Let null contribute its nullability to method type re-inference

[jcouv]

The scenario is Infer(s, null); with void Infer<T>(T t1, T t2). We currently infer string! but want to infer string? instead, so that no warnings are produced.
The problem is that when we re-infer type arguments with nullability, typeless expressions (like the null stripped of its conversions) do not contribute its type or nullability.

Alternatives considered:

1. If we had a solution for typeless expressions to contribute to method type inference, I would prefer that, but in the meantime, I propose a solution scoped to only affects nullability analysis: when we strip conversions off of arguments, we leave the conversion that gives null its type.
2. Another possible approach which avoid removing most conversions is what we do for default, switch and other expressions which get a different bound node once target-typed.

Fixes #43536

[cston]

string s [](start = 11, length = 8)

s is not used.

[cston]

public Optional(T value) => throw null!; [](start = 4, length = 40)

Not used.

[cston]

It feels like the current behavior of null is expected: we pass an untyped BoundLiteral to MethodTypeInferrer. But the behavior of default is unexpected: we pass a typed BoundDefaultExpression to MethodTypeInferrer where the type was only determined after running MethodTypeInferrer in initial binding.

[jcouv]

Just to clarify, you're saying we should keep the current behavior for null (Infer("", null) would infer string! and warn, rather than string?)?

[cston]

Not necessarily. I'm suggesting the arguments passed to MethodTypeInferrer in NullableWalker should match the typed/untyped-ness of the arguments passed to MethodTypeInferrer in initial binding. So null and default would both be passed as untyped arguments. Could NullableWalker fix up the nullability of the inferred type arguments after MethodTypeInferrer returns? Such an approach would also need to handle lambda return types so it might get complicated.

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