diff --git a/clang/lib/AST/ASTImporterLookupTable.cpp b/clang/lib/AST/ASTImporterLookupTable.cpp
--- a/clang/lib/AST/ASTImporterLookupTable.cpp
+++ b/clang/lib/AST/ASTImporterLookupTable.cpp
@@ -87,6 +87,18 @@
 ASTImporterLookupTable::ASTImporterLookupTable(TranslationUnitDecl &TU) {
   Builder B(*this);
   B.TraverseDecl(&TU);
+  // The VaList declaration may be created on demand only or not traversed.
+  // To ensure it is present and found during import, add it to the table now.
+  if (auto *D =
+          dyn_cast_or_null<NamedDecl>(TU.getASTContext().getVaListTagDecl())) {
+    // On some platforms (AArch64) the VaList declaration can be inside a 'std'
+    // namespace. This is handled specially and not visible by AST traversal.
+    // ASTImporter must be able to find this namespace to import the VaList
+    // declaration (and the namespace) correctly.
+    if (auto *Ns = dyn_cast<NamespaceDecl>(D->getDeclContext()))
+      add(&TU, Ns);
+    add(D->getDeclContext(), D);
+  }
 }
 
 void ASTImporterLookupTable::add(DeclContext *DC, NamedDecl *ND) {
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -8137,6 +8137,42 @@
   EXPECT_FALSE(SharedStatePtr->isNewDecl(ToBar));
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, VaListC) {
+  Decl *FromTU = getTuDecl(R"(typedef __builtin_va_list va_list;)", Lang_C99);
+
+  auto *FromVaList = FirstDeclMatcher<TypedefDecl>().match(
+      FromTU, typedefDecl(hasName("va_list")));
+  ASSERT_TRUE(FromVaList);
+
+  auto *ToVaList = Import(FromVaList, Lang_C99);
+  ASSERT_TRUE(ToVaList);
+
+  auto *ToBuiltinVaList = FirstDeclMatcher<TypedefDecl>().match(
+      ToAST->getASTContext().getTranslationUnitDecl(),
+      typedefDecl(hasName("__builtin_va_list")));
+
+  ASSERT_TRUE(ToAST->getASTContext().hasSameType(
+      ToVaList->getUnderlyingType(), ToBuiltinVaList->getUnderlyingType()));
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, VaListCpp) {
+  Decl *FromTU = getTuDecl(R"(typedef __builtin_va_list va_list;)", Lang_CXX03);
+
+  auto *FromVaList = FirstDeclMatcher<TypedefDecl>().match(
+      FromTU, typedefDecl(hasName("va_list")));
+  ASSERT_TRUE(FromVaList);
+
+  auto *ToVaList = Import(FromVaList, Lang_CXX03);
+  ASSERT_TRUE(ToVaList);
+
+  auto *ToBuiltinVaList = FirstDeclMatcher<TypedefDecl>().match(
+      ToAST->getASTContext().getTranslationUnitDecl(),
+      typedefDecl(hasName("__builtin_va_list")));
+
+  ASSERT_TRUE(ToAST->getASTContext().hasSameType(
+      ToVaList->getUnderlyingType(), ToBuiltinVaList->getUnderlyingType()));
+}
+
 INSTANTIATE_TEST_SUITE_P(ParameterizedTests, ASTImporterLookupTableTest,
                          DefaultTestValuesForRunOptions);