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
@@ -4794,6 +4794,11 @@
       Result = APValue((const FieldDecl *)nullptr);
       return true;
     }
+    // Can't access properties of an incomplete type.
+    if (!RD->hasDefinition()) {
+      Result = APValue();
+      return false;
+    }
     Result = APValue(APValue::UninitStruct(), RD->getNumBases(),
                      std::distance(RD->field_begin(), RD->field_end()));