Redirect destination-driven lowering logic to regular lowering for `SelectExpr` (#3219)

* Redirect destination-driven lowering logic to regular lowering for select expr

* Add test

2 files changed, 42 insertions, 0 deletions

diff --git a/tests/autodiff/nan-select.slang b/tests/autodiff/nan-select.slang
new file mode 100644
index 000000000..31a185e6b
--- /dev/null
+++ b/tests/autodiff/nan-select.slang
@@ -0,0 +1,36 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute -shaderobj -output-using-type
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+typedef DifferentialPair<float> dpfloat;
+typedef float.Differential dfloat;
+
+[BackwardDifferentiable]
+float f(float x)
+{
+    float k = x > 1e-8 ? atan2(x, x) : 0.0;
+    return k;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    {
+        dpfloat dpa = diffPair(2.0);
+
+        __bwd_diff(f)(dpa, 1.0);
+
+        outputBuffer[0] = dpa.d; // Expect: 0.0
+    }
+
+    {
+        dpfloat dpa = diffPair(0.0,);
+
+        __bwd_diff(f)(dpa, 1.0);
+
+        outputBuffer[1] = dpa.d; // Expect: 0.0, but if select is not turned into if-else,
+                                 // this will be 'inf' or 'nan'
+    }
+}
diff --git a/tests/autodiff/nan-select.slang.expected.txt b/tests/autodiff/nan-select.slang.expected.txt
new file mode 100644
index 000000000..e070cf84d
--- /dev/null
+++ b/tests/autodiff/nan-select.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+0.000000
+0.000000
+0.000000
+0.000000
+0.000000