Revert "Revert "test cases for the new features""

This reverts commit e1b668c868.

tests/compile/tgenericdefaults.nim

@@ -0,0 +1,29 @@
+type 
+  TFoo[T, U, R = int] = object
+    x: T
+    y: U
+    z: R
+
+  TBar[T] = TFoo[T, array[4, T], T]
+
+var x1: TFoo[int, float]
+
+static:
+  assert type(x1.x) is int
+  assert type(x1.y) is float
+  assert type(x1.z) is int
+  
+var x2: TFoo[string, R = float, U = seq[int]]
+
+static:
+  assert type(x2.x) is string
+  assert type(x2.y) is seq[int]
+  assert type(x2.z) is float
+
+var x3: TBar[float]
+
+static:
+  assert type(x3.x) is float
+  assert type(x3.y) is array[4, float]
+  assert type(x3.z) is float
+