diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1686,8 +1686,8 @@
   Module *DeclModule = SemaRef.getOwningModule(D);
   assert(DeclModule && "hidden decl has no owning module");
 
-  // If the owning module is visible, the decl is visible.
   if (SemaRef.isModuleVisible(DeclModule, D->isModulePrivate()))
+    // If the owning module is visible, the decl is visible.
     return true;
 
   // Determine whether a decl context is a file context for the purpose of
@@ -1761,6 +1761,21 @@
   if (ModulePrivate) {
     if (isInCurrentModule(M, getLangOpts()))
       return true;
+    else if (M->Kind == Module::ModuleKind::ModulePartitionImplementation &&
+             isModuleDirectlyImported(M))
+      // Unless a partition implementation is directly imported it is not
+      // counted as visible for lookup, although the contained decls might
+      // still be reachable.  It's a partition, so it must be part of the
+      // current module to be a valid import.
+      return true;
+    else if (getLangOpts().CPlusPlusModules && !ModuleScopes.empty() &&
+	     ModuleScopes[0].Module->Kind ==
+	     Module::ModuleKind::ModulePartitionImplementation &&
+	     ModuleScopes[0].Module->getPrimaryModuleInterfaceName() ==
+	     M->Name && isModuleDirectlyImported(M))
+      // We are building a module implementation partition and the TU imports
+      // the primary module interface unit.
+      return true;
   } else {
     if (VisibleModules.isVisible(M))
       return true;