diff --git a/clang-tools-extra/clang-tidy/modernize/ConcatNestedNamespacesCheck.cpp b/clang-tools-extra/clang-tidy/modernize/ConcatNestedNamespacesCheck.cpp
--- a/clang-tools-extra/clang-tidy/modernize/ConcatNestedNamespacesCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/ConcatNestedNamespacesCheck.cpp
@@ -82,9 +82,6 @@
   if (!locationsInSameFile(Sources, ND.getBeginLoc(), ND.getRBraceLoc()))
     return;
 
-  if (!Sources.isInMainFile(ND.getBeginLoc()))
-    return;
-
   if (anonymousOrInlineNamespace(ND))
     return;
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h b/clang-tools-extra/test/clang-tidy/checkers/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h
@@ -0,0 +1,8 @@
+namespace nn1 {
+namespace nn2 {
+// CHECK-FIXES: namespace nn1::nn2
+void t();
+} // namespace nn2
+} // namespace nn1
+// CHECK-FIXES: void t();
+// CHECK-FIXES-NEXT: } // namespace nn1
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize-concat-nested-namespaces.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize-concat-nested-namespaces.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize-concat-nested-namespaces.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize-concat-nested-namespaces.cpp
@@ -1,4 +1,12 @@
-// RUN: %check_clang_tidy -std=c++17-or-later %s modernize-concat-nested-namespaces %t
+// RUN: cp %S/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h %T/modernize-concat-nested-namespaces.h
+// RUN: %check_clang_tidy -std=c++17 %s modernize-concat-nested-namespaces %t -- -header-filter=".*" -- -I %T
+// RUN: FileCheck -input-file=%T/modernize-concat-nested-namespaces.h %S/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h -check-prefix=CHECK-FIXES
+// Restore header file and re-run with c++20:
+// RUN: cp %S/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h %T/modernize-concat-nested-namespaces.h
+// RUN: %check_clang_tidy -std=c++20 %s modernize-concat-nested-namespaces %t -- -header-filter=".*" -- -I %T
+// RUN: FileCheck -input-file=%T/modernize-concat-nested-namespaces.h %S/Inputs/modernize-concat-nested-namespaces/modernize-concat-nested-namespaces.h -check-prefix=CHECK-FIXES
+
+#include "modernize-concat-nested-namespaces.h"
 
 namespace n1 {}
 
@@ -159,3 +167,5 @@
 
   return 0;
 }
+
+// CHECK-MESSAGES: modernize-concat-nested-namespaces.h:1:1: warning: nested namespaces can be concatenated [modernize-concat-nested-namespaces]