[dllexport] Instantiate default ctor default args

clang/lib/Sema/SemaDeclCXX.cpp

for (Decl *Member : Class->decls()) {
 
     if (MD->isUserProvided()) {
       // Instantiate non-default class member functions ...
 
       // .. except for certain kinds of template specializations.
       if (TSK == TSK_ImplicitInstantiation && !ClassAttr->isInherited())
         continue;
 
+      // If this is an MS ABI dllexport default constructor, instantiate any
+      // default arguments.
+      if (S.Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+        auto CD = dyn_cast<CXXConstructorDecl>(MD);
+        if (CD && CD->isDefaultConstructor() && TSK == TSK_Undeclared) {
+          S.InstantiateDefaultCtorDefaultArgs(CD);
+        }
+      }
+
       S.MarkFunctionReferenced(Class->getLocation(), MD);
 
       // The function will be passed to the consumer when its definition is
       // encountered.
     } else if (MD->isExplicitlyDefaulted()) {
       // Synthesize and instantiate explicitly defaulted methods.
       S.MarkFunctionReferenced(Class->getLocation(), MD);
 

clang/test/CodeGenCXX/dllexport-ctor-closure-nested.cpp

+// RUN: %clang_cc1 -triple i686-windows-msvc -emit-llvm -std=c++14 \
+// RUN:    -fno-threadsafe-statics -fms-extensions -O1 -mconstructor-aliases \
+// RUN:    -disable-llvm-passes -o - %s -w -fms-compatibility-version=19.00 | \
+// RUN:    FileCheck %s
+
+struct HasDtor {
+  ~HasDtor();
+  int o;
+};
+struct HasImplicitDtor1 {
+  HasDtor o;
+};
+struct __declspec(dllexport) CtorClosureOuter {
+  struct __declspec(dllexport) CtorClosureInner {
+    CtorClosureInner(const HasImplicitDtor1 &v = {}) {}
+  };
+};
+
+// CHECK-LABEL: $"??1HasImplicitDtor1@@QAE@XZ" = comdat any
+// CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FCtorClosureInner@CtorClosureOuter@@QAEXXZ"({{.*}}) {{#[0-9]+}} comdat