Sema: Gate SPI operator fix behind EnforceSPIOperatorGroup

This new check is source breaking. Let's gate it behind an opt-in flag
for now and enable it by default on a new language mode or similar.

Author: xymus

include/swift/Basic/Features.def

@@ -561,6 +561,9 @@ EXPERIMENTAL_FEATURE(CheckImplementationOnly, true)
 /// report references to implementation-only imported modules.
 EXPERIMENTAL_FEATURE(CheckImplementationOnlyStrict, false)
 
+/// Check that use sites have the required @_spi import for operators.
+EXPERIMENTAL_FEATURE(EnforceSPIOperatorGroup, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/Decl.cpp

@@ -5537,7 +5537,9 @@ static bool checkAccess(const DeclContext *useDC, const ValueDecl *VD,
   }
   case AccessLevel::Public:
   case AccessLevel::Open: {
-    if (VD->isOperator() && VD->isSPI()) {
+    if (VD->getASTContext().LangOpts.hasFeature(
+            Feature::EnforceSPIOperatorGroup) &&
+        VD->isOperator() && VD->isSPI()) {
       const DeclContext *useFile = useDC->getModuleScopeContext();
       if (useFile->getParentModule() == sourceDC->getParentModule())
         return true;

lib/AST/FeatureSet.cpp

@@ -146,6 +146,7 @@ UNINTERESTING_FEATURE(SameElementRequirements)
 UNINTERESTING_FEATURE(SendingArgsAndResults)
 UNINTERESTING_FEATURE(CheckImplementationOnly)
 UNINTERESTING_FEATURE(CheckImplementationOnlyStrict)
+UNINTERESTING_FEATURE(EnforceSPIOperatorGroup)
 
 static bool findUnderscoredLifetimeAttr(Decl *decl) {
   auto hasUnderscoredLifetimeAttr = [](Decl *decl) {

test/SPI/public_client.swift

@@ -6,17 +6,21 @@
 // RUN: %target-swift-frontend -emit-module %S/Inputs/spi_helper.swift -module-name SPIHelper -emit-module-path %t/SPIHelper.swiftmodule -emit-module-interface-path %t/SPIHelper.swiftinterface -emit-private-module-interface-path %t/SPIHelper.private.swiftinterface -enable-library-evolution -swift-version 5 -parse-as-library
 
 /// Reading from swiftmodule
-// RUN: %target-typecheck-verify-swift -verify-ignore-unrelated -I %t -verify-ignore-unknown
+// RUN: %target-typecheck-verify-swift -verify-ignore-unrelated -I %t -verify-ignore-unknown \
+// RUN:   -enable-experimental-feature EnforceSPIOperatorGroup
 
 /// Reading from .private.swiftinterface
 // RUN: rm %t/SPIHelper.swiftmodule
-// RUN: %target-typecheck-verify-swift -verify-ignore-unrelated -I %t -verify-ignore-unknown
+// RUN: %target-typecheck-verify-swift -verify-ignore-unrelated -I %t -verify-ignore-unknown \
+// RUN:   -enable-experimental-feature EnforceSPIOperatorGroup
 
 /// Reading from the public .swiftinterface should raise errors on missing
 /// declarations.
 // RUN: rm %t/SPIHelper.private.swiftinterface
 // RUN: not %target-swift-frontend -typecheck -I %t %s
 
+// REQUIRES: swift_feature_EnforceSPIOperatorGroup
+
 import SPIHelper
 
 // Use the public API