[clang] [HLSL] Allow arrays to copy-initialize (PR #127557)

[Justin Bogner via cfe-commits]

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@@ -2927,6 +2898,53 @@ void CastOperation::CheckCXXCStyleCast(bool FunctionalStyle,
   }
 }
 
+// CheckHLSLCStyleCast - Returns `true` ihe cast is handled or errored as an
+// HLSL-specific cast. Returns false if the cast should be checked as a CXX
+// C-Style cast.
+bool CastOperation::CheckHLSLCStyleCast(CheckedConversionKind CCK) {
+  assert(Self.getLangOpts().HLSL && "Must be HLSL!");
+  QualType SrcTy = SrcExpr.get()->getType();
+  // HLSL has several unique forms of C-style casts which support aggregate to
+  // aggregate casting.
+  // This case should not trigger on regular vector cast, vector truncation
+  if (Self.HLSL().CanPerformElementwiseCast(SrcExpr.get(), DestType)) {
+    if (SrcTy->isConstantArrayType())
+      SrcExpr = Self.ImpCastExprToType(
+          SrcExpr.get(), Self.Context.getArrayParameterType(SrcTy),
+          CK_HLSLArrayRValue, VK_PRValue, nullptr, CCK);
+    Kind = CK_HLSLElementwiseCast;
+    return true;
+  }
+
+  // This case should not trigger on regular vector splat
+  // If the relative order of this and the HLSLElementWise cast checks
+  // are changed, it might change which cast handles what in a few cases
+  if (Self.HLSL().CanPerformAggregateSplatCast(SrcExpr.get(), DestType)) {
+    const VectorType *VT = SrcTy->getAs<VectorType>();
+    // change splat from vec1 case to splat from scalar
+    if (VT && VT->getNumElements() == 1)
+      SrcExpr = Self.ImpCastExprToType(
+          SrcExpr.get(), VT->getElementType(), CK_HLSLVectorTruncation,
+          SrcExpr.get()->getValueKind(), nullptr, CCK);
+    // Inserting a scalar cast here allows for a simplified codegen in
+    // the case the destTy is a vector
+    if (const VectorType *DVT = DestType->getAs<VectorType>())
+      SrcExpr = Self.ImpCastExprToType(
+          SrcExpr.get(), DVT->getElementType(),
+          Self.PrepareScalarCast(SrcExpr, DVT->getElementType()),
+          SrcExpr.get()->getValueKind(), nullptr, CCK);
+    Kind = CK_HLSLAggregateSplatCast;
+    return true;
+  }
+  if (DestType->isArrayType()) {
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bogner wrote:

A blank line here would be aesthetically pleasing

https://github.com/llvm/llvm-project/pull/127557