[SR-15218] Enhance interchangeable CGFloat/Double to allow interchange between optional

[mattyoung]

Previous ID	SR-15218
Radar	rdar://problem/83322839
Original Reporter	@mattyoung
Type	New Feature
Status	In Progress
Resolution

Additional Detail from JIRA

Votes	3
Component/s	Compiler
Labels	New Feature, StarterBug, TypeChecker
Assignee	ldhough (JIRA)
Priority	Medium

md5: f6e45c4a9212f2077155f5dc1ee322a7

Issue Description:

Currently passing Double? to parameter of CGFloat? do not work. Please add enhancement to allow such. Additionally, allow multiple level of optionality. Basically allow this:

Double?* <-> CGFloat?* (zero or more level of optionality)

See: https://forums.swift.org/t/why-double-can-pass-to-cgfloat-but-double-cannot/52217/5

[xedin]

@swift-ci create

[xedin]

I think this can make an interesting StarterBug.

[amritpan]

Hi @xedin, what should I look at if I'm interested in fixing this but don't have familiarity with where this is handled or what the fix will look like? I'd like to figure out if I can do this before assigning it to myself.

[xedin]

@amritpan This might be a bit tricky actually due to how conversion restrictions behave. CGFloat/Double conversion needs to wait until all Optional-to-Optional conversions are performed for a particular location, and if the optionality is the same on both sides (e.g. both types are no longer optional) allow Double/CGFloat conversion (Related methods are `matchTypes` and `simplifyRestrictedConstraintImpl` located in CSSimplify.cpp). This is relatively straight-forward to implement and it would result in a valid solution (you can double-check that using `-Xfrontend -debug-constraints` - more info about that @ https://github.com/apple/swift/blob/main/docs/DebuggingTheCompiler.md#debugging-the-type-checker). Next step is to add a number of implicit `.map` calls based on the number of `optional-to-optional` conversions performed for a given location during solution application (related code lives in CSApply.cpp - `coerceToType` method, you can search for ConversionRestrictionKind::DoubleToCGFloat to find relevant location). Here is another catch though - `ConstraintRestrictions` are not positional, so there is no way (currently) to determine how many did each location (identified by a ConstraintLocator) have, you'd have to change that to be able to insert appropriate number of `.map` calls or somehow figure out what was the optionality before first optional-to-optional conversion has been applied to a given location.

[mattyoung]

Cannot convert value of type 'UnsafeMutablePointer<Double>' to expected argument type 'UnsafeMutablePointer<CGFloat>'

extension Color {
    var luminance: Double {
        var (r, g, b, a): (Double, Double, Double, Double) = (0, 0, 0, 0)
        UIColor(self).getRed(&r, green: &g, blue: &b, alpha: &a)    // Cannot convert value of type 'UnsafeMutablePointer<Double>' to expected argument type 'UnsafeMutablePointer<CGFloat>'
        return 0.2126 * r + 0.7152 * g + 0.0722 * b
    }
}

is this the same problem? Or something different?

[xwu]

@mattyoung No, that is different and also not a problem, as it works as expected. It is logically impossible to make typed pointers to two types with possibly distinct representations in memory become interchangeable with each other.

[mattyoung]

Looks like this is fixed: #40047

[xwu]

@mattyoung No, that is a tailored diagnostic to explain that this is not supported.

[jreference]

@xedin Hello, I'm new to this project and tried working on this issue since it's a StarterBug. I went through the code with a debugger and did some experiments. I would like some help please.

1. How do we generate implicit map calls within ExprRewriter ? I looked at the AST generated when the compiler processes a regular someOptional.map call but it is quite large and complex. In particular, i'm not sure how I can get a reference to Optional::map I can use with a DotSyntaxCallExpr

2. There was a comment stating that CGFloat/Double conversion needs to wait until all Optional-to-Optional conversions are performed for a particular location. Does this comment mean waiting for all the existing coerceToOptional and DoubleToCGFloat logic within coerceToType() to finish first?