Explicitly detach derivative when forming a non-differentiable struct out of differentiable args (#4901)

* Explicitly detach derivative when forming a non-differentiable struct out of differentiable args

This fixes an issue where initializer lists get optimized out and lose information about non-differentiability.

There are 2 places where this could have been fixed:
1. When coercing initializer-list exprs, we can check for non-differentiable aggregate types and use a detach derivative on all the args.
2. Add an extra case in the peephole optimization step that adds detach-derivative when simplifying a make-struct of a non-differentiable type.

Even though solution 2 is more elegant, this PR goes with solution 1 simply to avoid having to use a differentiable-type-conformance-context that is used in the auto-diff IR passes to check for differentiability.

* Change test name + add expected vals

1 files changed, 21 insertions, 0 deletions

diff --git a/source/slang/slang-check-conversion.cpp b/source/slang/slang-check-conversion.cpp
index c135ada8d..85040dc55 100644
--- a/source/slang/slang-check-conversion.cpp
+++ b/source/slang/slang-check-conversion.cpp
@@ -498,6 +498,27 @@ namespace Slang
             toInitializerListExpr->type = QualType(toType);
             toInitializerListExpr->args = coercedArgs;
 
+            // Wrap initalizer list args if we're creating a non-differentiable struct within a 
+            // differentiable function.
+            // 
+            if (auto func = getParentFuncOfVisitor())
+            {
+                if (func->findModifier<DifferentiableAttribute>() && 
+                    !isTypeDifferentiable(toType))
+                {
+                    for (auto &arg : toInitializerListExpr->args)
+                    {
+                        if (isTypeDifferentiable(arg->type.type))
+                        {
+                            auto detachedArg = m_astBuilder->create<DetachExpr>();
+                            detachedArg->inner = arg;
+                            detachedArg->type = arg->type;
+                            arg = detachedArg;
+                        }
+                    }
+                }
+            }
+
             *outToExpr = toInitializerListExpr;
         }