diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 0079c49..e5cf31a 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3450,6 +3450,9 @@
   // C11 6.7.2.1p2:
   //   A struct-declaration that does not declare an anonymous structure or
   //   anonymous union shall contain a struct-declarator-list.
+  //
+  // This rule also existed in C89 and C99; the grammar for struct-declaration
+  // did not permit a struct-declaration without a struct-declarator-list.
   if (!getLangOpts().CPlusPlus && CurContext->isRecord() &&
       DS.getStorageClassSpec() == DeclSpec::SCS_unspecified) {
     // Check for Microsoft C extension: anonymous struct/union member.