improved-common-type.md

improved common type

• Proposed
• Prototype: None
• Implementation: None
• Specification: See below

Summary

There is a situation in which the current common-type algorithm results are counter-intuitive, and results 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?.

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 situation:

• If one expression is of a non-nullable value type T and the other is a null literal, the result is of type T?.

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?

Design meetings

None.