According to the Vulkan spec:

If the type of the values written by the fragment shader do not match the format of the corresponding color attachment, the resulting values are undefined for those components.

This would be the case if, for example, a fragment shader attempted to write to an R8G8B8A8_UINT attachment with an output of type ivec4. In this case, the signedness of the types does not match.

The dEQP-VK.api.fragment_shader_output.different_signedness.* tests explicitly test that this case works in the implementation, but if I'm correct, they are testing undefined behavior. In that case, these tests should be removed.

Am I missing something?