Steps to reproduce:
1. Create new "Standard Surface Shader"
2. Replace line 14 (#pragma surface surf Standard fullforwardshadows) with:
#pragma shader_feature _ALPHAPREMULTIPLY_ON
#pragma surface surf Standard alpha:premul

Actual result: Shader does not compile
Expected result: Shader does compile

Tested and reproduced on:
Windows 10
Unity 5.1.4f1, 5.2.4p1, 5.3.3f1, 5.4.0b7

Note: Using multi_compile instead of shader_feature does let you compile the shader