diff --git a/clang/include/clang/AST/ASTDiagnostic.h b/clang/include/clang/AST/ASTDiagnostic.h
--- a/clang/include/clang/AST/ASTDiagnostic.h
+++ b/clang/include/clang/AST/ASTDiagnostic.h
@@ -34,7 +34,8 @@
       ArrayRef<intptr_t> QualTypeVals);
 
   /// Returns a desugared version of the QualType, and marks ShouldAKA as true
-  /// whenever we remove significant sugar from the type.
+  /// whenever we remove significant sugar from the type. Make sure ShouldAKA
+  /// is initialized before passing it in.
   QualType desugarForDiagnostic(ASTContext &Context, QualType QT,
                                 bool &ShouldAKA);
 }  // end namespace clang
diff --git a/clang/lib/AST/ASTDiagnostic.cpp b/clang/lib/AST/ASTDiagnostic.cpp
--- a/clang/lib/AST/ASTDiagnostic.cpp
+++ b/clang/lib/AST/ASTDiagnostic.cpp
@@ -25,7 +25,8 @@
 using namespace clang;
 
 // Returns a desugared version of the QualType, and marks ShouldAKA as true
-// whenever we remove significant sugar from the type.
+// whenever we remove significant sugar from the type. Make sure ShouldAKA
+// is initialized before passing it in.
 QualType clang::desugarForDiagnostic(ASTContext &Context, QualType QT,
                                      bool &ShouldAKA) {
   QualifierCollector QC;