ABI decisions to be made

There are a few issues related to ABI decisions to make:

cc65: Invalid function declaration is not flagged as an error #2014:
ABI breakage Should we always pass arguments as default-promoted (eg. char to int, float to double) to a K&R-style function?
What if we pass a 1-byte struct/union by value to a K&R-style function?
Evaluate and restore ability to pass and return 3-byte structs by value #2086:
Should we pass 3-bytes structs/unions to a function like a 3-byte value or a 4-byte value (as before)?
cc65: allow larger structs to be passed by value #2127:
Should we push large structs (size > 4 bytes) by value just like other small types?

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