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Generic methods declared in protocols (and extensions thereof) cannot be used on existential values, because there is no way to specialize them for all potential types. Diagnose such cases in Embedded Swift mode and via `-Wwarning EmbeddedRestrictions`. This adds a bunch more warnings to the standard library that we'll need to clean up, probably by `#if`'ing more code out. Part of rdar://119383905.
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Embedded Swift language restrictions (EmbeddedRestrictions)
Embedded Swift is a subset of the Swift language that that introduces some restrictions on the use of language features to eliminate the need for the Swift runtime. These restrictions are captured by the EmbeddedRestrictions diagnostic group.
The Embedded Swift compilation model can produce extremely small binaries without external dependencies, suitable for restricted environments including embedded (microcontrollers) and baremetal setups (no operating system at all), and low-level environments (firmware, kernels, device drivers, low-level components of userspace OS runtimes). While the vast majority of Swift language features are available in Embedded Swift, there are some language features that require the full Swift standard library and runtime, which are not available in Embedded Swift.
Diagnostics in the EmbeddedRestrictions group describe those language features that cannot be used in Embedded Swift. These include:
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weakandunownedreferences, because Embedded Swift uses a simplified reference-counting model that cannot support them. For example:class Node { weak var parent: Node? // error: attribute 'weak' cannot be used in Embedded Swift }
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Non-final generic methods in a class, which are prohibited because they cannot be specialized for every possible call site. For example:
class MyGenericClass { func f(value: U) { } // warning: generic instance method 'f(value:)' in a class must be 'final' in Embedded Swift
func g() { } // okay, not generic relative to the class itself class func h() where T: P { } // warning: generic class method 'h()' in a class must be 'final' in Embedded Swift}
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Generic methods used on values of protocol type, which are prohibited because they cannot be specialized for every possible call site. For example:
protocol P: AnyObject { func doNothing() func doSomething(on value: T) }
func testGenerics<Value: P>(value: value, i: Int) { value.doNothing() // okay value.doSomething(on: i) // okay, always specialized }
func testValuesOfProtocolType(value: any P, i: Int) { value.doNothing() // okay value.doSomething(on: i) // warning: cannot use generic instance method 'doSomething(on:)' on a value of type 'any P' in Embedded Swift }