[AD] Add support for resolving custom derivatives where generic param…

…eters can't be automatically inferred (shader-slang#5630)

* [AD] Add support for resolving custom derivatives where generic parameters can't be automatically inferred

* Fix failing tests

* Update custom-derivative-generic.slang

source/slang/slang-check-decl.cpp

@@ -10915,7 +10915,61 @@ void checkDerivativeAttributeImpl(
     SemanticsContext::ExprLocalScope scope;
     auto ctx = visitor->withExprLocalScope(&scope);
     auto subVisitor = SemanticsVisitor(ctx);
-    auto checkedFuncExpr = visitor->dispatchExpr(attr->funcExpr, ctx);
+
+    auto exprToCheck = attr->funcExpr;
+
+    // If this is a generic, we want to wrap the call to the derivative method
+    // with the generic parameters of the source.
+    //
+    if (as<GenericDecl>(funcDecl->parentDecl) && !as<GenericAppExpr>(attr->funcExpr))
+    {
+        auto genericDecl = as<GenericDecl>(funcDecl->parentDecl);
+        auto substArgs = getDefaultSubstitutionArgs(ctx.getASTBuilder(), visitor, genericDecl);
+        auto appExpr = ctx.getASTBuilder()->create<GenericAppExpr>();
+
+        Index count = 0;
+        for (auto member : genericDecl->members)
+        {
+            if (as<GenericTypeParamDecl>(member) || as<GenericValueParamDecl>(member) ||
+                as<GenericTypePackParamDecl>(member))
+                count++;
+        }
+
+        appExpr->functionExpr = attr->funcExpr;
+
+        for (auto arg : substArgs)
+        {
+            if (count == 0)
+                break;
+
+            if (auto declRefType = as<DeclRefType>(arg))
+            {
+                auto baseTypeExpr = ctx.getASTBuilder()->create<SharedTypeExpr>();
+                baseTypeExpr->base.type = declRefType;
+                auto baseTypeType = ctx.getASTBuilder()->getOrCreate<TypeType>(declRefType);
+                baseTypeExpr->type.type = baseTypeType;
+
+                appExpr->arguments.add(baseTypeExpr);
+            }
+            else if (auto genericValParam = as<GenericParamIntVal>(arg))
+            {
+                auto declRef = genericValParam->getDeclRef();
+                appExpr->arguments.add(
+                    subVisitor
+                        .ConstructDeclRefExpr(declRef, nullptr, nullptr, SourceLoc(), nullptr));
+            }
+            else
+            {
+                SLANG_UNEXPECTED("Unhandled substitution arg type");
+            }
+
+            count--;
+        }
+
+        exprToCheck = appExpr;
+    }
+
+    auto checkedFuncExpr = visitor->dispatchExpr(exprToCheck, ctx);
     attr->funcExpr = checkedFuncExpr;
     if (attr->args.getCount())
         attr->args[0] = attr->funcExpr;
@@ -11427,6 +11481,26 @@ void checkDerivativeOfAttributeImpl(
     calleeDeclRef = calleeDeclRefExpr->declRef;
 
     auto calleeFunc = as<FunctionDeclBase>(calleeDeclRef.getDecl());
+
+    if (!calleeFunc)
+    {
+        // If we couldn't find a direct function, it might be a generic.
+        if (auto genericDecl = as<GenericDecl>(calleeDeclRef.getDecl()))
+        {
+            calleeFunc = as<FunctionDeclBase>(genericDecl->inner);
+
+            if (as<ErrorType>(resolved->type.type))
+            {
+                // If we can't resolve a type, something went wrong. If we're working with a generic
+                // decl, the most likely cause is a failure of generic argument inference.
+                //
+                visitor->getSink()->diagnose(
+                    derivativeOfAttr,
+                    Diagnostics::cannotResolveGenericArgumentForDerivativeFunction);
+            }
+        }
+    }
+
     if (!calleeFunc)
     {
         visitor->getSink()->diagnose(

tests/autodiff/custom-derivative-enum-param.slang

@@ -0,0 +1,57 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-vk -compute -shaderobj -output-using-type
+
+enum MyEnum { A, B, C };
+
+[BackwardDerivative(mDiff)]
+float m<let M : MyEnum>(float x) 
+{ 
+    switch (M)
+    {
+        case MyEnum.A:
+            return x * x;
+        case MyEnum.B:
+            return x;
+        case MyEnum.C:
+            return 3 * x;
+        default:
+            return 0;
+    }
+}
+
+void mDiff<let M : MyEnum>(inout DifferentialPair<float> x, float dResult)
+{
+    switch (M)
+    {
+        case MyEnum.A:
+            updateDiff(x, 2 * dResult * x.p);
+            break;
+        case MyEnum.B:
+            updateDiff(x, dResult);
+            break;
+        case MyEnum.C:
+            updateDiff(x, 3 * dResult);
+            break;
+        default:
+            updateDiff(x, 0);
+            break;
+    }
+}
+
+[Differentiable]
+float test(float x)
+{
+    return m<MyEnum.A>(x);
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    var a = diffPair(3.0);
+    __bwd_diff(test)(a, 1.0);
+    outputBuffer[dispatchThreadID.x] = a.d;
+    // CHECK: 6.0
+}

tests/diagnostics/custom-derivative-generic.slang

@@ -34,7 +34,7 @@ DifferentialPair<float> dd1(DifferentialPair<float> x)
 }
 
 // CHECK-DAG: {{.*}}(37): error 31151
-[BackwardDerivative(f)]
+[BackwardDerivativeOf(f)]
 DifferentialPair<float> df<let N:int>(inout DifferentialPair<float> x, float dOut)
 {
     var primal = x.p * x.p;