To reproduce:
1. Open attached project
2. In Hierarchy window, select LayoutPanel game object
3. In Inspector window, check Child Controls Size parameter in Layout Group component
4. In Hierarchy window, select LayoutChild
5. In Inspector window, check child's Rect Transform component

Expected: LayoutChild is not allowed to be resized when Child Controlls Size field is unticked (and vice versa)
Actual: LayoutChild can be resized when Child Controlls Size field is unticked (and vice versa)

Reproduced in: 2017.1.2p3, 2017.2.0p2, 2017.3.0b10, 2018.1.0a5
Not reproduced in: 5.5.5p1, 5.6.4p2

Note: field name was changed from "Control Child Size" to "Child Controls Size" and back, inverting it's meaning several times between 5.6 and 2017.1
Note: check .gif attached for issue reproduction