diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 341e0ab..1eea151 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -7622,6 +7622,7 @@
 
       return true;
     }
+    llvm_unreachable("The switch over PrevTSK must be exhaustive.");
 
   case TSK_ExplicitInstantiationDeclaration:
     switch (PrevTSK) {