fixes #836

compiler/sigmatch.nim

@@ -242,6 +242,9 @@ proc concreteType(c: TCandidate, t: PType): PType =
     addSonSkipIntLit(result, t.sons[1]) # XXX: semantic checking for the type?
   of tyNil:
     result = nil              # what should it be?
+  of tySequence, tySet:
+    if t.sons[0].kind == tyEmpty: result = nil
+    else: result = t
   of tyGenericParam, tyAnything:
     result = t
     while true: 

tests/generics/tcannot_pass_empty_seq_to_generic.nim

@@ -0,0 +1,17 @@
+discard """
+  errormsg: "type mismatch: got (seq[empty])"
+  line: 16
+"""
+
+# bug #836
+
+type
+  TOption*[T] = object
+    case FIsSome: bool
+    of false: nil
+    of true: FData: T
+
+proc some*[T](value: T): TOption[T] = TOption[T](FIsSome: true, FData: value)
+
+echo some(@[]).FIsSome
+