Allow use of typedesc as type converters (#8409)

Fixes #8403

compiler/semtypes.nim

@@ -1577,8 +1577,14 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
         result = prev
   of nkSym:
     let s = getGenSym(c, n.sym)
-    if s.kind == skType and s.typ != nil:
-      var t = s.typ
+    if s.kind == skType and s.typ != nil or
+      s.kind == skParam and s.typ.kind == tyTypeDesc:
+      var t =
+        if s.kind == skType:
+          s.typ
+        else:
+          internalAssert c.config, s.typ.base.kind != tyNone and prev == nil
+          s.typ.base
       let alias = maybeAliasType(c, t, prev)
       if alias != nil:
         result = alias

tests/generics/t8403.nim

@@ -0,0 +1,11 @@
+discard """
+  output: "6.0"
+"""
+
+proc sum*[T](s: seq[T], R: typedesc): R =
+  var sum: R = 0
+  for x in s:
+    sum += R(x)
+  return sum
+
+echo @[1, 2, 3].sum(float)