Fix issue with synthesizing `Differential` type for self-differential generic types (#4602)

* Fix issue with synthesizing `Differential` type for self-differential generic types

The problem was that we were using the type that was performing the lookup for `.Differential` which can have substitutions based on the local context where the decl is being referenced.

We need to synthesize the type local to the decl itself

* Update auto-differential-type-generic.slang

3 files changed, 76 insertions, 1 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index ee36a21fb..f60ead1e9 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -638,7 +638,12 @@ namespace Slang
                     auto typeDef = m_astBuilder->create<TypeAliasDecl>();
                     typeDef->nameAndLoc.name = item.declRef.getName();
                     typeDef->parentDecl = parent;
-                    typeDef->type.type = subType;
+
+                    // Compute the decl's type as if it is referred to from itself. This is important because
+                    // subType may have substitutions from the context it is used in, while this synthesis step
+                    // is local to the decl.
+                    // 
+                    typeDef->type.type = calcThisType(subType->getDeclRef().getDecl()->getDefaultDeclRef());
                     
                     synthesizedDecl = parent;
 
diff --git a/tests/autodiff/auto-differential-type-generic.slang b/tests/autodiff/auto-differential-type-generic.slang
new file mode 100644
index 000000000..060495e56
--- /dev/null
+++ b/tests/autodiff/auto-differential-type-generic.slang
@@ -0,0 +1,64 @@
+// Tests automatic synthesis of Differential type requirement for generic types.
+//
+// This specifically tests a synthesis path that occurs when the lookup of the Differential type happens before the conformance-check.
+// If this path doesn't construct the generic differential type correctly, it will throw an error when constructing the array
+// in this line: Feature<3>.Differential b = {0.2, 0.3, 0.4};
+// 
+
+//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;
+
+__generic<let C : int> 
+struct Feature: IDifferentiable
+{
+    float vals[C];
+}
+
+
+struct Linear<let C : int>
+{
+    typedef Feature<C> Input;
+    typedef Feature<C> Output;
+
+    [BackwardDerivative(eval_bwd)]
+    Output eval(Input in_feature)
+    {
+        Output out_feature;
+        for (int i = 0; i < C; i++)
+        {
+            out_feature.vals[i] = in_feature.vals[i] * 2.0;
+        }
+        return out_feature;
+    }
+
+    void eval_bwd(inout DifferentialPair<Input> in_feature_pair, Feature<C>.Differential d_output)
+    {
+        /* empty.. doesn't really matter */
+    }
+}
+
+[Differentiable]
+Feature<3> f(Feature<3> a, Linear<3> layer)
+{
+    return layer.eval(a);
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    Feature<3> a = {1.0, 2.0, 3.0};
+    Feature<3>.Differential b = {0.2, 0.3, 0.4};
+
+    Linear<3> layer;
+
+    var dpA = diffPair(a, b);
+
+    var result = fwd_diff(f)(dpA, layer).d;
+
+    outputBuffer[0] = result.vals[0];
+    outputBuffer[1] = result.vals[1];
+    outputBuffer[2] = result.vals[2];
+}
\ No newline at end of file
diff --git a/tests/autodiff/auto-differential-type-generic.slang.expected.txt b/tests/autodiff/auto-differential-type-generic.slang.expected.txt
new file mode 100644
index 000000000..c0d274b2b
--- /dev/null
+++ b/tests/autodiff/auto-differential-type-generic.slang.expected.txt
@@ -0,0 +1,6 @@
+type: float
+0.400000
+0.600000
+0.800000
+0.000000
+0.000000