Separate `LPat` from `Pat` on the type-level

Since the Trees That Grow effort started, we had `type LPat = Pat`.
This is so that `SrcLoc`s would only be annotated in GHC's AST, which is
the reason why all GHC passes use the extension constructor `XPat` to
attach source locations. See #15495 for the design discussion behind
that.

But now suddenly there are `XPat`s everywhere!
There are several functions which dont't cope with `XPat`s by either
crashing (`hsPatType`) or simply returning incorrect results
(`collectEvVarsPat`).

This issue was raised in #17330. I also came up with a rather clean and
type-safe solution to the problem: We define

```haskell
type family XRec p (f :: * -> *) = r | r -> p f
type instance XRec (GhcPass p) f = Located (f (GhcPass p))
type instance XRec TH          f =          f p
type LPat p = XRec p Pat
```

This is a rather modular embedding of the old "ping-pong" style, while
we only pay for the `Located` wrapper within GHC. No ping-ponging in
a potential Template Haskell AST, for example. Yet, we miss no case
where we should've handled a `SrcLoc`: `hsPatType` and
`collectEvVarsPat` are not callable at an `LPat`.

Also, this gets rid of one indirection in `Located` variants:
Previously, we'd have to go through `XPat` and `Located` to get from
`LPat` to the wrapped `Pat`. Now it's just `Located` again.

Thus we fix #17330.

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/GHC/Hs/Pat.hs-boot b/compiler/GHC/Hs/Pat.hs-boot
index fc5671c27a..b37bf187fd 100644
--- a/compiler/GHC/Hs/Pat.hs-boot
+++ b/compiler/GHC/Hs/Pat.hs-boot
@@ -10,10 +10,10 @@
 module GHC.Hs.Pat where
 
 import Outputable
-import GHC.Hs.Extension ( OutputableBndrId, GhcPass )
+import GHC.Hs.Extension ( OutputableBndrId, GhcPass, XRec )
 
 type role Pat nominal
 data Pat (i :: *)
-type LPat i = Pat i
+type LPat i = XRec i Pat
 
-instance (OutputableBndrId p) => Outputable (Pat (GhcPass p))
+instance OutputableBndrId p => Outputable (Pat (GhcPass p))