[Proposal] Target type inference for new() keyword in switch expressions

[I-Omnibus]

[ https://dotnetfiddle.net/Widget/CKXaCr ]

/// <summary>
/// Target type inference for new() keyword in switch expressions
/// </summary>
using System.Collections.Generic;
using System.Linq;

class CSharpLang {

	void NewKeywordInference() {
		PleaseMakeInference inferred;
		// Target type has been inferred with new() keyword
		inferred = new() { Direction = Move.Nowhere };

		// No inference available here despite the compiler knowing 
		// that it's explicitly dealing IEnumerable<PleaseMakeInference>
		var values = new int[] { -2, -1, 0, 1, 2 };
		IEnumerable<PleaseMakeInference> directions = values
		.Select(i => i switch {
			< 0 => new PleaseMakeInference() with { Direction = Move.Left },
			> 0 => new PleaseMakeInference() with { Direction = Move.Right },
			_ => new PleaseMakeInference() with { Direction = Move.Nowhere },
		});
		/*
		*************************************************
		Desired code
		*************************************************
		.Select(i => i switch {
			< 0 => new() with { Direction = Move.Left },
			> 0 => new() with { Direction = Move.Right },
			_ => new() with { Direction = Move.Nowhere },
		});
		*************************************************
		*/
	}
	record PleaseMakeInference {
		public Move Direction { get; init; }
	}
	enum Move : sbyte {
		Left = -1,
		Nowhere = 0,
		Right = +1,
	}
}

[PathogenDavid]

// No inference available here despite the compiler knowing 
// that it's explicitly dealing IEnumerable<PleaseMakeInference>

The compiler actually doesn't know that. (Well, the compiler might but the language doesn't.) The return type of Select is determined by the lambada rather than directions. So for your exact scenario you'd also need generic return-type target-typing. (See dotnet/csharplang#92)

---

That being said, this doesn't work even without the involvement of Select: (SharpLab)

int i = 2;
inferred = i switch {
    < 0 => new() with { Direction = Move.Left }, // error CS8754: There is no target type for 'new()'
    > 0 => new() with { Direction = Move.Right }, // error CS8754: There is no target type for 'new()'
    _ => new() with { Direction = Move.Nowhere }, // error CS8754: There is no target type for 'new()'
};

dotnet/csharplang#2389 was the original proposal for adding target-typing to switch expressions during C# 8. It was never formally specified, but based on the proposed spec it sounds like it would not help in this situation. As such, I'm fairly certain this would require a language change and therefore this request belongs in csharplang.

---

As an aside: If you add csharp to the start of your code block, it will add syntax highlighting to it. IE:

```csharp
Console.WriteLine("Hello, World!");
```

[alrz]

That being said, this doesn't work even without the involvement

new() and switch can be used together only if the target type could flow in. In that example with breaks the flow since it's not target typed.

dotnet/csharplang#2926 (comment)

[I-Omnibus]

Your example is much more concise and for what it's worth, the change to the final clause shows that the compiler has an idea of the target type for a new() keyword.

int i = 2;
inferred = i switch {
	< 0 => new PleaseMakeInference() with { Direction = Move.Left },
	> 0 => new PleaseMakeInference() with { Direction = Move.Right },
	_ => /*new()*/ String.Empty, //Error CS0029 Cannot implicitly convert type 'string' to 'CSharpLang.PleaseMakeInference'
};