[Serialization] Fix reporting a dependency cycle with a missing clang module

rdar://problem/57364033

Author: xymus

lib/Serialization/SerializedModuleLoader.cpp

@@ -747,7 +747,8 @@ void swift::serialization::diagnoseSerializedASTLoadFailure(
     auto circularDependencyIter =
         llvm::find_if(loadedModuleFile->getDependencies(),
                       [](const ModuleFile::Dependency &next) {
-                        return !next.Import.second->hasResolvedImports();
+                        return next.isLoaded() &&
+                               !next.Import.second->hasResolvedImports();
                       });
     assert(circularDependencyIter !=
                loadedModuleFile->getDependencies().end() &&

test/Serialization/Recovery/missing-clang-module.swift

@@ -0,0 +1,32 @@
+//// Report dependency cycles involving missing clang modules without crashing
+//// rdar://problem/57364033
+
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/custom-modules %t/
+
+// RUN: %target-swift-frontend -emit-module -DLIB_A %s -module-name A -emit-module-path %t/A.swiftmodule
+// RUN: %target-swift-frontend -emit-module -DLIB_B %s -module-name B -emit-module-path %t/B.swiftmodule -I %t -I %t/custom-modules
+// RUN: %target-swift-frontend -emit-module -DLIB_C %s -module-name C -emit-module-path %t/C.swiftmodule -I %t
+
+//// Delete the clang module
+// RUN: rm -r %t/custom-modules/
+
+// RUN: not %target-swift-frontend -emit-module -DLIB_D %s -module-name A -emit-module-path %t/D.swiftmodule -I %t 2>&1 | %FileCheck %s
+
+#if LIB_A
+
+#elseif LIB_B
+
+import IndirectImport // From custom-modules
+import A
+
+#elseif LIB_C
+
+import B
+
+#elseif LIB_D
+
+import C
+// CHECK: <unknown>:0: error: circular dependency between modules 'A' and 'B'
+
+#endif