Missing coverage: Vulkan shaders that alias the same binding with multiple shader variables

[jenatali]

Per the spec, section 15.8.3:

There are no limits on the number of shader variables that can have overlapping set and binding values in a shader; but which resources are statically used has an impact. If any shader variable identifying a resource is statically used in a shader, then the underlying descriptor bound at the declared set and binding must support the declared type in the shader when the shader executes.

If multiple shader variables are declared with the same set and binding values, and with the same underlying descriptor type, they can all be statically used within the same shader. However, accesses are not automatically synchronized, and Aliased decorations should be used to avoid data hazards (see section 2.18.2 Aliasing in the SPIR-V specification).

If multiple shader variables with the same set and binding values are declared in a single shader, but with different declared types, where any of those are not supported by the relevant bound descriptor, that shader can only be executed if the variables with the unsupported type are not statically used.

A noteworthy example of using multiple statically-used shader variables sharing the same descriptor set and binding values is a descriptor of type VK_DESCRIPTOR_TYPE_COMBINED_IMAGE_SAMPLER that has multiple corresponding shader variables in the UniformConstant storage class, where some could be OpTypeImage (Sampled=1), some could be OpTypeSampler, and some could be OpTypeSampledImage.

Real-world app usage shows shaders that use the same UBO or SSBO binding across different variables, with different struct types. These structs essentially just reinterpret the buffer memory. It'd be nice to get some test coverage for this usage scenario instead of just having to test specific apps.