[6.1] [SWT-0006] Return the thrown error from #expect(throws:) and #require(throws:). #857
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…ire(throws:)`. (#780) This PR changes the signatures of the various `throws:` overloads of `#expect` and `#require` so that on success they return the error that was thrown rather than `Void`. This then allows more ergonomic inspection of the error's properties: ```swift let error = try #require(throws: MyError.self) { try f() } #expect(error.hasWidget) #expect(error.userName == "John Smith") ``` For more information, see [the proposal document](https://github.com/swiftlang/swift-testing/blob/jgrynspan/return-errors-from-expect-throws/Documentation/Proposals/0006-return-errors-from-expect-throws.md). Resolves rdar://138235250. <details> <summary>Further PR Details</summary> It is not possible to overload a macro or function solely by return type without the compiler reporting `Ambiguous use of 'f()'`, so we are not able to stage this change in using `@_spi(Experimental)` without breaking test code that already imports our SPI. This change is potentially source-breaking for tests that inadvertently forward the result of these macro invocations to an enclosing scope. For example, the compiler will start emitting a warning here: ```swift func bar(_ pfoo: UnsafePointer<Foo>) throws { ... } withUnsafePointer(to: foo) { pfoo in //⚠️ Result of call to 'withUnsafePointer(to:_:)' is unused #expect(throws: BadFooError.self) { try bar(pfoo) } } ``` This warning can be suppressed by assigning the result of `#expect` (or of `withUnsafePointer(to:_:)`) to `_`: ```swift func bar(_ pfoo: UnsafePointer<Foo>) throws { ... } withUnsafePointer(to: foo) { pfoo in _ = #expect(throws: BadFooError.self) { try bar(pfoo) } } ``` Because `#expect` and `#require` are macros, they cannot be referenced by name like functions, so you cannot assign them to variables (and then run into trouble with the types of those variables.) Finally, this change deprecates the variants of `#expect` and `#require` that take two closures: ```swift #expect { //⚠️ 'expect(_:sourceLocation:performing:throws:)' is deprecated: Examine the result of '#expect(throws:)' instead. ... } throws: { error in guard let error = error as? FoodTruckError else { return false } return error.napkinCount == 0 } ``` These variants are no longer needed because you can simply examine the result of the other variants: ```swift let error = #expect(throws: FoodTruckError.self) { ... } #expect(error?.napkinCount == 0) ``` </details> ### Checklist: - [x] Code and documentation should follow the style of the [Style Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md). - [x] If public symbols are renamed or modified, DocC references should be updated.
grynspan added enhancement grynspan requested review from SeanROlszewski, briancroom, dennisweissmann, stmontgomery and suzannaratcliff as code owners 
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stmontgomery approved these changes Dec 13, 2024
briancroom approved these changes Dec 13, 2024
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#expect(throws:)and#require(throws:)to return the thrown error and deprecate the double-closure overloads.#expect(throws:)and#require(throws:). #780-Werror.