swift-testing: closure with try only inside #expect fails 'errors thrown from here are not handled'

Swift 6.2.4 / swift-testing case: try await helper { fixture in #expect(try await fixture.call() == x) }. The helper's parameter is typed (Fixture) async throws -> Void, yet every #expect site fails with 'errors thrown from here are not handled' (in the macro expansion Testing.__checkValue(try await ...)). This only happens when the closure's only try is inside the macro; a closure that also has a let v = try await ... outside the macro compiles. Fix: write the effects explicitly, { fixture async throws in ... }, or hoist the call out of #expect. A related error with the same root: #expect(value.mutatingMethod()) fails with 'cannot use mutating member on immutable value: $0', because the macro passes the receiver as an immutable $0; call it first and then check the result. If your runner rewrites #expect into plain checks, it won't see these errors: build the real target with swift build --build-tests.