Draft: Adapt to (but don't fully support) Or-patterns

TODO RGS: Explain why, cite issue

CHANGES.md

@@ -3,6 +3,7 @@
 
 Version 1.18 [????.??.??]
 -------------------------
+* Support GHC 9.12.
 * The `DLamE` and `DCaseE` data constructors (as well as the related
   `mkDLamEFromDPats` function) are now deprecated in favor of the new
   `DLamCasesE` data constructor. `DLamE`, `DCaseE`, and `mkDLamEFromDPats` will

Language/Haskell/TH/Desugar/Core.hs

@@ -686,6 +686,10 @@ dsPat (InvisP ty) = DInvisP <$> dsType ty
 #endif
 dsPat (ViewP _ _) =
   fail "View patterns are not supported in th-desugar. Use pattern guards instead."
+#if __GLASGOW_HASKELL__ >= 911
+dsPat (OrP _) =
+  fail "Or-patterns are not supported in th-desugar. Use pattern guards instead."
+#endif
 
 -- | Convert a 'DPat' to a 'DExp'. Fails on 'DWildP' and 'DInvisP'.
 dPatToDExp :: DPat -> DExp

Language/Haskell/TH/Desugar/Util.hs

@@ -1113,6 +1113,9 @@ extractBoundNamesPat (UnboxedSumP pat _ _) = extractBoundNamesPat pat
 extractBoundNamesPat (TypeP _)             = OS.empty
 extractBoundNamesPat (InvisP _)            = OS.empty
 #endif
+#if __GLASGOW_HASKELL__ >= 911
+extractBoundNamesPat (OrP pats)            = foldMap extractBoundNamesPat pats
+#endif
 
 ----------------------------------------
 -- General utility

README.md

@@ -363,10 +363,12 @@ potentially recognize this and perform a more sophisticated analysis to detect
 and remove such matches when desugaring guards, but it currently doesn't do
 such an analysis.
 
-## No support for view patterns
+## No support for view patterns or Or-patterns
+
+`th-desugar` does not support desugaring view patterns or Or-patterns. An
+alternative to using view patterns or Or-patterns in the patterns of a function
+is to use pattern guards.
 
-`th-desugar` does not support desugaring view patterns. An alternative to using
-view patterns in the patterns of a function is to use pattern guards.
 Currently, there is not a viable workaround for using view patterns in pattern
 synonym definitions—see [this `th-desugar`
 issue](https://github.com/goldfirere/th-desugar/issues/174).