From d9c57e613f2dacd221d9c46c10395cf373a8fcaf Mon Sep 17 00:00:00 2001
From: Theresa Foley <10618364+tangent-vector@users.noreply.github.com>
Date: Mon, 10 Jul 2023 17:48:51 -0700
Subject: Add support for texture footprint queries (#2970)

---
 source/slang/slang-check-conversion.cpp | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

(limited to 'source/slang/slang-check-conversion.cpp')

diff --git a/source/slang/slang-check-conversion.cpp b/source/slang/slang-check-conversion.cpp
index a6130d4e8..357b75cce 100644
--- a/source/slang/slang-check-conversion.cpp
+++ b/source/slang/slang-check-conversion.cpp
@@ -902,7 +902,23 @@ namespace Slang
                 if(auto witness = tryGetSubtypeWitness(fromType, toAggTypeDeclRef))
                 {
                     if (outToExpr)
+                    {
                         *outToExpr = createCastToSuperTypeExpr(toType, fromExpr, witness);
+
+                        // If the original expression was an l-value, then the result
+                        // of the cast may be an l-value itself. We want to be able
+                        // to invoke `[mutating]` methods on a value that is cast to
+                        // an interface it conforms to, and we also expect to be able
+                        // to pass a value of a derived `struct` type into methods that
+                        // expect a value of its base type.
+                        //
+                        // TODO: vet this logic for correctness.
+                        //
+                        if (fromExpr && fromExpr->type.isLeftValue)
+                        {
+                            (*outToExpr)->type.isLeftValue = true;
+                        }
+                    }
                     if (outCost)
                         *outCost = kConversionCost_CastToInterface;
                     return true;
-- 
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