fixes #5231 inheriting from partial specialized generic object (#5538)

tests/types/tinheritpartialgeneric.nim

@@ -0,0 +1,43 @@
+discard """
+  output: '''(c: hello, a: 10, b: 12.0)
+(a: 15.5, b: hello)
+(a: 11.75, b: 123)'''
+"""
+
+# bug #5231
+# generic object inheriting from
+# partial specialized generic object
+type
+  Curve1[T, X] = object of RootObj
+    a: T
+    b: X
+
+  Curve2[T] = Curve1[T, float64]
+
+  Curve3[T] = object of Curve2[T]
+    c: string
+
+  Curve4[T] = Curve1[float64, T]
+
+  Curve5[T] = object of Curve4[T]
+
+  Curve6[T] = object of T
+
+var x: Curve3[int]
+x.a = 10
+x.b = 12.0
+x.c = "hello"
+
+echo x
+
+var y: Curve5[string]
+y.b = "hello"
+y.a = 15.5
+
+echo y
+
+var z: Curve6[Curve4[int]]
+z.a = 11.75
+z.b = 123
+
+echo z