Steps to reproduce:
1. Open attached project ("New Unity Project1").
2. Notice that the "Console" window outputs:

"Assets/test.cs(11,16): error CS0172: Can not compute type of conditional expression as `int' and `ushort' convert implicitly to each other"

Expected result:
The compilation should allow implicit conversions in a ternary operator.

Note:
Creating a console application in Visual Studio 2015 with the same lines of code and running it works fine.

Reproduced with:
5.3.5p6, 5.4.0b25.

Workaround:
Implicitly cast the value to be of the same type. In this case:

int a = 0;
ushort b = 0;
a = true ? 0 : (int)b;
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The issue will be solved as part of compiler upgrade.