2718 was recently updated to assert that ORIGIN must equal CALLER of the first frame for reasons. Either we will need a new opcode for GAS_PAYER if we want that, or we will need to accept that sponsor payments will need to be handled some other way (not via the contract inspecting the transaction to figure out who the gas payer is at execution time).
I’m curious to hear people’s thoughts on this and how critical it is that the EVM is aware of who the gas payer is and whether we can live without it. Something to keep in mind is that for many/most transactions GAS_PAYER will equal ORIGIN, and introducing a GAS_PAYER opcode enable contracts to identify the type of transaction and potentially discriminate on it. Also, adding a new opcode is a pretty big deal since the space for them is somewhat limited, and additional opcodes add even more complexity to the EVM.
We could make it a precompile rather than an opcode, that would avoid contention over the limited opcode space, but that wouldn’t address the discrimination issue.