An instance compound assignment operator has no callable spelling: its call
sites can only be written as "x op= y" or "x++", and C# requires x to be an
assignable variable whose static type binds the operator the call names. The
reader therefore parks an object-typed receiver in a stack slot typed with the
operator's declaring type, and the passes that would substitute or retype such
a receiver hold back. CallInstruction vetoes unfit receiver replacements
through SatisfiesSlotRestrictionForInlining - not writable, not a storage
location, or of a type that declares a hiding operator of the same name or its
checked/unchecked sibling (a declaration-existence check on the type system,
deliberately not a binding question) - which covers inlining; copy propagation
asks the same check per receiver load, the using-transform hands receiver
loads a writable copy of the read-only using variable, and stack slots that
were typed on purpose keep their type when translated.
Assisted-by: Claude:claude-opus-5:Claude Code
Siegfried Pammer