Fix data-flow analysis not propagating diff property through differentiable calls (#3010)

* Add test for nodiff diagnostic for non-diff call propagated through diff call

* Add logic to disambiguate calls to differentiable and non-differentiable methods

* Add expected results for test

* Simplify test

* Update slang-ir-check-differentiability.cpp

* Added comments for TreatAsDifferentiableExpr flavors

---------

Co-authored-by: Yong He <yonghe@outlook.com>

2 files changed, 38 insertions, 0 deletions

diff --git a/tests/diagnostics/autodiff-data-flow-4.slang b/tests/diagnostics/autodiff-data-flow-4.slang
new file mode 100644
index 000000000..08fabf954
--- /dev/null
+++ b/tests/diagnostics/autodiff-data-flow-4.slang
@@ -0,0 +1,30 @@
+//DIAGNOSTIC_TEST:SIMPLE:
+
+float nonDiff(float x)
+{
+    return x;
+}
+
+[Differentiable]
+float f(float x)
+{
+    return x * x;
+}
+
+[Differentiable]
+float h(float x)
+{
+    float val = 0;
+
+    // call to non-differentiable method 
+    // (should error if the data-flow propagation works properly.)
+    // 
+    float y = nonDiff(x);
+
+    // call to a differentiable method, using the
+    // result of a non-differentiable call.
+    // 
+    val = f(y);
+
+    return val;
+}
diff --git a/tests/diagnostics/autodiff-data-flow-4.slang.expected b/tests/diagnostics/autodiff-data-flow-4.slang.expected
new file mode 100644
index 000000000..69f5d8707
--- /dev/null
+++ b/tests/diagnostics/autodiff-data-flow-4.slang.expected
@@ -0,0 +1,8 @@
+result code = -1
+standard error = {
+tests/diagnostics/autodiff-data-flow-4.slang(29): error 41020: derivative cannot be propagated through call to non-backward-differentiable function `nonDiff`, use 'no_diff' to clarify intention.
+    float y = nonDiff(x);
+                     ^
+}
+standard output = {
+}