[SR-3317] Literal protocol for decimal literals should support precise decimal accuracy

[jckarter]

Previous ID	SR-3317
Radar	rdar://problem/31414910
Original Reporter	@jckarter
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, AffectsABI, StarterProposal
Assignee	None
Priority	Medium

md5: 9de0502502b261886bad92d969cec324

duplicates:

• SR-920 Re-design builtin compiler protocols for literal convertible types

is duplicated by:

• SR-5678 Decimal initializer result is unexpected

relates to:

• SR-3970 Rework ExpressibleBy(Integer|Float)Literal protocols

Issue Description:

A decimal literal such as 1.23 currently constructs a value via the ExpressibleByFloatLiteral protocol, which eagerly parses the literal into a binary floating-point value, losing accuracy. ExpressibleByFloatLiteral is unsuitable for supporting decimal float or fixed-point types. We should introduce an ExpressibleByDecimalLiteral protocol that can receive the significand, exponent, and sign components of a decimal literal as precise integers, so that a conforming type can produce a precise result. It could look something like this:

protocol ExpressibleByDecimalLiteral {
  associatedtype Exponent: ExpressibleByIntegerLiteral
  associatedtype Significand: ExpressibleByIntegerLiteral
  init(negative: Bool, significand: Significand, exponent: Exponent)
}

ExpressibleByFloatLiteral could be made into a refinement of ExpressibleByDecimalLiteral as well.

[bob-wilson]

@swift-ci create

[swift-ci]

Comment by Ky (JIRA)

This is marked resolved; in what version of Swift was it introduced? I'm assuming 4.1...?

[bob-wilson]

It is resolved as a duplicate of SR-920