Target-typed "default" literal

Proposed
Prototype
Implementation
Specification

Summary

The target-typed default feature is a shorter form variation of the default(T) operator, which allows the type to be omitted. Its type is inferred by target-typing instead. Aside from that, it behaves like default(T).

Motivation

The main motivation is to avoid typing redundant information.

For instance, when invoking void Method(ImmutableArray<SomeType> array), the default literal allows M(default) in place of M(default(ImmutableArray<SomeType>)).

This is applicable in a number of scenarios, such as:

declaring locals (ImmutableArray<SomeType> x = default;)
ternary operations (var x = flag ? default : ImmutableArray<SomeType>.Empty;)
returning in methods and lambdas (return default;)
declaring default values for optional parameters (void Method(ImmutableArray<SomeType> arrayOpt = default))
including default values in array creation expressions (var x = new[] { default, ImmutableArray.Create(y) };)

Detailed design

A new expression is introduced, the default literal. An expression with this classification can be implicitly converted to any type, by a default-or-null literal conversion.

The inference of the type for the default literal works the same as that for the null literal, except that any type is allowed (not just reference types).

This conversion produces the default value of the inferred type.

The default literal may have a constant value, depending on the inferred type. So const int x = default; is legal, but const int? y = default; is not.

The default literal can be the operand of equality operators, as long as the other operand has a type. So default == x and x == default are valid expressions, but default == default is illegal.

Drawbacks

A minor drawback is that default literal can be used in place of null literal in most contexts. Two of the exceptions are throw null; and null == null, which are allowed for the null literal, but not the default literal.

Alternatives

There are a couple of alternatives to consider:

The status quo: The feature is not justified on its own merits and developers continue to use the default operator with an explicit type.
Extending the null literal: This is the VB approach with Nothing. We could allow int x = null;.

Unresolved questions

Should default be allowed as the operand of the is or as operators? Answer: disallow default is T, allow x is default, allow default as RefType (with always-null warning)

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