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, 10 insertions, 0 deletions

diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 02c4fae68..b9d7a898f 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -3929,6 +3929,16 @@ struct ExprLoweringVisitorBase : public ExprVisitor<Derived, LoweredValInfo>
                 baseVal.val));
     }
 
+    LoweredValInfo visitDetachExpr(DetachExpr* expr)
+    {
+        auto baseVal = lowerRValueExpr(context, expr->inner);
+
+        return LoweredValInfo::simple(
+            getBuilder()->emitDetachDerivative(
+                lowerType(context, expr->type),
+                getSimpleVal(context, baseVal)));
+    }
+
     LoweredValInfo visitPrimalSubstituteExpr(PrimalSubstituteExpr* expr)
     {
         auto baseVal = lowerSubExpr(expr->baseFunction);