Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -175,6 +175,7 @@
 COMPATIBLE_LANGOPT(CPlusPlusModules, 1, 0, "C++ modules syntax")
 BENIGN_ENUM_LANGOPT(CompilingModule, CompilingModuleKind, 3, CMK_None,
                     "compiling a module interface")
+COMPATIBLE_LANGOPT(AllowNonInlineExternalDefInHU, 1, 0, "allow non inline external definitions in header units")
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")
 BENIGN_LANGOPT(BuildingPCHWithObjectFile, 1, 0, "building a pch which has a corresponding object file")
 BENIGN_LANGOPT(CacheGeneratedPCH, 1, 0, "cache generated PCH files in memory")
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13088,6 +13088,7 @@
   // C++ [module.import/6] external definitions are not permitted in header
   // units.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
+      VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::ExternalLinkage &&
       !VDecl->isInline()) {
     Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);
Index: clang/test/CXX/module/module.import/p6.cpp
===================================================================
--- clang/test/CXX/module/module.import/p6.cpp
+++ clang/test/CXX/module/module.import/p6.cpp
@@ -22,3 +22,9 @@
 
 int bad_var_definition = 3;  // expected-error {{non-inline external definitions are not permitted in C++ header units}}
 
+class A {
+public:
+    // This is a declaration instead of definition.
+    static const int value = 43; 
+};
+