Fix erroneous diagnostic when checking a generic differentiable mutable method. (#3192)

* Add a unit test

* Fix.

---------

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

2 files changed, 54 insertions, 0 deletions

diff --git a/tests/autodiff/struct-this-parameter.slang b/tests/autodiff/struct-this-parameter.slang
new file mode 100644
index 000000000..9c8ddc724
--- /dev/null
+++ b/tests/autodiff/struct-this-parameter.slang
@@ -0,0 +1,50 @@
+//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], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+struct A : IDifferentiable
+{
+    float data[3];
+
+    [Differentiable]
+    __init()
+    {
+        [ForceUnroll] for (uint i = 0; i < 3; i++) data[i] = 0.f;
+    }
+
+    [mutating]
+    [Differentiable]
+    void write(float value, inout uint offset)
+    {
+        data[offset++] = value;
+    }
+
+    [mutating]
+    [Differentiable]
+    void write<let N : int>(vector<float, N> value, inout uint offset)
+    {
+        [ForceUnroll] for (uint i = 0; i < N; i++) write(value[i], offset);
+    }
+}
+
+[Differentiable]
+float3 run(float3 x)
+{
+    A a = A();
+    uint offset = 0;
+    a.write(x, offset);
+    return float3(a.data[0], a.data[1], a.data[2]);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    DifferentialPair<float3> dp = diffPair(float3(1.f, 2.f, 3.f), float3(0.f));
+    float3 dOut = float3(1.f, 0.5f, 0.1f);
+    bwd_diff(run)(dp, dOut);
+    outputBuffer[0] = dp.d[0];
+    outputBuffer[1] = dp.d[1];
+    outputBuffer[2] = dp.d[2];
+}
diff --git a/tests/autodiff/struct-this-parameter.slang.expected.txt b/tests/autodiff/struct-this-parameter.slang.expected.txt
new file mode 100644
index 000000000..bd23befb3
--- /dev/null
+++ b/tests/autodiff/struct-this-parameter.slang.expected.txt
@@ -0,0 +1,4 @@
+type: float
+1.000000
+0.500000
+0.100000