Index: clang/lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- clang/lib/Serialization/ASTReaderDecl.cpp
+++ clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1782,7 +1782,10 @@
   else
     DD = new (C) struct CXXRecordDecl::DefinitionData(D);
 
+  assert(!D->IsBeingDefined && "Should not overwrite definition in progress");
+  D->IsBeingDefined = true;
   ReadCXXDefinitionData(*DD, D);
+  D->IsBeingDefined = false;
 
   // We might already have a definition for this record. This can happen either
   // because we're reading an update record, or because we've already done some
@@ -2963,6 +2966,9 @@
     if (!DD)
       DD = RD->getCanonicalDecl()->DefinitionData;
 
+    if (!DD && RD->isBeingDefined())
+      return nullptr;
+
     // If there's no definition yet, then DC's definition is added by an update
     // record, but we've not yet loaded that update record. In this case, we
     // commit to DC being the canonical definition now, and will fix this when
Index: clang/test/Modules/Inputs/self-referencing-lambda/a.h
===================================================================
--- /dev/null
+++ clang/test/Modules/Inputs/self-referencing-lambda/a.h
@@ -0,0 +1,4 @@
+void f() {
+  int x = 0;
+  auto q = [xm = x]{};
+}
Index: clang/test/Modules/Inputs/self-referencing-lambda/module.modulemap
===================================================================
--- /dev/null
+++ clang/test/Modules/Inputs/self-referencing-lambda/module.modulemap
@@ -0,0 +1,4 @@
+module "a.h" {
+  header "a.h"
+  export *
+}
Index: clang/test/Modules/self-referencing-lambda.cpp
===================================================================
--- /dev/null
+++ clang/test/Modules/self-referencing-lambda.cpp
@@ -0,0 +1,5 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/self-referencing-lambda %s -verify -emit-obj -o %t2.o
+// expected-no-diagnostics
+
+#include "a.h"