Be less picky about reporing inaccessible code

Triggered by the discussion on Trac #12466, this patch
makes GHC less aggressive about reporting an error when
there are insoluble Givens.

Being so agressive was making some libraries fail to
compile, and is arguably wrong in at least some cases.
See the discussion on the ticket.

Several test now pass when they failed before; see
the files-modified list for this patch.

3 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T12466.hs b/testsuite/tests/typecheck/should_compile/T12466.hs
new file mode 100644
index 0000000000..7940d99a59
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T12466.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TypeFamilies #-}
+module T12466 where
+
+class Foo a where
+  foo :: (a ~ Int => Int) -> a -> a
+  foo _ a2 = a2
+
+instance Foo Char
diff --git a/testsuite/tests/typecheck/should_compile/T12466a.hs b/testsuite/tests/typecheck/should_compile/T12466a.hs
new file mode 100644
index 0000000000..d0749e6b0a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T12466a.hs
@@ -0,0 +1,26 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+module T12466a where
+
+import GHC.TypeLits (Nat)
+import Unsafe.Coerce (unsafeCoerce)
+
+data Dict a where
+  Dict :: a => Dict a
+
+newtype a :- b = Sub (a => Dict b)
+
+axiom :: forall a b. Dict (a ~ b)
+axiom = unsafeCoerce (Dict :: Dict (a ~ a))
+
+type Divides n m = n ~ Gcd n m
+type family Gcd :: Nat -> Nat -> Nat where
+
+dividesGcd :: forall a b c. (Divides a b, Divides a c) :- Divides a (Gcd b c)
+dividesGcd = Sub axiom
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index b9f452d15c..40d986a617 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -539,3 +539,5 @@ test('T12381', normal, compile, [''])
 test('T12082', normal, compile, [''])
 test('T10635', normal, compile, [''])
 test('T12170b', normal, compile, [''])
+test('T12466', normal, compile, [''])
+test('T12466a', normal, compile, [''])