Discard non-indexed relations instead of entire references.

lib/Index/Index.cpp

@@ -549,7 +549,9 @@ class IndexSwiftASTWalker : public SourceEntityWalker {
     assert(D);
     if (auto *VD = dyn_cast<ValueDecl>(D)) {
       if (!shouldIndex(VD, /*IsRef*/ true))
-        return true;
+        // Let the caller continue while discarding the relation to a symbol
+        // that won't appear in the index.
+        return false;
     }
     auto Match = std::find_if(Info.Relations.begin(), Info.Relations.end(),
                               [D](IndexRelation R) { return R.decl == D; });

test/Index/index_private_type_receiver.swift

@@ -0,0 +1,66 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/SDK)
+// RUN: split-file %s %t
+//
+// RUN: mkdir -p %t/SDK/Frameworks/FakeSystem.framework/Modules/FakeSystem.swiftmodule
+// RUN: %target-swift-frontend \
+// RUN:     -emit-module \
+// RUN:     -module-name FakeSystem \
+// RUN:     -o %t/SDK/Frameworks/FakeSystem.framework/Modules/FakeSystem.swiftmodule/%module-target-triple.swiftmodule \
+// RUN:     -swift-version 5 \
+// RUN:     %t/FakeSystem.swift
+//
+// RUN: %empty-directory(%t/idx)
+// RUN: %empty-directory(%t/modulecache)
+//
+// --- Built with indexing
+// RUN: %target-swift-frontend \
+// RUN:     -typecheck \
+// RUN:     -index-system-modules \
+// RUN:     -index-ignore-stdlib \
+// RUN:     -index-store-path %t/idx \
+// RUN:     -sdk %t/SDK \
+// RUN:     -Fsystem %t/SDK/Frameworks \
+// RUN:     -module-cache-path %t/modulecache \
+// RUN:     %t/view.swift
+//
+// --- Check the index.
+// RUN: c-index-test core -print-record %t/idx | %FileCheck %s
+//
+
+//--- FakeSystem.swift
+public protocol View {
+}
+
+public protocol ViewModifier {
+  associatedtype Body: View
+  typealias Content = _HiddenView<Self>
+  func body(content: Self.Content) -> Self.Body
+}
+
+public struct _HiddenView<M>: View where M: ViewModifier {
+  init(m: M) {}
+}
+
+public struct _HiddenModifier: ViewModifier {
+  public func body(content: Content) -> some View {
+    return _HiddenView(m: self)
+  }
+}
+
+extension View {
+  public func background() -> some View {
+    return _HiddenView(m: _HiddenModifier())
+  }
+}
+
+//--- view.swift
+import FakeSystem
+
+private struct Mod: FakeSystem.ViewModifier {
+  func body(content: Content) -> some View {
+    content
+      .background()
+    // CHECK: 6:8 | instance-method/Swift | s:10FakeSystem4ViewPAAE10backgroundQryF | Ref,Call,Dyn{{.*}}
+  }
+}