Allow SPI access within the same module in checkAccess

lib/AST/Decl.cpp

@@ -3500,7 +3500,10 @@ static bool checkAccess(const DeclContext *useDC, const ValueDecl *VD,
   case AccessLevel::Public:
   case AccessLevel::Open: {
     if (useDC && VD->isSPI()) {
-      auto *useSF = dyn_cast<SourceFile>(useDC->getModuleScopeContext());
+      auto useModuleScopeContext = useDC->getModuleScopeContext();
+      if (useModuleScopeContext == sourceDC->getModuleScopeContext()) return true;
+
+      auto *useSF = dyn_cast<SourceFile>(useModuleScopeContext);
       return !useSF || useSF->isImportedAsSPI(VD);
     }
     return true;

test/SPI/equatable_and_conformances.swift

@@ -0,0 +1,37 @@
+// Test how SPI affects access control in protocol conformance lookup.
+
+// RUN: %target-build-swift %s -swift-version 5
+
+// rdar://61043406
+
+@_spi(Private)
+public struct SPIEquatable : Equatable {
+  public static func ==(lhs: SPIEquatable, rhs: SPIEquatable) -> Bool {
+    return true
+  }
+}
+
+// rdar://61987739
+
+@_spi(Private)
+public struct SPIStruct {}
+
+public protocol PublicProto {
+  func thingOne()
+
+  @_spi(Private)
+  func thingTwo(_ data: SPIStruct)
+}
+
+extension PublicProto {
+  @_spi(Private)
+  public func thingTwo(_ data: SPIStruct) {
+    // default implementation
+  }
+}
+
+fileprivate struct FilePrivateStruct: PublicProto {
+  func thingOne() {
+    // OK
+  }
+}