diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index a32c529832bec..5438376242de5 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -753,23 +753,11 @@ void importer::getNormalInvocationArguments(
   invocationArgStrs.push_back("-fapinotes-swift-version=" +
                               languageVersion.asAPINotesVersionString());
 
-  // Prefer `-sdk` paths.
-  if (!searchPathOpts.getSDKPath().empty()) {
-    llvm::SmallString<261> path{searchPathOpts.getSDKPath()};
-    llvm::sys::path::append(path, "usr", "lib", "swift", "apinotes");
-
-    invocationArgStrs.push_back("-iapinotes-modules");
-    invocationArgStrs.push_back(path.str().str());
-  }
-
-  // Fallback to "legacy" `-resource-dir` paths.
-  {
     llvm::SmallString<261> path{searchPathOpts.RuntimeResourcePath};
     llvm::sys::path::append(path, "apinotes");
 
     invocationArgStrs.push_back("-iapinotes-modules");
     invocationArgStrs.push_back(path.str().str());
-  }
 }
 
 static void
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index d3681b51d1614..bc8cfc49ca8cf 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -2166,6 +2166,7 @@ static bool validateSwiftModuleFileArgumentAndAdd(const std::string &swiftModule
 
 static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
                                 DiagnosticEngine &Diags,
+                                const llvm::Triple &Triple,
                                 const CASOptions &CASOpts,
                                 const FrontendOptions &FrontendOpts,
                                 StringRef workingDirectory) {
@@ -2300,6 +2301,22 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
 
   if (const Arg *A = Args.getLastArg(OPT_resource_dir))
     Opts.RuntimeResourcePath = A->getValue();
+  else if (!Triple.isOSDarwin() && Args.hasArg(OPT_sdk)) {
+    llvm::SmallString<128> SDKResourcePath(Opts.getSDKPath());
+    llvm::sys::path::append(
+        SDKResourcePath, "usr", "lib",
+        FrontendOpts.UseSharedResourceFolder ? "swift" : "swift_static");
+    // Check for eg <sdkRoot>/usr/lib/swift/
+    if (llvm::sys::fs::exists(SDKResourcePath))
+      Opts.RuntimeResourcePath = SDKResourcePath.str();
+    else {
+      llvm::outs() << "You passed in an external -sdk without a Swift runtime.\n";
+      llvm::outs() << "Either specify a directory containing the runtime libraries with\n";
+      llvm::outs() << "the -resource-dir flag, or use -sysroot instead to point at a C/C++\n";
+      llvm::outs() << "sysroot alone. Falling back to << Opts.RuntimeResourcePath << for\n";
+      llvm::outs() << "the Swift runtime modules and libraries.\n";
+    }
+  }
 
   Opts.SkipRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdimport);
   Opts.ExcludeSDKPathsFromRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdlibimport);
@@ -3908,7 +3925,7 @@ bool CompilerInvocation::parseArgs(
 
   ParseSymbolGraphArgs(SymbolGraphOpts, ParsedArgs, Diags, LangOpts);
 
-  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags,
+  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags, LangOpts.Target,
                           CASOpts, FrontendOpts, workingDirectory)) {
     return true;
   }