tests/gadt/T15558.hs
new file mode 100644
index 0000000000..91dff3f956
--- /dev/null
+++ b/testsuite/tests/gadt/T15558.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE RankNTypes #-}
+module T15558 where
+
+data T a where
+  MkT :: T Int
+
+data Foo a = MkFoo ((a ~ Bool) => ())
+
+f :: T a -> Foo a
+f MkT = MkFoo ()
+
+-- g :: Foo Int
+-- g = f MkT
diff --git a/testsuite/tests/gadt/T15558.stderr b/testsuite/tests/gadt/T15558.stderr
new file mode 100644
index 0000000000..ee70f0373b
--- /dev/null
+++ b/testsuite/tests/gadt/T15558.stderr
@@ -0,0 +1,9 @@
+
+T15558.hs:11:15: warning: [-Winaccessible-code (in -Wdefault)]
+    • Couldn't match type ‘Int’ with ‘Bool’
+      Inaccessible code in
+        a type expected by the context:
+          (a ~ Bool) => ()
+    • In the first argument of ‘MkFoo’, namely ‘()’
+      In the expression: MkFoo ()
+      In an equation for ‘f’: f MkT = MkFoo ()
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 27210312c3..395cce8c12 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -118,3 +118,4 @@ test('T14320', normal, compile, [''])
 test('T14719', normal, compile_fail, ['-fdiagnostics-show-caret'])
 test('T14808', normal, compile, [''])
 test('T15009', normal, compile, [''])
+test('T15558', normal, compile, [''])
diff --git a/testsuite/tests/polykinds/T7594.stderr b/testsuite/tests/polykinds/T7594.stderr
index 1b4b9017f7..5632e97707 100644
--- a/testsuite/tests/polykinds/T7594.stderr
+++ b/testsuite/tests/polykinds/T7594.stderr
@@ -5,7 +5,7 @@ T7594.hs:37:12: error:
           inside the constraints: (:&:) c0 Real a
           bound by a type expected by the context:
                      forall a. (:&:) c0 Real a => a -> b
-          at T7594.hs:37:8-19
+          at T7594.hs:37:12-16
       ‘b’ is a rigid type variable bound by
         the inferred type of bar2 :: b
         at T7594.hs:37:1-19
diff --git a/testsuite/tests/typecheck/should_fail/tcfail068.stderr b/testsuite/tests/typecheck/should_fail/tcfail068.stderr
index 4318021213..299fc7b8a7 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail068.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail068.stderr
@@ -4,7 +4,7 @@ tcfail068.hs:14:9: error:
       ‘s1’ is a rigid type variable bound by
         a type expected by the context:
           forall s1. GHC.ST.ST s1 (IndTree s a)
-        at tcfail068.hs:(13,9)-(14,31)
+        at tcfail068.hs:(13,15)-(14,31)
       ‘s’ is a rigid type variable bound by
         the type signature for:
           itgen :: forall a s.
@@ -27,7 +27,7 @@ tcfail068.hs:19:9: error:
       ‘s1’ is a rigid type variable bound by
         a type expected by the context:
           forall s1. GHC.ST.ST s1 (IndTree s a)
-        at tcfail068.hs:(18,9)-(21,19)
+        at tcfail068.hs:(18,15)-(21,19)
       ‘s’ is a rigid type variable bound by
         the type signature for:
           itiap :: forall a s.
@@ -58,7 +58,7 @@ tcfail068.hs:24:36: error:
       ‘s1’ is a rigid type variable bound by
         a type expected by the context:
           forall s1. GHC.ST.ST s1 (IndTree s a)
-        at tcfail068.hs:24:29-46
+        at tcfail068.hs:24:35-46
       ‘s’ is a rigid type variable bound by
         the type signature for:
           itrap :: forall a s.
@@ -95,7 +95,7 @@ tcfail068.hs:36:46: error:
       ‘s1’ is a rigid type variable bound by
         a type expected by the context:
           forall s1. GHC.ST.ST s1 (c, IndTree s b)
-        at tcfail068.hs:36:40-63
+        at tcfail068.hs:36:45-63
       ‘s’ is a rigid type variable bound by
         the type signature for:
           itrapstate :: forall b a c s.
diff --git a/testsuite/tests/typecheck/should_fail/tcfail076.stderr b/testsuite/tests/typecheck/should_fail/tcfail076.stderr
index d4368a4cf5..52fcebb927 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail076.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail076.stderr
@@ -4,11 +4,11 @@ tcfail076.hs:18:82: error:
       ‘res1’ is a rigid type variable bound by
         a type expected by the context:
           forall res1. (b -> m res1) -> m res1
-        at tcfail076.hs:18:64-88
+        at tcfail076.hs:18:71-88
       ‘res’ is a rigid type variable bound by
         a type expected by the context:
           forall res. (a -> m res) -> m res
-        at tcfail076.hs:18:28-96
+        at tcfail076.hs:18:35-96
       Expected type: m res1
         Actual type: m res
     • In the expression: cont a