diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4316,7 +4316,8 @@
   if (auto *RD = T->getAsCXXRecordDecl()) {
     if (RD->isUnion())
       return APValue((const FieldDecl*)nullptr);
-
+    if (!RD->hasDefinition())
+      return APValue();
     APValue Struct(APValue::UninitStruct(), RD->getNumBases(),
                    std::distance(RD->field_begin(), RD->field_end()));
 
diff --git a/clang/test/SemaCXX/constant-expression-cxx2a.cpp b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
--- a/clang/test/SemaCXX/constant-expression-cxx2a.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
@@ -1401,3 +1401,15 @@
   //   decreasing address
   static_assert(f(6) == 543210);
 }
+
+namespace NoCrash {
+struct ForwardDecl; // expected-note {{forward declaration of}}
+struct Foo {        // expected-note 2{{candidate constructor}}
+  ForwardDecl f;    // expected-error {{field has incomplete type}}
+};
+
+constexpr Foo getFoo() {
+  Foo e = 123; // expected-error {{no viable conversion from 'int' to 'NoCrash::Foo'}}
+  return e;
+}
+}