[Sema] Only warn when exporting implementation-only types in SPI

Downgrade to a warning the diagnostics on the use of an
implemenetation-only type in SPI to allow support for
-experimental-spi-imports.

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -2697,6 +2697,12 @@ ERROR(decl_from_hidden_module,none,
       "it is an SPI imported from %3|"
       "it is SPI}4",
       (DescriptiveDeclKind, DeclName, unsigned, Identifier, unsigned))
+WARNING(decl_from_hidden_module_warn,none,
+      "cannot use %0 %1 %select{in SPI|as property wrapper in SPI|"
+      "in an extension with public or '@usableFromInline' members|"
+      "in an extension with conditional conformances}2; "
+      "%select{%3 has been imported as implementation-only}4",
+      (DescriptiveDeclKind, DeclName, unsigned, Identifier, unsigned))
 ERROR(conformance_from_implementation_only_module,none,
       "cannot use conformance of %0 to %1 %select{here|as property wrapper here|"
       "in an extension with public or '@usableFromInline' members|"

lib/Sema/TypeCheckAccess.cpp

@@ -1467,14 +1467,25 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
 
   // If there's an exportability problem with \p typeDecl, get its origin kind.
   static DisallowedOriginKind getDisallowedOriginKind(
-      const TypeDecl *typeDecl, const SourceFile &SF, const Decl *context) {
+      const TypeDecl *typeDecl, const SourceFile &SF, const Decl *context,
+      DowngradeToWarning &downgradeToWarning) {
+    downgradeToWarning = DowngradeToWarning::No;
     ModuleDecl *M = typeDecl->getModuleContext();
-    if (SF.isImportedImplementationOnly(M))
+    if (SF.isImportedImplementationOnly(M)) {
+      // Temporarily downgrade implementation-only exportability in SPI to
+      // a warning.
+      if (context->isSPI())
+        downgradeToWarning = DowngradeToWarning::Yes;
+
+      // Implementation-only imported, cannot be reexported.
       return DisallowedOriginKind::ImplementationOnly;
-    else if (typeDecl->isSPI() && !context->isSPI())
+    } else if (typeDecl->isSPI() && !context->isSPI()) {
+      // SPI can only be exported in SPI.
       return context->getModuleContext() == M ?
         DisallowedOriginKind::SPILocal :
         DisallowedOriginKind::SPIImported;
+    }
+
     return DisallowedOriginKind::None;
   };
 
@@ -1494,9 +1505,10 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
       const_cast<TypeRepr *>(typeRepr)->walk(TypeReprIdentFinder(
           [&](const ComponentIdentTypeRepr *component) {
         TypeDecl *typeDecl = component->getBoundDecl();
-        auto originKind = getDisallowedOriginKind(typeDecl, SF, context);
+        auto downgradeToWarning = DowngradeToWarning::No;
+        auto originKind = getDisallowedOriginKind(typeDecl, SF, context, downgradeToWarning);
         if (originKind != DisallowedOriginKind::None) {
-          diagnoser.diagnoseType(typeDecl, component, originKind);
+          diagnoser.diagnoseType(typeDecl, component, originKind, downgradeToWarning);
           foundAnyIssues = true;
         }
 
@@ -1524,9 +1536,10 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
         : SF(SF), context(context), diagnoser(diagnoser) {}
 
       void visitTypeDecl(const TypeDecl *typeDecl) {
-        auto originKind = getDisallowedOriginKind(typeDecl, SF, context);
+        auto downgradeToWarning = DowngradeToWarning::No;
+        auto originKind = getDisallowedOriginKind(typeDecl, SF, context, downgradeToWarning);
         if (originKind != DisallowedOriginKind::None)
-          diagnoser.diagnoseType(typeDecl, /*typeRepr*/nullptr, originKind);
+          diagnoser.diagnoseType(typeDecl, /*typeRepr*/nullptr, originKind, downgradeToWarning);
       }
 
       void visitSubstitutionMap(SubstitutionMap subs) {
@@ -1641,9 +1654,13 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
 
     void diagnoseType(const TypeDecl *offendingType,
                       const TypeRepr *complainRepr,
-                      DisallowedOriginKind originKind) const {
+                      DisallowedOriginKind originKind,
+                      DowngradeToWarning downgradeToWarning) const {
       ModuleDecl *M = offendingType->getModuleContext();
-      auto diag = D->diagnose(diag::decl_from_hidden_module,
+      auto errorOrWarning = downgradeToWarning == DowngradeToWarning::Yes?
+                            diag::decl_from_hidden_module_warn:
+                            diag::decl_from_hidden_module;
+      auto diag = D->diagnose(errorOrWarning,
                               offendingType->getDescriptiveKind(),
                               offendingType->getName(),
                               static_cast<unsigned>(reason), M->getName(),

test/SPI/implementation_only_spi_import_exposability.swift

@@ -26,8 +26,8 @@ public protocol IOIProtocol {}
 
 @_spi(A) @_implementationOnly import Lib
 
-@_spi(B) public func leakSPIStruct(_ a: SPIStruct) -> SPIStruct { fatalError() } // expected-error 2 {{cannot use struct 'SPIStruct' here; 'Lib' has been imported as implementation-only}}
-@_spi(B) public func leakIOIStruct(_ a: IOIStruct) -> IOIStruct { fatalError() } // expected-error 2 {{cannot use struct 'IOIStruct' here; 'Lib' has been imported as implementation-only}}
+@_spi(B) public func leakSPIStruct(_ a: SPIStruct) -> SPIStruct { fatalError() } // expected-warning 2 {{cannot use struct 'SPIStruct' in SPI; 'Lib' has been imported as implementation-only}}
+@_spi(B) public func leakIOIStruct(_ a: IOIStruct) -> IOIStruct { fatalError() } // expected-warning 2 {{cannot use struct 'IOIStruct' in SPI; 'Lib' has been imported as implementation-only}}
 
 public struct PublicStruct : IOIProtocol, SPIProtocol { // expected-error {{cannot use protocol 'IOIProtocol' here; 'Lib' has been imported as implementation-only}}
 // expected-error @-1 {{cannot use protocol 'SPIProtocol' here; 'Lib' has been imported as implementation-only}}
@@ -43,4 +43,9 @@ public struct PublicStruct : IOIProtocol, SPIProtocol { // expected-error {{cann
   }
 }
 
+@_spi(B)
+public struct LocalSPIStruct : IOIProtocol, SPIProtocol { // expected-warning {{cannot use protocol 'IOIProtocol' in SPI; 'Lib' has been imported as implementation-only}}
+// expected-warning @-1 {{cannot use protocol 'SPIProtocol' in SPI; 'Lib' has been imported as implementation-only}}
+}
+
 #endif