Index: clang/lib/AST/TextNodeDumper.cpp
===================================================================
--- clang/lib/AST/TextNodeDumper.cpp
+++ clang/lib/AST/TextNodeDumper.cpp
@@ -1693,7 +1693,11 @@
       FLAG(hasTrivialDestructor, trivial);
       FLAG(hasNonTrivialDestructor, non_trivial);
       FLAG(hasUserDeclaredDestructor, user_declared);
-      FLAG(hasConstexprDestructor, constexpr);
+      // Avoid calls to the external source.
+      if (!D->hasExternalVisibleStorage()) {
+        FLAG(hasConstexprDestructor, constexpr);
+      } else
+        OS << "unknown_constexpr";
       FLAG(needsImplicitDestructor, needs_implicit);
       FLAG(needsOverloadResolutionForDestructor, needs_overload_resolution);
       if (!D->needsOverloadResolutionForDestructor())
Index: clang/unittests/AST/ASTDumpTest.cpp
===================================================================
--- /dev/null
+++ clang/unittests/AST/ASTDumpTest.cpp
@@ -0,0 +1,140 @@
+//===- unittests/AST/ASTDumpTest.cpp --- Declaration tests ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Tests Decl::dump().
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclCXX.h"
+#include "clang/AST/DeclObjC.h"
+#include "clang/Basic/Builtins.h"
+#include "clang/Basic/FileManager.h"
+#include "clang/Basic/SourceManager.h"
+#include "gtest/gtest.h"
+
+using namespace clang;
+
+namespace clang {
+namespace ast {
+
+namespace {
+/// An ExternalASTSource that asserts if it is queried for information about
+/// any declaration.
+class TrappingExternalASTSource : public ExternalASTSource {
+  ~TrappingExternalASTSource() override = default;
+  bool FindExternalVisibleDeclsByName(const DeclContext *,
+                                      DeclarationName) override {
+    assert(false && "Unexpected call to FindExternalVisibleDeclsByName");
+    return true;
+  }
+
+  void FindExternalLexicalDecls(const DeclContext *,
+                                llvm::function_ref<bool(Decl::Kind)>,
+                                SmallVectorImpl<Decl *> &) override {
+    assert(false && "Unexpected call to FindExternalLexicalDecls");
+  }
+
+  void completeVisibleDeclsMap(const DeclContext *) override {
+    assert(false && "Unexpected call to completeVisibleDeclsMap");
+  }
+
+  void CompleteRedeclChain(const Decl *) override {
+    assert(false && "Unexpected call to CompleteRedeclChain");
+  }
+
+  void CompleteType(TagDecl *) override {
+    assert(false && "Unexpected call to CompleteType(Tag Decl*)");
+  }
+
+  void CompleteType(ObjCInterfaceDecl *) override {
+    assert(false && "Unexpected call to CompleteType(ObjCInterfaceDecl *)");
+  }
+};
+
+/// Tests that Decl::dump doesn't load additional declarations from the
+/// ExternalASTSource.
+class ExternalASTSourceDumpTest : public ::testing::Test {
+protected:
+  ExternalASTSourceDumpTest()
+      : FileMgr(FileMgrOpts), DiagID(new DiagnosticIDs()),
+        Diags(DiagID, new DiagnosticOptions, new IgnoringDiagConsumer()),
+        SourceMgr(Diags, FileMgr), Idents(LangOpts, nullptr),
+        Ctxt(LangOpts, SourceMgr, Idents, Sels, Builtins) {
+    Ctxt.setExternalSource(new TrappingExternalASTSource());
+  }
+
+  FileSystemOptions FileMgrOpts;
+  FileManager FileMgr;
+  IntrusiveRefCntPtr<DiagnosticIDs> DiagID;
+  DiagnosticsEngine Diags;
+  SourceManager SourceMgr;
+  LangOptions LangOpts;
+  IdentifierTable Idents;
+  SelectorTable Sels;
+  Builtin::Context Builtins;
+  ASTContext Ctxt;
+};
+} // unnamed namespace
+
+/// Set all flags that activate queries to the ExternalASTSource.
+static void setExternalStorageFlags(DeclContext *DC) {
+  DC->setHasExternalLexicalStorage();
+  DC->setHasExternalVisibleStorage();
+  DC->setMustBuildLookupTable();
+}
+
+/// Dumps the given Decl.
+static void dumpDecl(Decl *D) {
+  // Try dumping the decl which shouldn't trigger any calls to the
+  // ExternalASTSource.
+
+  std::string Out;
+  llvm::raw_string_ostream OS(Out);
+  D->dump(OS);
+}
+
+TEST_F(ExternalASTSourceDumpTest, DumpObjCInterfaceDecl) {
+  // Define an Objective-C interface.
+  ObjCInterfaceDecl *I = ObjCInterfaceDecl::Create(
+      Ctxt, Ctxt.getTranslationUnitDecl(), SourceLocation(),
+      &Ctxt.Idents.get("c"), nullptr, nullptr);
+  Ctxt.getTranslationUnitDecl()->addDecl(I);
+
+  setExternalStorageFlags(I);
+  dumpDecl(I);
+}
+
+TEST_F(ExternalASTSourceDumpTest, DumpRecordDecl) {
+  // Define a struct.
+  RecordDecl *R = RecordDecl::Create(
+      Ctxt, TagDecl::TagKind::TTK_Class, Ctxt.getTranslationUnitDecl(),
+      SourceLocation(), SourceLocation(), &Ctxt.Idents.get("c"));
+  R->startDefinition();
+  R->completeDefinition();
+  Ctxt.getTranslationUnitDecl()->addDecl(R);
+
+  setExternalStorageFlags(R);
+  dumpDecl(R);
+}
+
+TEST_F(ExternalASTSourceDumpTest, DumpCXXRecordDecl) {
+  // Define a class.
+  CXXRecordDecl *R = CXXRecordDecl::Create(
+      Ctxt, TagDecl::TagKind::TTK_Class, Ctxt.getTranslationUnitDecl(),
+      SourceLocation(), SourceLocation(), &Ctxt.Idents.get("c"));
+  R->startDefinition();
+  R->completeDefinition();
+  Ctxt.getTranslationUnitDecl()->addDecl(R);
+
+  setExternalStorageFlags(R);
+  dumpDecl(R);
+}
+
+} // end namespace ast
+} // end namespace clang
Index: clang/unittests/AST/CMakeLists.txt
===================================================================
--- clang/unittests/AST/CMakeLists.txt
+++ clang/unittests/AST/CMakeLists.txt
@@ -9,6 +9,7 @@
 
 add_clang_unittest(ASTTests
   ASTContextParentMapTest.cpp
+  ASTDumpTest.cpp
   ASTImporterFixtures.cpp
   ASTImporterTest.cpp
   ASTImporterGenericRedeclTest.cpp