[lldb] Fix crash when adding members to an "incomplete" type

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

@@ -269,8 +269,15 @@ static void PrepareContextToReceiveMembers(TypeSystemClang &ast,
   }
 
   // We don't have a type definition and/or the import failed, but we need to
-  // add members to it. Start the definition to make that possible.
-  tag_decl_ctx->startDefinition();
+  // add members to it. Start the definition to make that possible. If the type
+  // has no external storage we also have to complete the definition. Otherwise,
+  // that will happen when we are asked to complete the type
+  // (CompleteTypeFromDWARF).
+  ast.StartTagDeclarationDefinition(type);
+  if (!tag_decl_ctx->hasExternalLexicalStorage()) {
+    ast.SetDeclIsForcefullyCompleted(tag_decl_ctx);
+    ast.CompleteTagDeclarationDefinition(type);
+  }
 }
 
 ParsedDWARFTypeAttributes::ParsedDWARFTypeAttributes(const DWARFDIE &die) {

lldb/test/Shell/SymbolFile/DWARF/x86/typedef-in-incomplete-type.cpp

@@ -0,0 +1,23 @@
+// RUN: %clangxx --target=x86_64-pc-linux -flimit-debug-info -o %t -c %s -g
+// RUN: %lldb %t -o "target var a" -o "expr -- var" -o exit | FileCheck %s
+
+// This forces lldb to attempt to complete the type A. Since it has no
+// definition it will fail.
+// CHECK: target var a
+// CHECK: (A) a = <incomplete type "A">
+
+// Now attempt to display the second variable, which will try to add a typedef
+// to the incomplete type. Make sure that succeeds. Use the expression command
+// to make sure the resulting AST can be imported correctly.
+// CHECK: expr -- var
+// CHECK: (A::X) $0 = 0
+
+struct A {
+  // Declare the constructor, but don't define it to avoid emitting the
+  // definition in the debug info.
+  A();
+  using X = int;
+};
+
+A a;
+A::X var;