Nullable-Enhanced Common Type

Champion issue: https://github.com/dotnet/csharplang/issues/33

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?, and an expression such as condition ? x : 1.0 where x is of type int? would result in a value of type double?.

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 §11.6.3.15 to affect the following situations:

• 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 one expression is of a nullable value type T? and the other is of a value type U, and there is an implicit conversion from T to U, then the result is of type U?.

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

• the ternary expression §11.15
• implicitly typed array creation expression §11.7.15.5
• inferring the return type of a lambda §11.6.3.13 for type inference
• cases involving generics, such as invoking M<T>(T a, T b) as M(1, null).

More precisely, we change the following sections of the specification (insertions in bold, deletions in strikethrough):

Output type inferences

An output type inference is made from an expression E to a type T in the following way:

• If E is an anonymous function with inferred return type U (§11.6.3.13) and T is a delegate type or expression tree type with return type Tb, then a lower-bound inference (§11.6.3.10) is made from U to Tb.
• Otherwise, if E is a method group and T is a delegate type or expression tree type with parameter types T1...Tk and return type Tb, and overload resolution of E with the types T1...Tk yields a single method with return type U, then a lower-bound inference is made from U to Tb.
• **Otherwise, if E is an expression with nullable value type U?, then a lower-bound inference is made from U to T and a null bound is added to T. **
• Otherwise, if E is an expression with type U, then a lower-bound inference is made from U to T.
• Otherwise, if E is a constant expression with value null, then a null bound is added to T
• Otherwise, no inferences are made.

Fixing

An unfixed type variable Xi with a set of bounds is fixed as follows:

• The set of candidate types Uj starts out as the set of all types in the set of bounds for Xi.
• We then examine each bound for Xi in turn: For each exact bound U of Xi all types Uj which are not identical to U are removed from the candidate set. For each lower bound U of Xi all types Uj to which there is not an implicit conversion from U are removed from the candidate set. For each upper bound U of Xi all types Uj from which there is not an implicit conversion to U are removed from the candidate set.
• If among the remaining candidate types Uj there is a unique type V from which there is an implicit conversion to all the other candidate types, then Xi is fixed to V.
  • If V is a value type and there is a null bound for Xi, then Xi is fixed to V?
  • Otherwise Xi is fixed to V
• Otherwise, type inference fails.

Drawbacks

There may be some incompatibilities introduced by this proposal.

Alternatives

None.

Unresolved questions

• What is the severity of incompatibility introduced by this proposal, if any, and how can it be moderated?

Design meetings

None.