Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -171,6 +171,7 @@
 LANGOPT(Modules           , 1, 0, "modules semantics")
 COMPATIBLE_LANGOPT(ModulesTS  , 1, 0, "C++ Modules TS syntax")
 COMPATIBLE_LANGOPT(CPlusPlusModules, 1, 0, "C++ modules syntax")
+COMPATIBLE_LANGOPT(AllAdditionalTUReachable, 1, 0, "C++ modules syntax")
 BENIGN_ENUM_LANGOPT(CompilingModule, CompilingModuleKind, 3, CMK_None,
                     "compiling a module interface")
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1428,6 +1428,10 @@
   NegFlag<SetFalse, [CC1Option], "Disable">, PosFlag<SetTrue, [], "Enable">,
   BothFlags<[NoXarchOption], " modules for C++">>,
   ShouldParseIf<cplusplus.KeyPath>;
+defm all_additional_TU_reachable : BoolFOption<"all-additional-TU-reachable",
+  LangOpts<"AllAdditionalTUReachable">, DefaultFalse,
+  NegFlag<SetFalse, [CC1Option], "Disable">, PosFlag<SetTrue, [CC1Option], "Enable">,
+  BothFlags<[NoXarchOption], " treat all additional variable as reachable">>;
 def fdebug_pass_arguments : Flag<["-"], "fdebug-pass-arguments">, Group<f_Group>;
 def fdebug_pass_structure : Flag<["-"], "fdebug-pass-structure">, Group<f_Group>;
 def fdepfile_entry : Joined<["-"], "fdepfile-entry=">,
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -3640,6 +3640,12 @@
                    options::OPT_fno_implicit_module_maps, HaveClangModules))
     CmdArgs.push_back("-fimplicit-module-maps");
 
+  // -fall-additional-TU-reachable enables to treat all translation unit as
+  // reachable.
+  if (Args.hasFlag(options::OPT_fall_additional_TU_reachable,
+                   options::OPT_fno_all_additional_TU_reachable, false))
+    CmdArgs.push_back("-fall-additional-TU-reachable");
+
   // -fmodules-decluse checks that modules used are declared so (off by default)
   Args.addOptInFlag(CmdArgs, options::OPT_fmodules_decluse,
                     options::OPT_fno_modules_decluse);
Index: clang/lib/Sema/SemaLookup.cpp
===================================================================
--- clang/lib/Sema/SemaLookup.cpp
+++ clang/lib/Sema/SemaLookup.cpp
@@ -1953,9 +1953,9 @@
   //   considered reachable, but it is unspecified which are and under what
   //   circumstances.
   //
-  // The decision here is to treat all additional tranditional units as
-  // unreachable.
-  return false;
+  // The default value for AllAdditionalTUReachable is false. It implies all
+  // additional translation unit is not considered as reachable for portability.
+  return SemaRef.getLangOpts().AllAdditionalTUReachable;
 }
 
 bool Sema::isAcceptableSlow(const NamedDecl *D, Sema::AcceptableKind Kind) {
Index: clang/test/CXX/module/module.reach/ex1.cpp
===================================================================
--- clang/test/CXX/module/module.reach/ex1.cpp
+++ clang/test/CXX/module/module.reach/ex1.cpp
@@ -10,6 +10,7 @@
 //
 // RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/M.cppm -o %t/M.pcm
 // RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %t/X.cppm -fsyntax-only -verify
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %t/X-relax.cppm -fsyntax-only -verify -fall-additional-TU-reachable
 //
 //--- M-A.cppm
 export module M:A;
@@ -41,3 +42,11 @@
       // expected-note@* {{definition here is not reachable}} expected-note@* {{}}
 // FIXME: We should emit an error for unreachable definition of B.
 void g() { f(); }
+
+//--- X-relax.cppm
+// Tests that it wouldn't complain if we treat all additional TU as reachable TU.
+// expected-no-diagnostics
+export module X;
+import M;
+B b3;
+void g() { f(); }
Index: clang/test/CXX/module/module.reach/p2.cpp
===================================================================
--- clang/test/CXX/module/module.reach/p2.cpp
+++ clang/test/CXX/module/module.reach/p2.cpp
@@ -3,6 +3,7 @@
 // RUN: split-file %s %t
 // RUN: %clang_cc1 -std=c++20 %t/impl.cppm -emit-module-interface -o %t/M-impl.pcm
 // RUN: %clang_cc1 -std=c++20 %t/M.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/M.pcm
+// RUN: %clang_cc1 -std=c++20 %t/Use.cpp -fprebuilt-module-path=%t -verify -fsyntax-only -fall-additional-TU-reachable
 // RUN: %clang_cc1 -std=c++20 %t/UseStrict.cpp -fprebuilt-module-path=%t -verify -fsyntax-only
 
 //--- impl.cppm
@@ -14,6 +15,13 @@
 import :impl;
 export A f();
 
+//--- Use.cpp
+// expected-no-diagnostics
+import M;
+void test() {
+  auto a = f();
+}
+
 //--- UseStrict.cpp
 import M;
 void test() {