Fix lowering of associated types in generic interfaces (#6600)

* Fix lowering of associated types in generic interfaces.

* Update diff-assoctype-generic-interface.slang

* Fix-up lowering of differentiable witnesses for implicit ops

* Update slang-ir-autodiff-transcriber-base.cpp

* Fix issue with differentiating type-packs

1 files changed, 110 insertions, 0 deletions

diff --git a/tests/autodiff/diff-assoctype-generic-interface.slang b/tests/autodiff/diff-assoctype-generic-interface.slang
new file mode 100644
index 000000000..79e0eff08
--- /dev/null
+++ b/tests/autodiff/diff-assoctype-generic-interface.slang
@@ -0,0 +1,110 @@
+// Test calling differentiable function through dynamic dispatch.
+
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-vk -compute -shaderobj -output-using-type -compile-arg -skip-spirv-validation -emit-spirv-directly
+
+//TEST_INPUT:ubuffer(data=[2 0 0 0 0 0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+interface IGetter : IDifferentiable
+{
+    [Differentiable]
+    float get(uint id);
+}
+
+struct GetterImpl : IGetter
+{
+    float[8] data;
+
+    __init(float[8] data)
+    { this.data = data; }
+
+    [Differentiable]
+    float get(uint id)
+    {
+        return data[id];
+    }
+}
+interface IFoo<int N>
+{
+    associatedtype Params : IGetter;
+
+    [Differentiable]
+    Params bar();
+}
+
+[BackwardDerivative(load_bwd)]
+[ForwardDerivative(load_fwd)]
+float load(uint id)
+{
+    return outputBuffer[id] + 2;
+}
+
+DifferentialPair<float> load_fwd(uint id)
+{
+    return DifferentialPair<float>(load(id), 3.f);
+}
+
+void load_bwd(uint id, float.Differential dOut)
+{
+    outputBuffer[id + 8] = dOut;
+}
+
+struct FooImpl1: IFoo<8>
+{   
+    typealias Params = GetterImpl;
+
+    __init()
+    { }
+
+    [Differentiable]
+    Params bar()
+    {
+        float x = load(0);
+        return GetterImpl({x, x+1, x+2, x+3, x+4, x+5, x+6, x+7});
+    }
+}
+
+/*
+// There's a slight issue with dynamic dispatch over generic interfaces. Uncomment after that is fixed.
+
+struct FooImpl2: IFoo<8>
+{
+    typealias Params = GetterImpl;
+
+    __init()
+    { }
+
+    [Differentiable]
+    Params bar()
+    {
+        float x = 2 * load(0);
+        return GetterImpl({x, x+5, x+7, x+9, x+11, x+13, x+15, x+17});
+    }
+}
+*/
+
+IFoo<8> getFoo(uint id)
+{
+    /*if (id == 0)
+        return FooImpl1();
+    else
+        return FooImpl2();*/
+    return FooImpl1();
+}
+
+[Differentiable]
+float doThing(uint id)
+{
+    IFoo<8> foo = getFoo(id);
+    return foo.bar().get(0);
+}
+
+[shader("compute")]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    outputBuffer[0] = doThing(0); // CHECK: 2.0
+    outputBuffer[1] = doThing(1); // CHECK: 4.0
+
+    outputBuffer[2] = fwd_diff(doThing)(0).d; // CHECK: 3.0
+    outputBuffer[3] = fwd_diff(doThing)(1).d; // CHECK: 3.0
+}
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