Various fixes around differentiable member associations `[DerivativeMember(<diff-member>)]` (#4525)

* Add diagnostic for missing diff-member associations

+ Automatically create diff member associations if differential type is the same as the primal type.
+ Move diff-member attribute checking to conformance-checking phase to avoid circularity issues.

Fixes #4103

* Update slang-check-decl.cpp

---------

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

1 files changed, 43 insertions, 0 deletions

diff --git a/tests/autodiff/struct-without-diff-associations.slang b/tests/autodiff/struct-without-diff-associations.slang
new file mode 100644
index 000000000..dea7b9630
--- /dev/null
+++ b/tests/autodiff/struct-without-diff-associations.slang
@@ -0,0 +1,43 @@
+//TEST:SIMPLE(filecheck=CHECK): -target hlsl -line-directive-mode none -entry computeMain -stage compute
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+struct Foo2 : IDifferentiable
+{
+    float dx[3];
+}
+
+struct Foo : IDifferentiable
+{
+    typealias Differential = Foo2;
+
+    float x[3];
+    // CHECK: tests/autodiff/struct-without-diff-associations.slang(15): error 30102: differentiable member 'x' should have a corresponding field in 'Foo2'. Use [DerivativeMember(Foo2.<field-name>)] or mark as no_diff
+    // CHECK-NEXT: float x[3];
+    // CHECK-NEXT:       ^
+};
+
+[Differentiable]
+Foo getFoo(float x)
+{
+    return { { x, x, x } };
+}
+
+[Differentiable]
+float foobar(float x)
+{
+    int i = 3 * int(floor(x));
+    Foo foo = getFoo(x);
+    return foo.x[i] * foo.x[i];
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    {
+        float a = 0.5;
+        var d = fwd_diff(foobar)(diffPair(a, 1.0)).d;
+        outputBuffer[0] = d;
+    }
+}
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