Improve diagnostic on differentiablitiy check. (#2687)

Co-authored-by: Yong He <yhe@nvidia.com>

2 files changed, 17 insertions, 1 deletions

diff --git a/tests/diagnostics/autodiff-data-flow-3.slang b/tests/diagnostics/autodiff-data-flow-3.slang
index 0a8e3e58a..21dd9f76c 100644
--- a/tests/diagnostics/autodiff-data-flow-3.slang
+++ b/tests/diagnostics/autodiff-data-flow-3.slang
@@ -18,9 +18,22 @@ float g(float x)
 }
 
 [BackwardDifferentiable]
+void diffOut(inout float x)
+{
+    x = 2;
+}
+
+float noDiffFunc(float x)
+{
+    return 0.0;
+}
+
+[BackwardDifferentiable]
 float h(float x)
 {
     NoDiffField obj;
     obj.fp.f = detach(x * x); // OK.
+    obj.fp.f = noDiffFunc(x); // OK.
+    diffOut(obj.fp.f); // Error.
     return obj.fp.f;
 }
diff --git a/tests/diagnostics/autodiff-data-flow-3.slang.expected b/tests/diagnostics/autodiff-data-flow-3.slang.expected
index 73381cf58..817b595a6 100644
--- a/tests/diagnostics/autodiff-data-flow-3.slang.expected
+++ b/tests/diagnostics/autodiff-data-flow-3.slang.expected
@@ -1,8 +1,11 @@
 result code = -1
 standard error = {
-tests/diagnostics/autodiff-data-flow-3.slang(16): error 41024: derivative is lost during assignment to non-differentiable location. Use 'detach()' to clarify intention.
+tests/diagnostics/autodiff-data-flow-3.slang(16): error 41024: derivative is lost during assignment to non-differentiable location, use 'detach()' to clarify intention.
     obj.fp.f = x * x; // Error, this location cannot hold derivative.
              ^
+tests/diagnostics/autodiff-data-flow-3.slang(37): error 41025: derivative is lost when passing a non-differentiable location to an `out` or `inout` parameter, consider passing a temporary variable instead.
+    diffOut(obj.fp.f); // Error.
+                   ^
 }
 standard output = {
 }