[ModuleInterface] Only skip extensions without public members

Additional restriction to keep printing some extensions to
implementation-only imported types. This will be accepted only if the
extension is marked SPI.

Author: xymus

lib/AST/ASTPrinter.cpp

@@ -169,12 +169,22 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(bool preferTypeRepr,
         if (!shouldPrint(ED->getExtendedNominal(), options))
           return false;
 
-        // Skip extensions to implementation-only imported types.
+        // Skip extensions to implementation-only imported types that have
+        // no public members.
         auto localModule = ED->getParentModule();
         auto nominalModule = ED->getExtendedNominal()->getParentModule();
         if (localModule != nominalModule &&
-            localModule->isImportedImplementationOnly(nominalModule))
-          return false;
+            localModule->isImportedImplementationOnly(nominalModule)) {
+
+          bool shouldPrintMembers = llvm::any_of(
+                                      ED->getMembers(),
+                                      [&](const Decl *member) -> bool {
+            return shouldPrint(member, options);
+          });
+
+          if (!shouldPrintMembers)
+            return false;
+        }
 
         for (const Requirement &req : ED->getGenericRequirements()) {
           if (!isPublicOrUsableFromInline(req.getFirstType()))

test/SPI/experimental_spi_imports_swiftinterface.swift

@@ -4,7 +4,7 @@
 
 /// Generate 3 empty modules.
 // RUN: touch %t/empty.swift
-// RUN: %target-swift-frontend -emit-module %t/empty.swift -module-name ExperimentalImported -emit-module-path %t/ExperimentalImported.swiftmodule -swift-version 5 -enable-library-evolution
+// RUN: %target-swift-frontend -emit-module %S/Inputs/ioi_helper.swift -module-name ExperimentalImported -emit-module-path %t/ExperimentalImported.swiftmodule -swift-version 5 -enable-library-evolution
 // RUN: %target-swift-frontend -emit-module %t/empty.swift -module-name IOIImported -emit-module-path %t/IOIImported.swiftmodule -swift-version 5 -enable-library-evolution
 // RUN: %target-swift-frontend -emit-module %t/empty.swift -module-name SPIImported -emit-module-path %t/SPIImported.swiftmodule -swift-version 5 -enable-library-evolution
 
@@ -24,3 +24,10 @@
 @_spi(dummy) import SPIImported
 // CHECK-PUBLIC: {{^}}import SPIImported
 // CHECK-PRIVATE: @_spi{{.*}} import SPIImported
+
+@_spi(X)
+extension IOIPublicStruct {
+  public func foo() {}
+}
+// CHECK-PUBLIC-NOT: IOIPublicStruct
+// CHECK-PRIVATE: @_spi{{.*}} extension IOIPublicStruct