How to reproduce:
1. Download user's attached project "ScreenPosBug.zip"
2. Open the Scene "ClipTest" and play it
3. Now open the custom surface shader "ClipShader" and change line 32 to "#ifndef INSTANCING_ON"
4. Notice that when using "UNITY_INSTANCING_ENABLED" the value of the perspective corrected screenPos is not displayed as the color of the quad

Expected result: The value of the perspective corrected screenPos should be displayed as the color of the quad. The bottom left should be black, the right red, the top green, and the top right yellow

Actual result: The quad is blue. The generated vertex and fragment shader do not show that the screenPos is required.

Reproducible with: 2017.4.11f1, 2018.1.9f2, 2018.2.8f1, 2018.3.0b1

Note: The line that changes the color to green is disabled with an "#ifndef UNITY_INSTANCING_ENABLED" (ClipShader.shader L32-33). When viewing the generated vertex and fragment shaders, the "INSTANCING_ON" should inform that the screenPos is required.