diff --git a/clang/include/clang/Basic/Module.h b/clang/include/clang/Basic/Module.h
--- a/clang/include/clang/Basic/Module.h
+++ b/clang/include/clang/Basic/Module.h
@@ -469,6 +469,9 @@
   bool isUnimportable(const LangOptions &LangOpts, const TargetInfo &Target,
                       Requirement &Req, Module *&ShadowingModule) const;
 
+  /// Determine whether this module can be built in this compilation.
+  bool isForBuilding(const LangOptions &LangOpts) const;
+
   /// Determine whether this module is available for use within the
   /// current translation unit.
   bool isAvailable() const { return IsAvailable; }
diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -148,6 +148,26 @@
   llvm_unreachable("could not find a reason why module is unimportable");
 }
 
+// The -fmodule-name option tells the compiler to textually include headers in
+// the specified module, meaning Clang won't build the specified module. This
+// is useful in a number of situations, for instance, when building a library
+// that vends a module map, one might want to avoid hitting intermediate build
+// products containing the module map or avoid finding the system installed
+// modulemap for that library.
+bool Module::isForBuilding(const LangOptions &LangOpts) const {
+  StringRef TopLevelName = getTopLevelModuleName();
+  StringRef CurrentModule = LangOpts.CurrentModule;
+
+  // When building framework Foo, we want to make sure that Foo *and*
+  // Foo_Private are textually included and no modules are built for both.
+  if (getTopLevelModule()->IsFramework &&
+      CurrentModule == LangOpts.ModuleName &&
+      !CurrentModule.endswith("_Private") && TopLevelName.endswith("_Private"))
+    TopLevelName = TopLevelName.drop_back(8);
+
+  return TopLevelName == CurrentModule;
+}
+
 bool Module::isAvailable(const LangOptions &LangOpts, const TargetInfo &Target,
                          Requirement &Req,
                          UnresolvedHeaderDirective &MissingHeader,
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -109,25 +109,6 @@
   PED_Elifndef
 };
 
-// The -fmodule-name option tells the compiler to textually include headers in
-// the specified module, meaning clang won't build the specified module. This is
-// useful in a number of situations, for instance, when building a library that
-// vends a module map, one might want to avoid hitting intermediate build
-// products containimg the module map or avoid finding the system installed
-// modulemap for that library.
-static bool isForModuleBuilding(Module *M, StringRef CurrentModule,
-                                StringRef ModuleName) {
-  StringRef TopLevelName = M->getTopLevelModuleName();
-
-  // When building framework Foo, we wanna make sure that Foo *and* Foo_Private
-  // are textually included and no modules are built for both.
-  if (M->getTopLevelModule()->IsFramework && CurrentModule == ModuleName &&
-      !CurrentModule.endswith("_Private") && TopLevelName.endswith("_Private"))
-    TopLevelName = TopLevelName.drop_back(8);
-
-  return TopLevelName == CurrentModule;
-}
-
 static MacroDiag shouldWarnOnMacroDef(Preprocessor &PP, IdentifierInfo *II) {
   const LangOptions &Lang = PP.getLangOpts();
   if (isReservedInAllContexts(II->isReserved(Lang))) {
@@ -2219,14 +2200,13 @@
       alreadyIncluded(*File))
     Action = IncludeLimitReached;
 
-  bool MaybeTranslateInclude = Action == Enter && File && SuggestedModule &&
-                               !isForModuleBuilding(SuggestedModule.getModule(),
-                                                    getLangOpts().CurrentModule,
-                                                    getLangOpts().ModuleName);
-
   // FIXME: We do not have a good way to disambiguate C++ clang modules from
   // C++ standard modules (other than use/non-use of Header Units).
   Module *SM = SuggestedModule.getModule();
+
+  bool MaybeTranslateInclude =
+      Action == Enter && File && SM && !SM->isForBuilding(getLangOpts());
+
   // Maybe a usable Header Unit
   bool UsableHeaderUnit = false;
   if (getLangOpts().CPlusPlusModules && SM && SM->isHeaderUnit()) {
@@ -2556,9 +2536,7 @@
     // that behaves the same as the header would behave in a compilation using
     // that PCH, which means we should enter the submodule. We need to teach
     // the AST serialization layer to deal with the resulting AST.
-    if (getLangOpts().CompilingPCH &&
-        isForModuleBuilding(SM, getLangOpts().CurrentModule,
-                            getLangOpts().ModuleName))
+    if (getLangOpts().CompilingPCH && SM->isForBuilding(getLangOpts()))
       return {ImportAction::None};
 
     assert(!CurLexerSubmodule && "should not have marked this as a module yet");
diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -541,7 +541,7 @@
   // of the same top-level module. Until we do, make it an error rather than
   // silently ignoring the import.
   // FIXME: Should we warn on a redundant import of the current module?
-  if (Mod->getTopLevelModuleName() == getLangOpts().CurrentModule &&
+  if (Mod->isForBuilding(getLangOpts()) &&
       (getLangOpts().isCompilingModule() || !getLangOpts().ModulesTS)) {
     Diag(ImportLoc, getLangOpts().isCompilingModule()
                         ? diag::err_module_self_import
diff --git a/clang/test/Modules/implementation-of-module-private.m b/clang/test/Modules/implementation-of-module-private.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/implementation-of-module-private.m
@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+//--- frameworks/FW.framework/Modules/module.modulemap
+framework module FW {}
+//--- frameworks/FW.framework/Modules/module.private.modulemap
+framework module FW_Private {}
+
+//--- tu.m
+@import FW_Private; // expected-error{{@import of module 'FW_Private' in implementation of 'FW'; use #import}}
+
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t/cache -fimplicit-module-maps \
+// RUN:   -fmodule-name=FW -F %t/frameworks %t/tu.m -verify