fixes #20807, refs #20450, regression with seq inference (#20818)

compiler/semexprs.nim

@@ -2402,12 +2402,16 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags; expectedType: P
     markUsed(c, n.info, s)
     result = semSizeof(c, setMs(n, s))
   of mArrToSeq, mOpenArrayToSeq:
-    if n.len == 2 and expectedType != nil and (
+    if expectedType != nil and (
         let expected = expectedType.skipTypes(abstractRange-{tyDistinct});
         expected.kind in {tySequence, tyOpenArray}):
       # seq type inference
       var arrayType = newType(tyOpenArray, nextTypeId(c.idgen), expected.owner)
       arrayType.rawAddSon(expected[0])
+      if n[0].kind == nkSym and sfFromGeneric in n[0].sym.flags:
+        # may have been resolved to `@`[empty] at some point,
+        # reset to `@` to deal with this
+        n[0] = newSymNode(n[0].sym.owner, n[0].info)
       n[1] = semExpr(c, n[1], flags, arrayType)
     result = semDirectOp(c, n, flags, expectedType)
   else:

tests/types/ttopdowninference.nim

@@ -244,3 +244,14 @@ block: # bug #11777
   type S = set[0..5]
   var s: S = {1, 2}
   doAssert 1 in s
+
+block: # regression #20807
+  var s: seq[string]
+  template fail =
+    s = @[]
+  template test(body: untyped) =
+    body
+  proc test(a: string) = discard
+  test: fail()
+  doAssert not (compiles do:
+    let x: seq[int] = `@`[string]([]))