Champion "default" expression (C# 7.1)

[gafter]

• Proposal added https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.1/target-typed-default.md
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive) [done]
• Spec'ed

See also dotnet/roslyn#7737
A working implementation was merged to master branch and is now in C# 7.1.

LDM history:

• https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-03-07.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-05-31.md#default-in-operators
• There were further revisions on default in binary operators, but can't find the notes

[jcouv]

Some LDM notes: https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-05-31.md#default-in-operators

[franzalex]

@gafter Can you please fix the broken links in your post?

[odalet]

I've noted that the following code does not compile (I'm using VS2017 15.5.2); I suspect there is a good reason for it; however I suppose it's worth mentioning it:

        public static (TR, TL) ToTuple<TR, TL>(this Either<TR, TL> source) =>
            source.IsRight ? (source.Right, default) : (default, source.Left);

It says that there is no conversion between (TR, default) and (default, TL); and this reminds me of the similar error we get when we need casting at leat one arm to a common base type... Similarly here, I just need to qualify one arm with a default(TL|TR) explicitely, not both...
However, I wonder whether, in this case, the type inference system could match on the return type?

[jcouv]

@odalet From a quick check at the spec for conditional expressions, I see that the return type of cond ? x : y is always either the type of x or the type of y.
But in your example, (source.Right, default) or (default, source.Left) by themselves (absent the context of a conditional) don't have determinable types.
In C#, type information for inference is not completely free-flowing, it only flows certain ways.

[odalet]

Thanks for pointing this. It makes sense. Maybe, at one time in the future, the C# compiler could infer some type information from lvalues (as Rust does)