Make note of #340 in the README

Partial pattern matches in `do`-notation are difficult to single
because they desugar down to `case` expressions with overlapping
patterns. This is somewhat non-obvious, so make a note of this in
the `README`.

Bumps the `th-desugar` submodule (and updates a reference to an issue
number) while I'm in town.

README.md

@@ -641,6 +641,28 @@ The following constructs are supported for promotion but not singleton generatio
   Overlap is caused by `otherwise` catch-all guard, which is always true and thus
 overlaps with `pred x` guard.
 
+  Another non-obvious source of overlapping patterns comes from partial pattern
+  matches in `do`-notation. For example:
+
+    ```haskell
+    f :: [()]
+    f = do
+      Just () <- [Nothing]
+      return ()
+    ```
+
+  This has overlap because the partial pattern match desugars to the following:
+
+    ```haskell
+    f :: [()]
+    f = case [Nothing] of
+          Just () -> return ()
+          _ -> fail "Partial pattern match in do notation"
+    ```
+
+  Here, it is more evident that the catch-all pattern `_` overlaps with the
+  one above it.
+
 The following constructs are not supported:
 
 * datatypes that store arrows, `Nat`, or `Symbol`

src/Data/Singletons/Prelude/List/Internal.hs

@@ -538,7 +538,7 @@ $(singletonsOnly [d|
   replicate n x           = if n == 0 then [] else x : replicate (n-1) x
 
 -- Uses partial pattern-matching in a list comprehension
--- (see https://github.com/goldfirere/th-desugar/issues/80)
+-- (see https://github.com/goldfirere/singletons/issues/340)
 --  transpose               :: [[a]] -> [[a]]
 --  transpose []             = []
 --  transpose ([]   : xss)   = transpose xss