csharplang/proposals/target-typed-conditional-expression.md

Target-Typed Conditional Expression

For a conditional expression c ? e1 : e2, when

1. there is no common type for e1 and e2, or
2. for which a common type exists but one of the expressions e1 or e2 has no implicit conversion to that type

we define a new conditional expression conversion that permits an implicit conversion from the conditional expression to any type T for which there is a conversion-from-expression from e1 to T and also from e2 to T. It is an error if a conditional expression neither has a common type between e1 and e2 nor is subject to a conditional expression conversion.

Open Issues

We would like to extend this target typing to cases in which the conditional expression has a common type for e1 and e2 but there is no conversion from that common type to the target type. That would bring target typing of the conditional expression into alignment of target typing of the switch expression. However we are concerned that would be a breaking change:

M(b ? 1 : 2); // calls M(long) without this feature; calls M(short) with this feature

void M(short);
void M(long);

We could reduce the scope of the breaking change by modifying the rules for better conversion from expression: the conversion from a conditional expression to T1 is a better conversion from expression than the conversion to T2 if the conversion to T1 is not a conditional expression conversion and the conversion to T2 is a conditional expression conversion. That resolves the breaking change in the above program (it calls M(long) with or without this feature). This approach does have two small downsides. First, it is not quite the same as the switch expression:

M(b ? 1 : 2); // calls M(long)
M(b switch { true => 1, false => 2 }); // calls M(short)

This is still a breaking change, but its scope is less likely to affect real programs:

M(b ? 1 : 2, 1); // calls M(long, long) without this feature; ambiguous with this feature.

M(short, short);
M(long, long);

This becomes ambiguous because the conversion to long is better for the first argument (because it does not use the conditional expression conversion), but the conversion to short is better for the second argument (because short is a better conversion target than long). This breaking change seems less serious because it does not silently change the behavior of an existing program.

Should we elect to make this change to the proposal, we would change

Better conversion from expression

Given an implicit conversion C1 that converts from an expression E to a type T1, and an implicit conversion C2 that converts from an expression E to a type T2, C1 is a better conversion than C2 if E does not exactly match T2 and at least one of the following holds:

• E exactly matches T1 (Exactly matching Expression)
• T1 is a better conversion target than T2 (Better conversion target)

to

Better conversion from expression

Given an implicit conversion C1 that converts from an expression E to a type T1, and an implicit conversion C2 that converts from an expression E to a type T2, C1 is a better conversion than C2 if E does not exactly match T2 and at least one of the following holds:

• E exactly matches T1 (Exactly matching Expression)
• T1 is a better conversion target than T2 (Better conversion target) and either C1 is not a conditional expression conversion or C2 is a conditional expression conversion.