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LDM-2018-08-22.md

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C# Language Design Notes for August 22, 2018

Agenda

  1. Target-typed new
  2. Clarification on constraints with nullable reference types enabled

Discussion

Target-typed new

Proposal: https://github.com/dotnet/csharplang/blob/master/proposals/target-typed-new.md

Question: Should target-typed new be allowed for tuple types?

We currently don't allow the constructor syntax new (int, int)(0, 0). Should we allow (int, int) t = new(0, 0)? Would this mean the same thing as a tuple literal, or a call to a constructor on System.ValueTuple? This would also expose some of the differences between ValueTuple and tuple types, in that there is no constructor for a tuple type with greater than 7 elements.

Decision: Let's allow it, as long as that doesn't require a lot of extra work. The meaning would be to call the underlying System.ValueTuple constructors. This would expose differences in tuples with a lot of elements, but this seems like a very rare and unimportant case.

Question: Allow throw new()? It would convert to bare Exception by the spec.

Decision: Disallow. Fundamentally, we don't like this stylistically.

Question: Allow new() with user-defined comparison and arithmetic operators?

Decision: Allow.

Generic constraints with nullable reference types

Question: In the following snippet, is U a non-nullable reference type?

void M<T, U>() where T : class, U : T` {}

Answer: Yes

Question: In the following snippet, is I a non-nullable reference type?

interface I {}
void M<T>() where T : I {}

Answer: Yes

Q: Do we want to warn for redundant constraints?

A: We don't currently. Let's stay with that decision for now.