Fix crash on overloaded custom derivative function (#2782)

* Fix issue with resolving overloaded custom forward derivative methods.

* Add test

2 files changed, 43 insertions, 0 deletions

diff --git a/tests/autodiff/overloaded-custom-deriv.slang b/tests/autodiff/overloaded-custom-deriv.slang
new file mode 100644
index 000000000..81f91974f
--- /dev/null
+++ b/tests/autodiff/overloaded-custom-deriv.slang
@@ -0,0 +1,41 @@
+//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], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[ForwardDerivative(diff_f)]
+float f(float v)
+{
+    return v * v;
+}
+
+DifferentialPair<float> diff_f(DifferentialPair<float> v)
+{
+    return diffPair(v.p * v.p, 2 * v.d * v.p);
+}
+
+[ForwardDerivative(diff_f)]
+float2 f(float2 v)
+{
+    return v * v;
+}
+
+DifferentialPair<float2> diff_f(DifferentialPair<float2> v)
+{
+    return diffPair(v.p * v.p, float2(2 * v.d.x * v.p.x, 0.0));
+}
+
+[ForwardDifferentiable]
+float test(float v)
+{
+    return f(v) + f(float2(v, v)).x;
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    var p = diffPair(3.0, 1.0);
+    let rs = __fwd_diff(test)(p);
+    outputBuffer[0] = rs.d;
+}
diff --git a/tests/autodiff/overloaded-custom-deriv.slang.expected.txt b/tests/autodiff/overloaded-custom-deriv.slang.expected.txt
new file mode 100644
index 000000000..7da9c9037
--- /dev/null
+++ b/tests/autodiff/overloaded-custom-deriv.slang.expected.txt
@@ -0,0 +1,2 @@
+type: float
+12.000000