Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -11136,6 +11136,7 @@
   // Emitting members of dllexported classes is delayed until the class
   // (including field initializers) is fully parsed.
   SmallVector<CXXRecordDecl*, 4> DelayedDllExportClasses;
+  SmallVector<CXXMethodDecl*, 4> DelayedDllExportMemberFunctions;
 
 private:
   class SavePendingParsedClassStateRAII {
Index: clang/lib/Sema/Sema.cpp
===================================================================
--- clang/lib/Sema/Sema.cpp
+++ clang/lib/Sema/Sema.cpp
@@ -961,6 +961,7 @@
 
   // All dllexport classes should have been processed already.
   assert(DelayedDllExportClasses.empty());
+  assert(DelayedDllExportMemberFunctions.empty());
 
   // Remove file scoped decls that turned out to be used.
   UnusedFileScopedDecls.erase(
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -6283,8 +6283,8 @@
         M->dropAttr<DLLExportAttr>();
 
       if (M->hasAttr<DLLExportAttr>()) {
-        DefineImplicitSpecialMember(*this, M, M->getLocation());
-        ActOnFinishInlineFunctionDef(M);
+        // Define after any fields with in-class initializers have been parsed.
+        DelayedDllExportMemberFunctions.push_back(M);
       }
     }
   };
@@ -11537,6 +11537,15 @@
 
 void Sema::ActOnFinishCXXNonNestedClass(Decl *D) {
   referenceDLLExportedClassMethods();
+
+  if (!DelayedDllExportMemberFunctions.empty()) {
+    SmallVector<CXXMethodDecl*, 4> WorkList;
+    std::swap(DelayedDllExportMemberFunctions, WorkList);
+    for (CXXMethodDecl *M : WorkList) {
+      DefineImplicitSpecialMember(*this, M, M->getLocation());
+      ActOnFinishInlineFunctionDef(M);
+    }
+  }
 }
 
 void Sema::referenceDLLExportedClassMethods() {
Index: clang/test/CodeGenCXX/dllexport.cpp
===================================================================
--- clang/test/CodeGenCXX/dllexport.cpp
+++ clang/test/CodeGenCXX/dllexport.cpp
@@ -851,6 +851,15 @@
 // Baz's operator=, causing instantiation of Foo<int> after which
 // ActOnFinishCXXNonNestedClass is called, and we would bite our own tail.
 // M32-DAG: define weak_odr dso_local dllexport x86_thiscallcc dereferenceable(1) %"struct.InClassInits::Baz"* @"??4Baz@InClassInits@@QAEAAU01@ABU01@@Z"
+
+// Trying to define the explicitly defaulted ctor must be delayed until the
+// in-class initializer for x has been processed.
+struct PR40006 {
+  __declspec(dllexport) PR40006() = default;
+  int x = 42;
+};
+// M32-DAG: define weak_odr dso_local dllexport x86_thiscallcc %"struct.InClassInits::PR40006"* @"??0PR40006@InClassInits@@QAE@XZ"
+
 }
 
 // We had an issue where instantiating A would force emission of B's delayed