Implicit conversions

[zygoloid]

Proposal to support a limited set of implicit conversions. This would generally permit only implicit conversions that are lossless and semantics-preserving. In particular, this proposal allows:

• Conversion from an integer type to a wider integer type of the same signedness, and from an unsigned integer type to a wider signed integer type.
• Conversion from an integer type to a floating-point type that has enough mantissa bits to exactly represent all integers in the source type.
• Conversion from integer literals to integer and floating-point types that can represent them.
• Conversion from floating-point literals to floating-point types that can represent them.
• Conversions required for generics: conversions of values between facet types, and conversions of types between type-of-types, as described in the generics proposals.
• Conversions required for inheritance: derived-to-base conversions for class pointers and class values.

Other conversions, such as lossy conversions between arithmetic types and conversions between bool and other types are not supported.

A mechanism to permit user-defined types to support implicit conversion is provided:

class StringView { ... }
impl String as ImplicitAs(StringView) {
  fn Convert[me: Self]() -> StringView { ... }
}
fn F(s: StringView);
fn G(s: String) {
  // OK, calls F(s.(ImplicitAs(StringView).Convert)())
  F(s);
}

[chandlerc]

Really like this overall BTW. Also somewhat happy to defer most of the detailed review to @josh11b. =] A few minor wording / clarification comments inline, nothing super substantial though.

[chandlerc]

Still some useful iteration I think on lossless and semantically the same... happy to chat in person to try to reach a good solution there...

[chandlerc]

The phrase "used interchangably" seems a bit confusing here. The subsequent explanation is clear, but maybe just jump to that?

Suggested change

	Two types are equivalent if they can be used interchangeably, implicitly: they
	Two types are equivalent if they

[chandlerc]

LGTM, ship it. I think this has converged enough, and if we get more issues that need to be clarified, I think we can fix this forward and still unblock subsequent design work.