diff --git a/clang-tools-extra/clangd/IncludeCleaner.cpp b/clang-tools-extra/clangd/IncludeCleaner.cpp
--- a/clang-tools-extra/clangd/IncludeCleaner.cpp
+++ b/clang-tools-extra/clangd/IncludeCleaner.cpp
@@ -122,16 +122,20 @@
   void add(const Decl *D) {
     if (!D || !isNew(D->getCanonicalDecl()))
       return;
-    // If we see a declaration in the mainfile, skip all non-definition decls.
-    // We only do this for classes because forward declarations are common and
-    // we don't want to include every header that forward-declares the symbol
-    // because they're often unrelated.
+    // Special case RecordDecls, as it is common for them to be forward
+    // declared multiple times. The most common cases are:
+    // - Definition available in TU, only mark that one as usage. The rest is
+    //   likely to be unnecessary. This might result in false positives when an
+    //   internal definition is visible.
+    // - There's a forward declaration in the main file, no need for other
+    //   redecls.
     if (const auto *RD = llvm::dyn_cast<RecordDecl>(D)) {
-      if (SM.isInMainFile(RD->getMostRecentDecl()->getBeginLoc())) {
-        if (const auto *Definition = RD->getMostRecentDecl()->getDefinition())
-          Result.insert(Definition->getLocation());
+      if (const auto *Definition = RD->getDefinition()) {
+        Result.insert(Definition->getLocation());
         return;
       }
+      if (SM.isInMainFile(RD->getMostRecentDecl()->getLocation()))
+        return;
     }
     for (const Decl *Redecl : D->redecls())
       Result.insert(Redecl->getLocation());
diff --git a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
--- a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
@@ -39,10 +39,10 @@
           "class ^X;",
           "X *y;",
       },
-      // FIXME(kirillbobyrev): When definition is available, we don't need to
-      // mark forward declarations as used.
+      // When definition is available, we don't need to mark forward
+      // declarations as used.
       {
-          "class ^X {}; class ^X;",
+          "class ^X {}; class X;",
           "X *y;",
       },
       // We already have forward declaration in the main file, the other
@@ -51,17 +51,24 @@
           "class ^X {}; class X;",
           "class X; X *y;",
       },
+      // Nested class definition can occur outside of the parent class
+      // definition. Bar declaration should be visible to its definition but
+      // it will always be because we will mark Foo definition as used.
+      {
+          "class ^Foo { class Bar; };",
+          "class Foo::Bar {};",
+      },
       // TypedefType and UsingDecls
       {
           "using ^Integer = int;",
           "Integer x;",
       },
       {
-          "namespace ns { struct ^X; struct ^X {}; }",
-          "using ns::X;",
+          "namespace ns { void ^foo(); void ^foo() {} }",
+          "using ns::foo;",
       },
       {
-          "namespace ns { struct X; struct X {}; }",
+          "namespace ns { void foo(); void foo() {}; }",
           "using namespace ns;",
       },
       {
@@ -88,8 +95,8 @@
       },
       // Redecls
       {
-          "class ^X; class ^X{}; class ^X;",
-          "X *y;",
+          "void ^foo(); void ^foo() {} void ^foo();",
+          "void bar() { foo(); }",
       },
       // Constructor
       {