Merge pull request #4654 from unisonweb/travis/4280/use-blanks

Fix parsing bug involving `use` and underscore-prefixed names

unison-src/transcripts/fix4280.md

@@ -0,0 +1,12 @@
+```ucm:hide
+.> builtins.merge
+```
+
+```unison
+foo.bar._baz = 5
+
+bonk : Nat
+bonk =
+  use foo.bar _baz
+  _baz
+```

unison-src/transcripts/fix4280.output.md

@@ -0,0 +1,23 @@
+```unison
+foo.bar._baz = 5
+
+bonk : Nat
+bonk =
+  use foo.bar _baz
+  _baz
+```
+
+```ucm
+
+  Loading changes detected in scratch.u.
+
+  I found and typechecked these definitions in scratch.u. If you
+  do an `add` or `update`, here's how your codebase would
+  change:
+  
+    ⍟ These new definitions are ok to `add`:
+    
+      bonk         : Nat
+      foo.bar._baz : Nat
+
+```

unison-syntax/src/Unison/Syntax/Parser.hs

@@ -323,6 +323,7 @@ wordyDefinitionName = queryToken $ \case
 importWordyId :: Ord v => P v m (L.Token Name)
 importWordyId = queryToken \case
   L.WordyId (HQ'.NameOnly n) -> Just n
+  L.Blank s | not (null s) -> Just $ Name.unsafeFromString ("_" <> s)
   _ -> Nothing
 
 -- The `+` in: use Foo.bar + as a Name