I keep thinking about how the crux of the challenge we're having is that marking a specific OSPlatform as supported implies that no other platforms are supported, but we don't always want that behavior.

[MinimumOSPlatform("windows7.0")]
public void WindowsOnlyAPI() {}

[MinimumOSPlatform("ios14")]
public void WorksEverywhereButOnIosVersion14IsRequired() {}

If we set that problem aside for a moment, and we focus purely on excluding an OSPlatform, then I think we're otherwise OK. The following would just work, wouldn't it?

[UnsupportedOSPlatform("browser")]
public void WorksEverywhereExceptBrowser() {}

I believe the ranges were being introduced so that we could use [UnsupportedOSPlatform] to denote a combination of supported and unsupported, specifying which version ranges are unsupported, leaving anything outside the range as supported without implying exclusive platform support. Using [UnsupportedOSPlatform] in that way feels awkward to me, and less expressive than using [MinimumOSPlatform] to indicate support.

I'm really concerned about the range support, regardless of the syntax. It makes the data harder to reason over as a human code reader, and potentially more brittle when reasoning over in the analyzer implementation. I think we'd introduce a higher likelihood that the data will be incomprehensible and then the analyzer will need to introduce more diagnostics for when that's the case.

So what if we did the following?

1. Rename RemovedInOSPlatformAttribute to UnsupportedOSPlatformAttribute, but otherwise leave it as-is
   • This specifies the lower-bound version in which an API became unsupported
2. Add a property to MinimumOSPlatformAttribute that indicates whether or not the platform should be treated as exclusive

I think we could then satisfy our scenarios:

[MinimumOSPlatform("windows7.0")]
public void WindowsOnlyAPI() {}

[MinimumOSPlatform("ios14.0", IsExclusive = false)]
public void WorksEverywhereButOnIosVersion14IsRequired() {}

[UnsupportedOSPlatform("browser")]
public void WorksEverywhereExceptBrowser() {}

The evolution of an API going from unsupported to having a minimum version required for support would involve changing [UnsupportedOSPlatform("ios")] to [MinimumOSPlatform("ios14", IsExclusive = false)]. If there are multiple platforms that add support over time, then they would each need to specify IsExclusive = false.

To show an edge case where something comes in and out of support for a platform repeatedly:

[MinimumOSPlatform("ios12.0", IsExclusive = false)]
[UnsupportedOSPlatform("ios14.0")]
[MinimumOSPlatform("ios12.0", IsExclusive = false)]
[MinimumOSPlatform("ipados16.0", IsExclusive = false)]
public void Worked_In_12_And_13_But_Not_14_Or_15_But_Works_Again_In_16() {}

If this approach has merit, then I'd suggest that we also rename MinimumOSPlatformAttribute to SupportedOSPlatformAttribute. Both SupportedOSPlatform and UnsupportedOSPlatform would accept a platform name with an optional version such that the version is always treated as the lower-bound.

By default, a [SupportedOSPlatform] attribute denotes exclusive support on that platform. But specifying IsExclusive = false on the attribute allows support to be turned on for the platform without affecting support on other platforms. An [UnsupportedOSPlatform] attribute never implies anything about any other platforms.

To address the [concern of introducing another means for obsoleting an API]([MinimumOSPlatform("ios12.0", IsExclusive = false)]), I suggest we remove the ObsoletedInOSPlatformAttribute that we introduced.

To summarize, I propose that we:

1. Rename RemovedInOSPlatformAttribute to UnsupportedOSPlatformAttribute
2. Rename MinimumOSPlatformAttribute to SupportedOSPlatformAttribute
3. Remove ObsoletedInOSPlatformAttribute
4. Add an IsExclusive Boolean property to SupportedOSPlatformAttribute, that defaults to true

2. I think we had problems with the || based guards; I'll need @buyaa-n to weigh in there.

This is a more meaningful scenario than the other || example we discussed before, for this one we should try to add support