For mat2x2, the stride between columns is 16 bytes in In a uniform buffer and 8 bytes in storage buffer.
As part of the resolution to #922, we decided to avoid having the user tell us the matrix stride.

Then consider code like this:

[[block]]  struct S {
   [[offset(0)]]  M : mat2x2<f32>;
};

[[set(0), binding(1)]] var<storage> sbuf : S;
[[set(0), binding(1)]] var<storage> ubuf : S;

Then:

• ubuf.M[1][0] is 16 bytes into the buffer, and
• sbuf.M[1][0] is 8 bytes into the buffer

This is ... confusing for users.

Also, when generating SPIR-V code for the type, we have to split S into two types, which causes a lot of complexity.

I suggest adding a rule:

• If a type contains a mat2x2, then that type may not be used as the store type or inside the store type for both a uniform buffer and a storage buffer.

We already have a note in 3.4.6.2. Internal Layout of Values that says:

Note: Matrix values are laid out more compactly in the storage storage class than in the uniform storage class.

Thanks to @Kangz who raised this to prominence.