Merge pull request #61954 from tshortli/spi-witness-availability

Sema: Relax the availability requirements for `@_spi` protocol witnesses

Author: swift-ci

lib/Sema/TypeCheckDecl.cpp

@@ -1707,6 +1707,20 @@ bool TypeChecker::isAvailabilitySafeForConformance(
   AvailabilityContext infoForConformingDecl =
       overApproximateAvailabilityAtLocation(dc->getAsDecl()->getLoc(), dc);
 
+  // Relax the requirements for @_spi witnesses by treating the requirement as
+  // if it were introduced at the deployment target. This is not strictly sound
+  // since clients of SPI do not necessarily have the same deployment target as
+  // the module declaring the requirement. However, now that the public
+  // declarations in API libraries are checked according to the minimum possible
+  // deployment target of their clients this relaxation is needed for source
+  // compatibility with some existing code and is reasonably safe for the
+  // majority of cases.
+  if (witness->isSPI()) {
+    AvailabilityContext deploymentTarget =
+        AvailabilityContext::forDeploymentTarget(Context);
+    requirementInfo.constrainWith(deploymentTarget);
+  }
+
   // Constrain over-approximates intersection of version ranges.
   witnessInfo.constrainWith(infoForConformingDecl);
   requirementInfo.constrainWith(infoForConformingDecl);

test/Interpreter/spi_witness_availability.swift

@@ -0,0 +1,69 @@
+// RUN: %target-run-simple-swift(-target %target-cpu-macos10.15) | \
+// RUN:   %FileCheck %s --check-prefix=CHECK-CONFORMANCES-NOT-AVAILABLE
+// RUN: %target-run-simple-swift(-target %target-cpu-macos10.15.4) | \
+// RUN:   %FileCheck %s --check-prefix=CHECK-CONFORMANCES-AVAILABLE
+// RUN: %target-run-simple-swift(-target %target-cpu-macos10.15.4 -library-level api) | \
+// RUN:   %FileCheck %s --check-prefix=CHECK-CONFORMANCES-AVAILABLE-API
+
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+
+@available(macOS 10.15, *)
+public protocol BaseProto {
+  static var foo: String { get }
+}
+
+@available(macOS 10.15, *)
+extension BaseProto {
+  public static var foo: String { return "Base" }
+}
+
+@available(macOS 10.15.4, *)
+public protocol DerivedProto: BaseProto {}
+
+@available(macOS 10.15.4, *)
+extension DerivedProto {
+  public static var foo: String { return "Derived" }
+}
+
+@_spi(Private)
+@available(macOS 10.15.4, *)
+public protocol SecretDerivedProto: BaseProto {
+}
+
+@_spi(Private)
+@available(macOS 10.15.4, *)
+extension SecretDerivedProto {
+  public static var foo: String { return "Secret" }
+}
+
+@available(macOS 10.15, *)
+public struct NoSecrets: BaseProto {
+  public init() {}
+}
+
+@available(macOS 10.15.4, *)
+extension NoSecrets: DerivedProto {}
+
+@available(macOS 10.15, *)
+public struct HasSecrets: BaseProto {
+  public init() {}
+}
+
+@_spi(Private)
+@available(macOS 10.15.4, *)
+extension HasSecrets: SecretDerivedProto {}
+
+@available(macOS 10.15, *)
+func doIt<T: BaseProto>(_: T) {
+  print(T.foo)
+}
+
+// CHECK-CONFORMANCES-NOT-AVAILABLE:  Base
+// CHECK-CONFORMANCES-AVAILABLE:      Derived
+// CHECK-CONFORMANCES-AVAILABLE-API:  Base
+doIt(NoSecrets())
+// CHECK-CONFORMANCES-NOT-AVAILABLE:  Base
+// CHECK-CONFORMANCES-AVAILABLE:      Secret
+// CHECK-CONFORMANCES-AVAILABLE-API:  Secret
+doIt(HasSecrets())