exec: prevent VtArray from being used as a computation result type

To help avoid confusion around Vdf's array flattening behavior,
prevent plugin authors from defining computations that return VtArray
values or registering VtArray types with execution.

(Internal change: 2368583)

Author: jloy

pxr/exec/exec/computationBuilders.h

@@ -26,6 +26,7 @@
 
 #include "pxr/base/tf/token.h"
 #include "pxr/base/tf/type.h"
+#include "pxr/base/vt/traits.h"
 #include "pxr/exec/vdf/context.h"
 #include "pxr/usd/sdf/path.h"
 
@@ -844,6 +845,9 @@ Exec_PrimComputationBuilder&
 Exec_PrimComputationBuilder::Callback(
     ExecCallbackFn &&callback)
 {
+    static_assert(!VtIsArray<ResultType>::value,
+                  "VtArray is not a supported result type");
+
     _AddCallback(
         std::move(callback),
         ExecTypeRegistry::GetInstance().CheckForRegistration<ResultType>());
@@ -860,6 +864,9 @@ Exec_PrimComputationBuilder::Callback(
 {
     static_assert(!std::is_reference_v<ResultType>,
                   "Callback functions must return by value");
+    static_assert(!VtIsArray<ResultType>::value,
+                  "VtArray is not a supported result type");
+
     _AddCallback(
         [callback](const VdfContext& ctx) { ctx.SetOutput(callback(ctx)); },
         ExecTypeRegistry::GetInstance().CheckForRegistration<ResultType>());

pxr/exec/exec/testenv/testExecTypeRegistration.cpp

@@ -62,9 +62,6 @@ TestBasicRegistration()
 
     reg.RegisterType(TestExecTypeRegistrationValue{});
     reg.CheckForRegistration<TestExecTypeRegistrationValue>();
-
-    reg.RegisterType(VtArray<TestExecTypeRegistrationValue>{});
-    reg.CheckForRegistration<VtArray<TestExecTypeRegistrationValue>>();
 }
 
 static void
@@ -189,23 +186,6 @@ TestCreateVector()
         // define only what is necessary for VtValue & VdfVector.
         TF_AXIOM(accessor[0] == TestExecTypeRegistrationValue{});
     }
-
-    // VtArray<TestExecTypeRegistrationValue> is not known to Vt and is not an
-    // Sdf value type.
-    {
-        static_assert(!VtIsKnownValueType<
-                      VtArray<TestExecTypeRegistrationValue>>());
-        static_assert(!SdfValueTypeTraits<
-                      VtArray<TestExecTypeRegistrationValue>>::IsValueType);
-        const VtArray<TestExecTypeRegistrationValue> arr = { {}, {} };
-        const VdfVector vec = reg.CreateVector(VtValue(arr));
-        TF_AXIOM(vec.Holds<TestExecTypeRegistrationValue>());
-        const auto accessor = vec.GetReadAccessor<
-            TestExecTypeRegistrationValue>();
-        ASSERT_EQ(accessor.GetNumValues(), 2);
-        TF_AXIOM(accessor[0] == arr[0]);
-        TF_AXIOM(accessor[1] == arr[1]);
-    }
 }
 
 static void

pxr/exec/exec/typeRegistry.h

@@ -75,6 +75,8 @@ class ExecTypeRegistry
     ///
     template <typename ValueType>
     static void RegisterType(const ValueType &fallback) {
+        static_assert(!VtIsArray<ValueType>::value,
+                      "VtArray is not a supported execution value type");
         _GetInstanceForRegistration()._RegisterType(fallback);
     }