Turn "inaccessible code" error into a warning

With GADTs, it is possible to write programs such that the type
constraints make some code branches inaccessible.

Take, for example, the following program ::

    {-# LANGUAGE GADTs #-}

    data Foo a where
     Foo1 :: Foo Char
     Foo2 :: Foo Int

    data TyEquality a b where
            Refl :: TyEquality a a

    checkTEQ :: Foo t -> Foo u -> Maybe (TyEquality t u)
    checkTEQ x y = error "unimportant"

    step2 :: Bool
    step2 = case checkTEQ Foo1 Foo2 of
             Just Refl -> True -- Inaccessible code
             Nothing -> False

Clearly, the `Just Refl` case cannot ever be reached, because the `Foo1`
and `Foo2` constructors say `t ~ Char` and `u ~ Int`, while the `Refl`
constructor essentially mandates `t ~ u`, and thus `Char ~ Int`.

Previously, GHC would reject such programs entirely; however, in
practice this is too harsh. Accepting such code does little harm, since
attempting to use the "impossible" code will still produce errors down
the chain, while rejecting it means we cannot legally write or generate
such code at all.

Hence, we turn the error into a warning, and provide
`-Winaccessible-code` to control GHC's behavior upon encountering this
situation.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #11066

Differential Revision: https://phabricator.haskell.org/D4744

1 files changed, 10 insertions, 17 deletions

diff --git a/testsuite/tests/gadt/T3651.stderr b/testsuite/tests/gadt/T3651.stderr
index 14216eb149..62e3bf16d7 100644
--- a/testsuite/tests/gadt/T3651.stderr
+++ b/testsuite/tests/gadt/T3651.stderr
@@ -1,21 +1,14 @@
 
-T3651.hs:11:11: error:
-    • Couldn't match type ‘Bool’ with ‘()’
-      Inaccessible code in
-        a pattern with constructor: U :: Z (), in an equation for ‘unsafe1’
-    • In the pattern: U
+T3651.hs:11:15: error:
+    • Couldn't match type ‘()’ with ‘Bool’
+      Expected type: a
+        Actual type: ()
+    • In the expression: ()
       In an equation for ‘unsafe1’: unsafe1 B U = ()
 
-T3651.hs:14:11: error:
-    • Couldn't match type ‘Bool’ with ‘()’
-      Inaccessible code in
-        a pattern with constructor: U :: Z (), in an equation for ‘unsafe2’
-    • In the pattern: U
+T3651.hs:14:15: error:
+    • Couldn't match type ‘()’ with ‘Bool’
+      Expected type: a
+        Actual type: ()
+    • In the expression: ()
       In an equation for ‘unsafe2’: unsafe2 B U = ()
-
-T3651.hs:17:11: error:
-    • Couldn't match type ‘Bool’ with ‘()’
-      Inaccessible code in
-        a pattern with constructor: U :: Z (), in an equation for ‘unsafe3’
-    • In the pattern: U
-      In an equation for ‘unsafe3’: unsafe3 B U = True