Compiler can't determine best common type for `switch` expression

[gurustron]

Probably my understanding of the spec in this part is incorrect, but according to switch expression feature spec:

The type of the switch_expression is the best common type (§11.6.3.15) of the expressions appearing to the right of the => tokens of the switch_expression_arms if such a type exists and the expression in every arm of the switch expression can be implicitly converted to that type.

And the natural function type has the following:

Function_types are used in a few specific contexts only:

• implicit and explicit conversions
• method type inference (§11.6.3) and best common type (§11.6.3.15)
• var initializers

And:

Best common type (§11.6.3.15) is defined in terms of type inference so the type inference changes above apply to best common type as well.

With following sample which compiles:

var fs = new[] { (string s) => s.Length, (string s) => int.Parse(s) };

So for array construction compiler is able to determine the best common type.

Version Used: .NET 8.0.101, C# 12

Steps to Reproduce:

Create a swtich expression with anonymous lambdas:

var func = (args.Length == 1) switch
{
    false => () => "Hello World",
    true => () => "Foo Bar"
};

A minimal repro, with source-code provided, is ideal. Repro @sharplab

Expected Behavior:

Compiles

Actual Behavior:

error CS8506: No best type was found for the switch expression.

[PoseidonEnergy]

I would also like to highlight another discrepancy related to functions.

This does not compile:

var str = (() => "Hello World")(); // does not compile

However, this compiles:

var str = ((Func<string>)(() => "Hello World"))(); // compiles

[RikkiGibson]

@cston for visibility. I think this is a subtle issue relating to lambda natural type.

[jaredpar]

@cston ping for comment on whether or not this is about lambda natural type

[cston]

It looks like the best common type calculation for switch expressions currently ignores function types.

There is a similar issue with conditional expressions:

// error CS0173: Type of conditional expression cannot be determined
var func = (args.Length == 1) ? () => "Hello" : () => "World";

For switch expressions and conditional expressions, we could potentially use function types in the best common type calculation of the overall expression type.

[jaredpar]

Moving to csharplang as this is a design question vs. compiler impl.

[CyrusNajmabadi]

Definitely seems odd. I can't come up with a rational as to why array BCT works differently than switch or conditional BCT vis-a-vis function-types.

[HaloFour]

I would also like to highlight another discrepancy related to functions.

IIRC, the team decided against supporting IIFE in those scenarios: https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-07-26.md#direct-invocation-of-lambdas

[PrinceSatish]

By explicitly casting the lambdas to Func, you provide the necessary type information for the compiler to determine the return type of the switch expression, and it will compile without errors.