fix #20588 (#21104)

(cherry picked from commit 8054be6e52)

compiler/semexprs.nim

@@ -286,6 +286,8 @@ proc semConv(c: PContext, n: PNode): PNode =
       return evaluated
     else:
       targetType = targetType.base
+  of tyAnything, tyUntyped, tyTyped:
+    localError(c.config, n.info, "illegal type conversion to '$1'" % typeToString(targetType))
   else: discard
 
   maybeLiftType(targetType, c, n[0].info)

tests/arc/t20588.nim

@@ -0,0 +1,22 @@
+discard """
+  cmd: "nim check --warnings:off --hints:off $file"
+  errormsg: ""
+  nimout: '''
+t20588.nim(20, 12) Error: illegal type conversion to 'auto'
+t20588.nim(21, 14) Error: illegal type conversion to 'typed'
+t20588.nim(22, 16) Error: illegal type conversion to 'untyped'
+'''
+"""
+
+
+
+
+
+
+
+
+
+
+discard 0.0.auto
+discard typed("abc")
+discard untyped(4)