Fix issue of double lowering issue a differentiable function (#8182)

Close #8054.
For detailed root cause is at:
https://github.com/shader-slang/slang/issues/8054#issuecomment-3189579508

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/autodiff/avoid-double-lowering.slang b/tests/autodiff/avoid-double-lowering.slang
new file mode 100644
index 000000000..354d1d27b
--- /dev/null
+++ b/tests/autodiff/avoid-double-lowering.slang
@@ -0,0 +1,26 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+// CHECK-NOT: an internal error threw
+// CHECK-NOT: error:
+
+interface IFoo<T, int N>: IDifferentiable
+    where T : __BuiltinFloatingPointType
+{
+    This myFunc(This other);
+}
+
+struct Foo<T, int N>
+    where T : __BuiltinFloatingPointType
+{
+    [BackwardDifferentiable]
+    This myFunc(This other)
+    {
+        [MaxIters(N)]
+        for (int i = 0; i < N; i++) {}
+
+        return this;
+    }
+}
+
+extension <T, int N> Foo<T, N> : IFoo<T, N>
+    where T : __BuiltinFloatingPointType {}
+