Dumping the implicit/explicit classification of all pairs from a 120-type
universe and diffing it against what csc actually compiles surfaced two
divergences (16 affected pairs), both proven by a compiling-and-running
snippet and captured here as ignored known-bug tests:
1. Nullable conversions derived from tuple conversions are missing:
csc accepts "(long, object)? a = t;" for t of type (int, string) as
well as the lifted and explicit forms, CSharpConversions returns None.
The ECMA spec's 10.6.1 does not list tuple conversions as liftable,
so this is a case of Roslyn exceeding the spec.
2. An explicit user-defined conversion to a nullable target is rejected
when the operator result additionally needs an explicit numeric
conversion: csc accepts "(int?)new ImplicitToLong()" (operator to
long, then explicit long -> int?), CSharpConversions returns None.
The same sweep confirmed the SByte..Decimal TypeCode range in
ImplicitEnumerationConversion is correct: csc accepts zero constants of
any numeric type (0.0, 0f, 0m) for enum conversion and rejects '\0',
matching the implementation exactly.
Assisted-by: Claude:claude-fable-5:Claude Code
Siegfried Pammer
6932586827