Soften new unavailable conformance diagnostics

In #41054, we fixed an oversight which caused us to not notice when a user erased a concrete type to an existential using an unavailable conformance. Unfortunately, this is source-breaking and needs to be reduced to a warning in Swift 5 mode unless the user opts in.

Fixes rdar://91940820.

Author: Becca Royal-Gordon

lib/Sema/ResilienceDiagnostics.cpp

@@ -158,7 +158,8 @@ bool
 TypeChecker::diagnoseConformanceExportability(SourceLoc loc,
                                               const RootProtocolConformance *rootConf,
                                               const ExtensionDecl *ext,
-                                              const ExportContext &where) {
+                                              const ExportContext &where,
+                                              bool useConformanceAvailabilityErrorsOption) {
   if (!where.mustOnlyReferenceExportedDecls())
     return false;
 
@@ -178,6 +179,9 @@ TypeChecker::diagnoseConformanceExportability(SourceLoc loc,
                      rootConf->getProtocol()->getName(),
                      static_cast<unsigned>(*reason),
                      M->getName(),
-                     static_cast<unsigned>(originKind));
+                     static_cast<unsigned>(originKind))
+      .warnUntilSwiftVersionIf(useConformanceAvailabilityErrorsOption &&
+                               !ctx.LangOpts.EnableConformanceAvailabilityErrors,
+                               6);
   return true;
 }

lib/Sema/TypeCheckAvailability.cpp

@@ -2584,7 +2584,8 @@ bool swift::diagnoseExplicitUnavailability(const ValueDecl *D, SourceRange R,
 bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
                                            const RootProtocolConformance *rootConf,
                                            const ExtensionDecl *ext,
-                                           const ExportContext &where) {
+                                           const ExportContext &where,
+                                           bool useConformanceAvailabilityErrorsOption) {
   auto *attr = AvailableAttr::isUnavailable(ext);
   if (!attr)
     return false;
@@ -2641,7 +2642,10 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   diags.diagnose(loc, diag::conformance_availability_unavailable,
                  type, proto,
                  platform.empty(), platform, EncodedMessage.Message)
-      .limitBehavior(behavior);
+      .limitBehavior(behavior)
+      .warnUntilSwiftVersionIf(useConformanceAvailabilityErrorsOption &&
+                               !ctx.LangOpts.EnableConformanceAvailabilityErrors,
+                               6);
 
   switch (attr->getVersionAvailability(ctx)) {
   case AvailableVersionComparison::Available:
@@ -2995,7 +2999,8 @@ class ExprAvailabilityWalker : public ASTWalker {
 
     if (auto EE = dyn_cast<ErasureExpr>(E)) {
       for (ProtocolConformanceRef C : EE->getConformances()) {
-        diagnoseConformanceAvailability(E->getLoc(), C, Where);
+        diagnoseConformanceAvailability(E->getLoc(), C, Where, Type(), Type(),
+                                        /*useConformanceAvailabilityErrorsOpt=*/true);
       }
     }
 
@@ -3777,7 +3782,8 @@ bool
 swift::diagnoseConformanceAvailability(SourceLoc loc,
                                        ProtocolConformanceRef conformance,
                                        const ExportContext &where,
-                                       Type depTy, Type replacementTy) {
+                                       Type depTy, Type replacementTy,
+                                       bool useConformanceAvailabilityErrorsOption) {
   assert(!where.isImplicit());
 
   if (!conformance.isConcrete())
@@ -3811,12 +3817,14 @@ swift::diagnoseConformanceAvailability(SourceLoc loc,
   };
 
   if (auto *ext = dyn_cast<ExtensionDecl>(rootConf->getDeclContext())) {
-    if (TypeChecker::diagnoseConformanceExportability(loc, rootConf, ext, where)) {
+    if (TypeChecker::diagnoseConformanceExportability(loc, rootConf, ext, where,
+                                                      useConformanceAvailabilityErrorsOption)) {
       maybeEmitAssociatedTypeNote();
       return true;
     }
 
