Ignore unary constraint tuples during typechecking (#17511)

We deliberately avoid defining a magical `Unit%` class, for reasons
that I have expounded upon in the newly added
`Note [Ignore unary constraint tuples]` in `TcHsType`. However, a
sneaky user could try to insert `Unit%` into their program by way of
Template Haskell, leading to the interface-file error observed
in #17511. To avoid this, any time we encounter a unary constraint
tuple during typechecking, we drop the surrounding constraint tuple
application. This is safe to do since `Unit% a` and `a` would be
semantically equivalent (unlike other forms of unary tuples).

Fixes #17511.

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs
index de7ec7ec81..ba427efee7 100644
--- a/compiler/prelude/TysWiredIn.hs
+++ b/compiler/prelude/TysWiredIn.hs
@@ -722,9 +722,9 @@ created in Template Haskell or in, e.g., `deriving` code. There is
 nothing special about one-tuples in Core; in particular, they have no
 custom pretty-printing, just using `Unit`.
 
-NB (Feb 16): for /constraint/ one-tuples I have 'Unit%' but no class
-decl in GHC.Classes, so I think this part may not work properly. But
-it's unused I think.
+Note that there is *not* a unary constraint tuple, unlike for other forms of
+tuples. See [Ignore unary constraint tuples] in TcHsType for more
+details.
 
 See also Note [Flattening one-tuples] in MkCore and
 Note [Don't flatten tuples from HsSyn] in MkCore.