hacky fix for generic constraints matching

tests/reject/tgenconstraints.nim

@@ -0,0 +1,30 @@
+discard """
+  file: "tgenconstraints.nim"
+  line: 25
+  errormsg: "cannot instantiate T2"
+"""
+
+type
+  T1[T: int|string] = object
+    x: T
+
+  T2[T: Ordinal] = object
+    x: T
+
+var x1: T1[int]
+var x2: T1[string]
+var x3: T2[int]
+
+proc foo[T](x: T): T2[T] {.discardable.} =
+  var o: T1[T]
+
+foo(10)
+
+proc bar(x: string|TNumber): T1[type(x)] {.discardable.} =
+  when type(x) is TNumber:
+    var o: T2[type(x)]
+
+bar "test"
+bar 100
+bar 1.1
+