Add testcase for #12571 (#14955)

tests/statictypes/tstatictypes.nim

@@ -9,6 +9,7 @@ output: '''
 b is 2 times a
 17
 ['\x00', '\x00', '\x00', '\x00']
+heyho
 '''
 """
 
@@ -223,3 +224,18 @@ block: # issue #14802
     12
   const myConst = static(fn(1))
   doAssert myConst == 12
+
+
+# bug #12571
+type
+  T[K: static bool] = object of RootObj
+    when K == true:
+      foo: string
+    else:
+      bar: string
+  U[K: static bool] = object of T[K]
+
+let t = T[true](foo: "hey")
+let u = U[false](bar: "ho")
+echo t.foo, u.bar
+