• Proposal added (below)

  Discussed in LDM 2019-04-22

  Decision in LDM (Approved for C# 8)

  Finalized (done, rejected, inactive)

  Spec'ed
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@333fred asked me

Given the following code, we cannot determine a best type for the switch expression and the code does not compile. Why can't we determine that the best type is A, given that's the target type of the expression?

class A
{
}

class B : A
{
}

class C : A
{
}

class D
{
    void M()
    {
        A a = null;
        a = a switch
        {
            B b => b,
            C c => c,
            _ => throw new System.Exception(),
        };
    }
}

Casting one branch to A fixes the code

My initial reaction was

We use the same inference algorithm for new []{ b, c } and () => { if (b) return b; else return c; } and c ? b : c and M(b, c) where T M<T>(T x, T y).  The general philosophy is that we avoid inferring a type that wasn't one of the input types.  What if they had a common interface but not a common base class?  What if they had two common interfaces?  We just don't do that. We use "target type" only for certain conversions.  We don't have a "conversion from array creation" or "conversion from conditional expression" and similarly we won't have a "conversion from switch expression".

However, my answer was not satisfactory to either @333fred or myself.

This is a proposal to "do that".

Proposal

Currently, a switch expression is specified to have a type that is the inferred common type of the expressions in the switch arms. It is (currently) required that there be such a common type.

Instead, I propose

1. We attempt to infer a common type among the arms of the switch expression. If there is such a common type, that is the (natural) type of the switch expression.
2. There is a switch expression conversion (a new implicit conversion-from-expression) from a switch expression to the type T if there is an implicit conversion from every arm's value expression to T.
3. Let T be the type of the switch expression, either the target type of the switch expression conversion, or if it was not subject to such a conversion the expression's natural type. It is an error if T does not exist. It is an error if a switch arm's expression cannot be implicitly converted to T.