fixes #21027; cast expressions need a type (#21029)

* fixes #21027; cast expressions need a type

* Apply suggestions from code review

Thanks to @beef331

compiler/semexprs.nim

@@ -394,6 +394,8 @@ proc semCast(c: PContext, n: PNode): PNode =
   let castedExpr = semExprWithType(c, n[1])
   if castedExpr.kind == nkClosedSymChoice:
     errorUseQualifier(c, n[1].info, castedExpr)
+  if targetType == nil:
+    localError(c.config, n.info, "Invalid usage of cast, cast requires a type to convert to, e.g., cast[int](0d).")
   if tfHasMeta in targetType.flags:
     localError(c.config, n[0].info, "cannot cast to a non concrete type: '$1'" % $targetType)
   if not isCastable(c, targetType, castedExpr.typ, n.info):

tests/types/t21027.nim

@@ -0,0 +1,5 @@
+discard """
+  errormsg: "Invalid usage of cast, cast requires a type to convert to, e.g., cast[int](0d)."
+"""
+# bug #21027
+let x: uint64 = cast(5)