Always display inferred variables using braceswip/display-inferred

We now always show "forall {a}. T" for inferred variables,
previously this was controlled by -fprint-explicit-foralls.

This implements part 1 of https://github.com/ghc-proposals/ghc-proposals/pull/179.

Part of GHC ticket #16320.

Furthermore, when printing a levity restriction error, we now display
the HsWrap of the expression. This lets users see the full elaboration with
-fprint-typechecker-elaboration (see also #17670)

1 files changed, 33 insertions, 0 deletions

diff --git a/testsuite/tests/codeGen/should_fail/T13233_elab.hs b/testsuite/tests/codeGen/should_fail/T13233_elab.hs
new file mode 100644
index 0000000000..87269769d9
--- /dev/null
+++ b/testsuite/tests/codeGen/should_fail/T13233_elab.hs
@@ -0,0 +1,33 @@
+-- Same as T13233, but we compile with -fprint-typechecker-elaboration.
+-- The error message should show an eta-expanded call to (#,#).
+-- See comments at https://gitlab.haskell.org/ghc/ghc/merge_requests/2540
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE MagicHash #-}
+module Bug where
+
+import GHC.Exts (TYPE, RuntimeRep, Weak#, State#, RealWorld, mkWeak# )
+
+class Foo (a :: TYPE rep) where
+  bar :: forall rep2 (b :: TYPE rep2). (a -> a -> b) -> a -> a -> b
+
+baz :: forall rep (a :: TYPE rep). Foo a => a -> a -> (# a, a #)
+baz = bar (#,#)
+
+obscure :: (forall (rep1 :: RuntimeRep) (rep2 :: RuntimeRep)
+                   (a :: TYPE rep1) (b :: TYPE rep2).
+                   a -> b -> (# a, b #)) -> ()
+obscure _ = ()
+
+quux :: ()
+quux = obscure (#,#)
+
+-- It used to be that primops has no binding. However, as described in
+-- Note [Primop wrappers] in PrimOp we now rewrite unsaturated primop
+-- applications to their wrapper, which allows safe use of levity polymorphism.
+primop :: forall (rep :: RuntimeRep) (a :: TYPE rep) b c.
+          a -> b -> (State# RealWorld -> (# State# RealWorld, c #))
+       -> State# RealWorld -> (# State# RealWorld, Weak# b #)
+primop = mkWeak#