Add a test case for dynamic dispatch with This type in interface decl. (shader-slang#1431)

* Add a test case for dynamic dispatch with `This` type in interface decl.

* Update comments

* fix typo in comments

Co-authored-by: Tim Foley <tfoleyNV@users.noreply.github.com>

Author: csyonghe

source/slang/slang-lower-to-ir.cpp

@@ -1629,20 +1629,13 @@ struct ValLoweringVisitor : ValVisitor<ValLoweringVisitor, LoweredValInfo, Lower
 
     LoweredValInfo visitThisType(ThisType* type)
     {
-        // TODO: In theory, we should only run into a `ThisType` when lowering a concrete
-        // declaration defined on an interface type (e.g., via an `extension`).
-        //
-        // There is an open question of how we should emit a concrete method (say) defined
-        // on an `interface` type. We could emit the code in "object-oriented" style,
-        // passing in a `this` parameter of type `IFoo`, or we could emit it in a "generic"
-        // type where the whole member is wrapped in a generic on `<This : IFoo>`.
-        //
-        // The generic option has the benefit of having a clear solution in the case of
-        // static members that don't have a `this` parameter, but might still need `This`,
-        // but we have so far favored the "object-oriented" lowering for code involving
-        // bare interface types.
-        //
-        // For now we punt and emit the `ThisType` of an interface `IFoo` as `IFoo`.
+        // A `This` type in an interface decl should lower to `IRThisType`,
+        // while `This` type in a concrete `struct` should lower to the `struct` type
+        // itself. A `This` type reference in a concrete type is already translated to that
+        // type in semantics checking in this setting.
+        // If we see `This` type here, we are dealing with `This` inside an interface decl.
+        // Therefore, `context->thisType` should have been set to `IRThisType`
+        // in `visitInterfaceDecl`, and we can just use that value here.
         //
         if (context->thisType != nullptr)
             return LoweredValInfo::simple(context->thisType);

tests/compute/dynamic-dispatch-5.slang

@@ -0,0 +1,39 @@
+//TEST(compute):COMPARE_COMPUTE:-cpu -xslang -allow-dynamic-code
+//DISABLE_TEST(compute):COMPARE_COMPUTE:-cuda -xslang -allow-dynamic-code
+
+// Test dynamic dispatch code gen for general `This` type.
+
+interface IInterface
+{
+	int Compute(int inVal, This other);
+};
+
+int GenericCompute<T:IInterface>(int inVal, T obj, T other)
+{
+	return obj.Compute(inVal, other);
+}
+
+struct Impl : IInterface
+{
+    int base;
+	int Compute(int inVal, This other) { return other.base + base + inVal; }
+};
+int test(int inVal)
+{
+    Impl obj1, obj2;
+    obj1.base = 1;
+    obj2.base = 2;
+	return GenericCompute<Impl>(inVal, obj1, obj2);
+}
+
+//TEST_INPUT:ubuffer(data=[0 1 2 3], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer : register(u0);
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+	uint tid = dispatchThreadID.x;
+	int inVal = outputBuffer[tid];
+	int outVal = test(inVal);
+	outputBuffer[tid] = outVal;
+}

tests/compute/dynamic-dispatch-5.slang.expected.txt

@@ -0,0 +1,4 @@
+3
+4
+5
+6