improved common type

Proposed
Prototype: None
Implementation: None
Specification: See below

Summary

There are two circumstances in which the current common-type algorithm results are counter-intuitive, and result in the programmer adding what feels like a redundant cast to the code. With this change, an expression such as condition ? 1 : null would result in a value of type int?, and an expression of the form condition ? new Dog() : new Cat() would result in a value of their common base type Animal.

Motivation

This is a common cause of what feels to the programmer like needless boilerplate code.

Detailed design

We modify the specification for finding the best common type of a set of expressions to affect the following two circumstances:

If one expression is of a non-nullable value type T and the other is a null literal, the result is of type T?.
If the two operands are class types and have a common base type other than object, the most specific such type is the result.

This is expected to affect the following aspects of the language:

the ternary expression
implicitly types array creation expression
inferring the return type of a lambda for type inference

Drawbacks

There may be some incompatibilities introduced by this proposal.

Alternatives

None.

Unresolved questions

What is the severity of incompatibility introduced by this proposal, and how can it be moderated?
Should we handle reference types other than class types?

Design meetings