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.

3 files changed, 24 insertions, 0 deletions

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, [''])