Updates based on feedback from @rjmccall

Author: llvm-beanz

clang/lib/CodeGen/CGExprScalar.cpp

@@ -2508,7 +2508,8 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     assert(DestTy->isVectorType() && "Expected dest type to be vector type");
     Value *Vec = Visit(const_cast<Expr *>(E));
     SmallVector<int, 16> Mask;
-    Mask.insert(Mask.begin(), DestTy->getAs<VectorType>()->getNumElements(), 0);
+    Mask.insert(Mask.begin(), DestTy->castAs<VectorType>()->getNumElements(),
+                0);
     return Builder.CreateShuffleVector(Vec, Mask, "trunc");
   }
 

clang/lib/Sema/SemaChecking.cpp

@@ -15609,8 +15609,8 @@ static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
         return;
       return DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_vector_scalar);
     } else if (S.getLangOpts().HLSL &&
-               Target->getAs<VectorType>()->getNumElements() <
-                   Source->getAs<VectorType>()->getNumElements()) {
+               Target->castAs<VectorType>()->getNumElements() <
+                   Source->castAs<VectorType>()->getNumElements()) {
       // Diagnose vector truncation but don't return. We may also want to
       // diagnose an element conversion.
       DiagnoseImpCast(S, E, T, CC, diag::warn_hlsl_impcast_vector_truncation);

clang/lib/Sema/SemaExprCXX.cpp

@@ -4764,11 +4764,13 @@ Sema::PerformImplicitConversion(Expr *From, QualType ToType,
                              From->getValueKind()).get();
     break;
   case ICK_HLSL_Vector_Truncation: {
-    // Note: HLSL vectors are ExtVectors. Since this truncates a vector to a
-    // smaller vector, this can only operate on arguments where the source and
-    // destination types are ExtVectors.
-    auto *FromVec = From->getType()->castAs<ExtVectorType>();
-    auto *ToVec = ToType->castAs<ExtVectorType>();
+    // Note: HLSL built-in vectors are ExtVectors. Since this truncates a vector
+    // to a smaller vector, this can only operate on arguments where the source
+    // and destination types are ExtVectors.
+    assert(From->getType()->isExtVectorType() && ToType->isExtVectorType() &&
+           "HLSL vector truncation should only apply to ExtVectors");
+    auto *FromVec = From->getType()->castAs<VectorType>();
+    auto *ToVec = ToType->castAs<VectorType>();
     QualType ElType = FromVec->getElementType();
     QualType TruncTy =
         Context.getExtVectorType(ElType, ToVec->getNumElements());