Compiler can't determine best common type for `switch` expression about csharplang HOT 8 OPEN

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

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

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

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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.

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

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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.

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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

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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.

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