-    if (diagnoseExplicitUnavailability(loc, rootConf, ext, where)) {
+    if (diagnoseExplicitUnavailability(loc, rootConf, ext, where,
+                                       useConformanceAvailabilityErrorsOption)) {
       maybeEmitAssociatedTypeNote();
       return true;
     }
@@ -3844,7 +3852,8 @@ swift::diagnoseConformanceAvailability(SourceLoc loc,
   SubstitutionMap subConformanceSubs =
       concreteConf->getSubstitutions(DC->getParentModule());
   if (diagnoseSubstitutionMapAvailability(loc, subConformanceSubs, where,
-                                          depTy, replacementTy))
+                                          depTy, replacementTy,
+                                          useConformanceAvailabilityErrorsOption))
     return true;
 
   return false;
@@ -3854,11 +3863,13 @@ bool
 swift::diagnoseSubstitutionMapAvailability(SourceLoc loc,
                                            SubstitutionMap subs,
                                            const ExportContext &where,
-                                           Type depTy, Type replacementTy) {
+                                           Type depTy, Type replacementTy,
+                                           bool useConformanceAvailabilityErrorsOption) {
   bool hadAnyIssues = false;
   for (ProtocolConformanceRef conformance : subs.getConformances()) {
     if (diagnoseConformanceAvailability(loc, conformance, where,
-                                        depTy, replacementTy))
+                                        depTy, replacementTy,
+                                        useConformanceAvailabilityErrorsOption))
       hadAnyIssues = true;
   }
   return hadAnyIssues;

lib/Sema/TypeCheckAvailability.h

@@ -220,14 +220,16 @@ diagnoseConformanceAvailability(SourceLoc loc,
                                 ProtocolConformanceRef conformance,
                                 const ExportContext &context,
                                 Type depTy=Type(),
-                                Type replacementTy=Type());
+                                Type replacementTy=Type(),
+                                bool useConformanceAvailabilityErrorsOption = false);
 
 bool
 diagnoseSubstitutionMapAvailability(SourceLoc loc,
                                     SubstitutionMap subs,
                                     const ExportContext &context,
                                     Type depTy=Type(),
-                                    Type replacementTy=Type());
+                                    Type replacementTy=Type(),
+                                    bool useConformanceAvailabilityErrorsOption = false);
 
 /// Diagnose uses of unavailable declarations. Returns true if a diagnostic
 /// was emitted.
@@ -261,7 +263,8 @@ bool diagnoseExplicitUnavailability(
     SourceLoc loc,
     const RootProtocolConformance *rootConf,
     const ExtensionDecl *ext,
-    const ExportContext &where);
+    const ExportContext &where,
+    bool useConformanceAvailabilityErrorsOption = false);
 
 /// Check if \p decl has a introduction version required by -require-explicit-availability
 void checkExplicitAvailability(Decl *decl);

lib/Sema/TypeChecker.h

@@ -999,7 +999,8 @@ bool diagnoseDeclRefExportability(SourceLoc loc,
 bool diagnoseConformanceExportability(SourceLoc loc,
                                       const RootProtocolConformance *rootConf,
                                       const ExtensionDecl *ext,
-                                      const ExportContext &where);
+                                      const ExportContext &where,
+                                      bool useConformanceAvailabilityErrorsOpt = false);
 
 /// \name Availability checking
 ///

test/Sema/conformance_availability_warn.swift

@@ -52,3 +52,33 @@ func passAvailableConformance1a(x: HasAvailableConformance1) {
   _ = x.isGalloping
   _ = UsesHorse<HasAvailableConformance1>.self
 }
+
+// Explicit unavailability
+public struct HasAvailableConformance2 {}
+
+@available(*, unavailable)
+extension HasAvailableConformance2 : Horse {} // expected-note 6 {{conformance of 'HasAvailableConformance2' to 'Horse' has been explicitly marked unavailable here}}
+
+// Some availability diagnostics become warnings in Swift 5 mode without
+// -enable-conformance-availability-errors because they were incorrectly
+// accepted before and rejecting them would break source compatibility. Others
+// are unaffected because they have always been rejected.
+
+func passAvailableConformance2(x: HasAvailableConformance2) {
+  takesHorse(x) // expected-error {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable}}
+  takesHorseExistential(x) // expected-warning {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable; this is an error in Swift 6}}
+  x.giddyUp() // expected-error {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable}}
+  _ = x.isGalloping // expected-error {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable}}
+  _ = x[keyPath: \.isGalloping] // expected-error {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable}}
+  _ = UsesHorse<HasAvailableConformance2>.self // expected-error {{conformance of 'HasAvailableConformance2' to 'Horse' is unavailable}}
+}
+
+@available(*, unavailable)
+func passAvailableConformance2a(x: HasAvailableConformance2) {
+  takesHorse(x)
+  takesHorseExistential(x)
+  x.giddyUp()
+  _ = x.isGalloping
+  _ = UsesHorse<HasAvailableConformance2>.self
+}
+