diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index 6dbf5de..71bbd82 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -1561,6 +1561,7 @@
   case diag::err_odr_non_type_parameter_type_inconsistent:
     return diag::warn_odr_non_type_parameter_type_inconsistent;
   }
+  llvm_unreachable("Diagnostic kind not handled in preceding switch");
 }
 
 bool StructuralEquivalenceContext::IsEquivalent(Decl *D1, Decl *D2) {