[SR-6671] Decimal.leastNonzeroMagnitude, leastNormalMagnitude should have -128 exponent

[swift-ci]

Previous ID	SR-6671
Radar	None
Original Reporter	takasek (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	@xwu
Priority	Medium

md5: f8e60ccf11f2fdc6d95d64e7bcbd0e41

relates to:

• SR-14974 Decimal.ulp produces inaccurate results

Issue Description:

`Decimal.leastNonzeroMagnitude` and `Decimal.leastNormalMagnitude` are set -127 as their exponents.
https://github.com/apple/swift-corelibs-foundation/blob/ee61823ae71f4c4f9ca157fd290c4b7de915b2a4/Foundation/Decimal.swift#L120

However, you can make less value than least magnitude by such code below.

func truelyLeastDecimal() -> Decimal {
    var x = Decimal(string: "0.1")!
    var r = Decimal()
    NSDecimalPower(&r, &x, 128, .plain)
    return r
}

func p(_ d: Decimal) {
    print("=====")
    print(d)
    print(d._mantissa)
    print(d._exponent)
}
p(Decimal.leastNormalMagnitude)
p(truelyLeastDecimal())

/*
 =====
 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 (1, 0, 0, 0, 0, 0, 0, 0)
 -127
 =====
 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 (1, 0, 0, 0, 0, 0, 0, 0)
 -128
 */

NSDecimalNumber defines its exponent's range is down to -128.
https://developer.apple.com/documentation/foundation/nsdecimalnumber

Thus `Foundation.Decimal` should follow it. Isn't it?

[swift-ci]

Comment by Yoshitaka Seki (JIRA)

I just submitted #1386 .

[weissi]

this still crashes on main:

 40> truelyLeastDecimal()
$R3: Decimal = 0.000000

 41> print(truelyLeastDecimal()) 
0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 42> p(truelyLeastDecimal())
Execution interrupted. Enter code to recover and continue.
Enter LLDB commands to investigate (type :help for assistance.)

[xwu]

#3067