Diagnose public protocol requirements witnessed by internal protocol extensions

include/swift/Basic/Features.def

@@ -564,6 +564,8 @@ EXPERIMENTAL_FEATURE(CheckImplementationOnlyStrict, false)
 /// Check that use sites have the required @_spi import for operators.
 EXPERIMENTAL_FEATURE(EnforceSPIOperatorGroup, true)
 
+EXPERIMENTAL_FEATURE(StrictSemaForTextualInterface, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -466,6 +466,7 @@ static bool usesFeatureTildeSendable(Decl *decl) {
 }
 
 UNINTERESTING_FEATURE(AnyAppleOSAvailability)
+UNINTERESTING_FEATURE(StrictSemaForTextualInterface)
 
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet

lib/Sema/TypeCheckProtocol.cpp

@@ -1762,11 +1762,14 @@ static bool checkWitnessAccess(DeclContext *dc,
 
   auto actualScopeToCheck = requiredAccessScope.first;
 
+  // without StrictTextualInterfaceChecking feature forConformance is set true
+  bool forConformance = not dc->getASTContext().LangOpts.hasFeature
+                            (Feature::StrictSemaForTextualInterface);
   // Setting the 'forConformance' flag means that we admit witnesses in
   // protocol extensions that we can see, but are not necessarily as
   // visible as the conforming type and protocol.
   if (!witness->isAccessibleFrom(actualScopeToCheck.getDeclContext(),
-                                 /*forConformance=*/true)) {
+                                 forConformance)) {
     // Special case: if we have `@testable import` of the witness's module,
     // allow the witness to match if it would have matched for just this file.
     // That is, if '@testable' allows us to see the witness here, it should
@@ -4453,7 +4456,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
                        diagKind, getProtocolRequirementKind(requirement),
                        witness, isSetter, requiredAccess,
                        protoAccessScope.accessLevelForDiagnostics(),
-                       proto);
+                       proto).warnUntilFutureSwiftVersionIf(DC->getASTContext().LangOpts.hasFeature(Feature::StrictSemaForTextualInterface));
 
         auto *decl = dyn_cast<AbstractFunctionDecl>(witness);
         if (decl && decl->isSynthesized())

test/Sema/diag_internal_protocol_access_check.swift

@@ -0,0 +1,31 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-typecheck-verify-swift -enable-library-evolution -enable-experimental-feature StrictSemaForTextualInterface
+
+public protocol P {
+  func foo() -> Int
+}
+internal protocol Q: P {
+  func fooImpl() -> Int32
+}
+extension Q {
+  public func foo() -> Int { // expected-note {{mark the instance method as 'public' to satisfy the requirement}}
+    return Int(self.fooImpl())
+  }
+}
+public struct S: Q { // expected-error {{method 'foo()' must be declared public because it matches a requirement in public protocol 'P'}}
+  internal func fooImpl() -> Int32 {
+    return 42
+  }
+}
+
+public struct Foo {
+  public init(value: Int) {}
+}
+public protocol PublicProtocol {
+  init?(integer: Int)
+}
+protocol InternalProtocol: PublicProtocol {}
+extension InternalProtocol {
+  public init(integer: Int) {} // expected-note {{mark the initializer as 'public' to satisfy the requirement}}
+}
+extension Foo: PublicProtocol, InternalProtocol {} // expected-error {{initializer 'init(integer:)' must be declared public because it matches a requirement in public protocol 'PublicProtocol'}}