Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7781,6 +7781,8 @@
   "%select{ or namespace|, namespace, or enumeration}1">;
 def err_invalid_declarator_scope : Error<"cannot define or redeclare %0 here "
   "because namespace %1 does not enclose namespace %2">;
+def err_invalid_declarator_in_export : Error<"cannot export %0 here "
+  "because it had be declared in %1.">;
 def err_invalid_declarator_global_scope : Error<
   "definition or redeclaration of %0 cannot name the global scope">;
 def err_invalid_declarator_in_function : Error<
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -5745,6 +5745,9 @@
     else if (isa<BlockDecl>(Cur))
       Diag(Loc, diag::err_invalid_declarator_in_block)
         << Name << SS.getRange();
+    else if (isa<ExportDecl>(Cur))
+      Diag(Loc, diag::err_invalid_declarator_in_export)
+          << Name << cast<NamedDecl>(DC) << SS.getRange();
     else
       Diag(Loc, diag::err_invalid_declarator_scope)
       << Name << cast<NamedDecl>(Cur) << cast<NamedDecl>(DC) << SS.getRange();
Index: clang/test/CXX/module/module.interface/p6.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.interface/p6.cpp
@@ -0,0 +1,52 @@
+// RUN: %clang_cc1 -std=c++20 %s -verify
+
+export module X;
+
+export template <typename T>
+struct X {
+  struct iterator {
+    T node;
+  };
+  void foo() {}
+  template <typename U>
+  U bar();
+};
+
+export template <typename T> X<T>::iterator; // expected-error {{cannot export 'iterator' here because it had be declared in 'X'}}
+export template <typename T> void X<T>::foo();  // expected-error {{cannot export 'foo' here because it had be declared in 'X'}}
+export template <typename T> template <typename U> U X<T>::bar();  // expected-error {{cannot export 'bar' here because it had be declared in 'X'}}
+
+export struct Y {
+  struct iterator {
+    int node;
+  };
+  void foo() {}
+  template <typename U>
+  U bar();
+};
+
+export struct Y::iterator; // expected-error {{cannot export 'iterator' here because it had be declared in 'Y'}}
+export void Y::foo(); // expected-error {{cannot export 'foo' here because it had be declared in 'Y'}}
+export template <typename U> U Y::bar(); // expected-error {{cannot export 'bar' here because it had be declared in 'Y'}}
+
+export {
+  template <typename T> X<T>::iterator; // expected-error {{cannot export 'iterator' here because it had be declared in 'X'}}
+  struct Y::iterator; // expected-error {{cannot export 'iterator' here because it had be declared in 'Y'}}
+}
+
+namespace A{
+  export struct X;
+  struct Y;
+  export void foo();
+  void bar();
+}
+
+// [module.interface]/p6
+// A redeclaration of an entity X is implicitly exported if X was introduced by an exported declaration
+void A::foo();
+export struct A::X; // expected-error {{cannot export 'X' here because it had be declared in 'A'}}
+export struct A::Y; // expected-error {{cannot export 'Y' here because it had be declared in 'A'.}}
+export void A::foo(); // expected-error {{cannot export 'foo' here because it had be declared in 'A'}}
+// The compiler couldn't export A::bar() here since A::bar() is declared above without exported.
+// See [module.interface]/p6 for details.
+export void A::bar(); // expected-error {{cannot export 'bar' here because it had be declared in 'A'}}