HandleAccessorCall had no way to express an omitted argument, so
CallBuilder asserted that none had been detected before it got there: any
assembly indexing through an indexer with an optional parameter hit that
assert in a Debug build, and Release wrote the defaults back out. Accessor
calls now go through the same ArgumentList helpers as an ordinary call.
Two things had to reach them. The assigned value of a setter is the last
argument of the accessor call but not an argument of the access - the
standard adds it only for the invocation (12.6.2.1) - so it neither ends
the run of optional arguments nor is written out with them, and whether an
accessor is written as an access at all is decided once, before the
arguments are translated, so the scan and the count cannot disagree. The
names have to stop where the argument list does, and they name the
indexer's parameters, which the type system takes from the getter rather
than from the accessor being called.
C# allows named arguments in an element access, but NamedArgumentTransform
refused to introduce one for any accessor, so an access whose arguments
the compiler reordered came out as a temporary. Only indexers gain this:
a property access has no argument list, an operator cannot take names,
and a setter's value stays unnamed on the right-hand side. Introducing a
name replaces the call with a block, so it is refused where the
surrounding instruction requires the call itself - a call-inline-assign
block, or the target of a compound assignment.
Whether the shortened access still binds to the same member is left to
IsUnambiguousAccess. If it does not, the omitted arguments are written out
again before any cast is tried, since restoring them cannot change what
the access means. A type declaring both this[int] and this[int, int = 10]
therefore keeps both arguments; the fixture pins that.
Not covered: params indexers, [Optional] without a constant,
[DateTimeConstant]-style defaults, default(T) at a value-type
instantiation, and omitting a middle optional argument - the last of which
plain calls do not do either.
Assisted-by: Claude:claude-opus-5[1m]:Claude Code
Siegfried Pammer
bd7f8f9e09