diff --git a/clang/lib/AST/Interp/Compiler.cpp b/clang/lib/AST/Interp/Compiler.cpp
index ef579bc5d8972..0fc93c14131e6 100644
--- a/clang/lib/AST/Interp/Compiler.cpp
+++ b/clang/lib/AST/Interp/Compiler.cpp
@@ -1698,10 +1698,8 @@ bool Compiler<Emitter>::VisitUnaryExprOrTypeTraitExpr(
   if (Kind == UETT_VectorElements) {
     if (const auto *VT = E->getTypeOfArgument()->getAs<VectorType>())
       return this->emitConst(VT->getNumElements(), E);
-
-    // FIXME: Apparently we need to catch the fact that a sizeless vector type
-    // has been passed and diagnose that (at run time).
     assert(E->getTypeOfArgument()->isSizelessVectorType());
+    return this->emitSizelessVectorElementSize(E);
   }
 
   if (Kind == UETT_VecStep) {
diff --git a/clang/lib/AST/Interp/Interp.h b/clang/lib/AST/Interp/Interp.h
index f86b787fb034e..ff2748f91adfe 100644
--- a/clang/lib/AST/Interp/Interp.h
+++ b/clang/lib/AST/Interp/Interp.h
@@ -2735,6 +2735,15 @@ inline bool InvalidDeclRef(InterpState &S, CodePtr OpPC,
   return CheckDeclRef(S, OpPC, DR);
 }
 
+inline bool SizelessVectorElementSize(InterpState &S, CodePtr OpPC) {
+  if (S.inConstantContext()) {
+    const SourceRange &ArgRange = S.Current->getRange(OpPC);
+    const Expr *E = S.Current->getExpr(OpPC);
+    S.CCEDiag(E, diag::note_constexpr_non_const_vectorelements) << ArgRange;
+  }
+  return false;
+}
+
 inline bool Assume(InterpState &S, CodePtr OpPC) {
   const auto Val = S.Stk.pop<Boolean>();
 
diff --git a/clang/lib/AST/Interp/Opcodes.td b/clang/lib/AST/Interp/Opcodes.td
index 49ebb156ab2fb..9f29fa9272711 100644
--- a/clang/lib/AST/Interp/Opcodes.td
+++ b/clang/lib/AST/Interp/Opcodes.td
@@ -733,6 +733,8 @@ def InvalidDeclRef : Opcode {
   let Args = [ArgDeclRef];
 }
 
+def SizelessVectorElementSize : Opcode;
+
 def Assume : Opcode;
 
 def ArrayDecay : Opcode;
diff --git a/clang/test/SemaCXX/builtin_vectorelements.cpp b/clang/test/SemaCXX/builtin_vectorelements.cpp
index 59ff09ac72e42..b23675ea0ac6a 100644
--- a/clang/test/SemaCXX/builtin_vectorelements.cpp
+++ b/clang/test/SemaCXX/builtin_vectorelements.cpp
@@ -1,7 +1,9 @@
 // RUN: %clang_cc1 -triple x86_64                       -std=c++20 -fsyntax-only -verify -disable-llvm-passes %s
+// RUN: %clang_cc1 -triple x86_64                       -std=c++20 -fsyntax-only -verify -disable-llvm-passes -fexperimental-new-constant-interpreter %s
 
 // REQUIRES: aarch64-registered-target
 // RUN: %clang_cc1 -triple aarch64 -target-feature +sve -std=c++20 -fsyntax-only -verify -disable-llvm-passes %s
+// RUN: %clang_cc1 -triple aarch64 -target-feature +sve -std=c++20 -fsyntax-only -verify -disable-llvm-passes -fexperimental-new-constant-interpreter %s
 
 template <typename T>
 using VecT __attribute__((vector_size(16))) = T;