Sema: Don't derive witnesses in swiftinterface files

lib/Sema/TypeCheckProtocol.cpp

@@ -3963,22 +3963,25 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
   // Determine whether we can derive a witness for this requirement.
   bool canDerive = false;
 
-  // Can a witness for this requirement be derived for this nominal type?
-  if (auto derivable = DerivedConformance::getDerivableRequirement(
-                         nominal,
-                         requirement)) {
-    if (derivable == requirement) {
-      // If it's the same requirement, we can derive it here.
-      canDerive = true;
-    } else {
-      // Otherwise, go satisfy the derivable requirement, which can introduce
-      // a member that could in turn satisfy *this* requirement.
-      auto derivableProto = cast<ProtocolDecl>(derivable->getDeclContext());
-      auto conformance =
-          TypeChecker::conformsToProtocol(Adoptee, derivableProto,
-                                          DC->getParentModule());
-      if (conformance.isConcrete()) {
-        (void)conformance.getConcrete()->getWitnessDecl(derivable);
+  auto *SF = DC->getParentSourceFile();
+  if (!(SF == nullptr || SF->Kind == SourceFileKind::Interface)) {
+    // Can a witness for this requirement be derived for this nominal type?
+    if (auto derivable = DerivedConformance::getDerivableRequirement(
+                           nominal,
+                           requirement)) {
+      if (derivable == requirement) {
+        // If it's the same requirement, we can derive it here.
+        canDerive = true;
+      } else {
+        // Otherwise, go satisfy the derivable requirement, which can introduce
+        // a member that could in turn satisfy *this* requirement.
+        auto derivableProto = cast<ProtocolDecl>(derivable->getDeclContext());
+        auto conformance =
+            TypeChecker::conformsToProtocol(Adoptee, derivableProto,
+                                            DC->getParentModule());
+        if (conformance.isConcrete()) {
+          (void)conformance.getConcrete()->getWitnessDecl(derivable);
+        }
       }
     }
   }
@@ -4288,6 +4291,10 @@ ResolveWitnessResult ConformanceChecker::resolveWitnessViaDerivation(
                        ValueDecl *requirement) {
   assert(!isa<AssociatedTypeDecl>(requirement) && "Use resolveTypeWitnessVia*");
 
+  auto *SF = DC->getParentSourceFile();
+  if (SF != nullptr && SF->Kind == SourceFileKind::Interface)
+    return ResolveWitnessResult::Missing;
+
   // Find the declaration that derives the protocol conformance.
   NominalTypeDecl *derivingTypeDecl = nullptr;
   auto *nominal = Adoptee->getAnyNominal();

test/ModuleInterface/DerivedWitnesses.swiftinterface

@@ -0,0 +1,44 @@
+// RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
+
+// swift-interface-format-version: 1.0
+// swift-compiler-version: Apple Swift version 5.5 (swiftlang-1300.0.29.102 clang-1300.0.28.1)
+// swift-module-flags: -target x86_64-apple-macosx11.0 -enable-objc-interop -enable-library-evolution -module-name DerivedWitnesses
+import Swift
+
+public enum HasSynthesizedEquals : Int {
+  case x
+  case y
+
+  public static func == (a: HasSynthesizedEquals, b: HasSynthesizedEquals) -> Bool
+  public func hash(into hasher: inout Hasher)
+  public var hashValue: Int {
+    get
+  }
+
+  public init?(rawValue: Int)
+  public typealias RawValue = Int
+  public var rawValue: Int {
+    get
+  }
+}
+
+public enum UsesDefaultEquals : Int {
+  case x
+  case y
+
+  public init?(rawValue: Int)
+  public typealias RawValue = Int
+  public var rawValue: Int {
+    get
+  }
+}
+
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s16DerivedWitnesses20HasSynthesizedEqualsOSQAASQ2eeoiySbx_xtFZTW : $@convention(witness_method: Equatable) (@in_guaranteed HasSynthesizedEquals, @in_guaranteed HasSynthesizedEquals, @thick HasSynthesizedEquals.Type) -> Bool {
+// CHECK: bb0(%0 : $*HasSynthesizedEquals, %1 : $*HasSynthesizedEquals, %2 : $@thick HasSynthesizedEquals.Type):
+// CHECK: function_ref @$s16DerivedWitnesses20HasSynthesizedEqualsO2eeoiySbAC_ACtFZ : $@convention(method) (HasSynthesizedEquals, HasSynthesizedEquals, @thin HasSynthesizedEquals.Type) -> Bool
+// CHECK: return
+
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s16DerivedWitnesses17UsesDefaultEqualsOSQAASQ2eeoiySbx_xtFZTW : $@convention(witness_method: Equatable) (@in_guaranteed UsesDefaultEquals, @in_guaranteed UsesDefaultEquals, @thick UsesDefaultEquals.Type) -> Bool {
+// CHECK: bb0(%0 : $*UsesDefaultEquals, %1 : $*UsesDefaultEquals, %2 : $@thick UsesDefaultEquals.Type):
+// CHECK: function_ref @$ss2eeoiySbx_xtSYRzSQ8RawValueRpzlF : $@convention(thin) <τ_0_0 where τ_0_0 : RawRepresentable, τ_0_0.RawValue : Equatable> (@in_guaranteed τ_0_0, @in_guaranteed τ_0_0) -> Bool
+// CHECK: return