fix: error recovery at sepBy combinator

src/Lean/Parser/Basic.lean

@@ -628,7 +628,7 @@ private partial def sepByFnAux (p : ParserFn) (sep : ParserFn) (allowTrailingSep
     let mut s := p c s
     if s.hasError then
       if s.pos > pos then
-        return s
+        return s.mkNode nullKind iniSz
       else if pOpt then
         let s := s.restore sz pos
         return s.mkNode nullKind iniSz

tests/lean/interactive/completion3.lean

@@ -0,0 +1,10 @@
+structure S where
+  x : Nat
+  y : String
+  b : Bool
+
+def f (s : S) : Nat :=
+  let rec foo (s : S) :=
+    if s. then 1 else 2
+       --^ textDocument/completion
+  foo s

tests/lean/interactive/completion3.lean.expected.out

@@ -0,0 +1,8 @@
+{"textDocument": {"uri": "file://completion3.lean"},
+ "position": {"line": 7, "character": 9}}
+{"items":
+ [{"label": "b", "detail": "S → Bool"},
+  {"label": "mk", "detail": "Nat → String → Bool → S"},
+  {"label": "x", "detail": "S → Nat"},
+  {"label": "y", "detail": "S → String"}],
+ "isIncomplete": true}