[clang] [HLSL] Allow truncation to scalar (PR #104844)

[Chris B via cfe-commits]

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@@ -2032,26 +2032,42 @@ static bool IsVectorConversion(Sema &S, QualType FromType, QualType ToType,
   if (S.Context.hasSameUnqualifiedType(FromType, ToType))
     return false;
 
+  // HLSL allows implicit truncation of vector types.
+  if (S.getLangOpts().HLSL) {
+    auto *ToExtType = ToType->getAs<ExtVectorType>();
+    auto *FromExtType = FromType->getAs<ExtVectorType>();
+
+    // If both arguments are vectors, handle possible vector truncation and
+    // element conversion.
+    if (ToExtType && FromExtType) {
+      unsigned FromElts = FromExtType->getNumElements();
+      unsigned ToElts = ToExtType->getNumElements();
+      if (FromElts < ToElts)
+        return false;
+      if (FromElts == ToElts)
+        ElConv = ICK_Identity;
+      else
+        ElConv = ICK_HLSL_Vector_Truncation;
+
+      QualType FromElTy = FromExtType->getElementType();
+      QualType ToElTy = ToExtType->getElementType();
+      if (S.Context.hasSameUnqualifiedType(FromElTy, ToElTy))
+        return true;
+      return IsVectorElementConversion(S, FromElTy, ToElTy, ICK, From);
+    }
+    if (FromExtType && nullptr == ToExtType) {
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llvm-beanz wrote:

Yoda conditions are a shockingly hard habit to break...

https://github.com/llvm/llvm-project/pull/104844