fix a crash when using type equality constaint (#8515)

Close #8193.

When constructing `TransitiveTypeWitness` node, we should check if there
is operand that represents two equal times. Currently, we only check
whether the operand is `TypeEqualityWitness`, which is not good enough,
because a `DeclaredSubtypeWitness` could also be representing two same
types, in that case, we should also const fold this kind of witness.

Fails to do so, we could finally ends up with a generating a lookup
witness IR on a generic parameter that is not supposed to be looked up.

1 files changed, 35 insertions, 0 deletions

diff --git a/tests/autodiff/self-differential-type-equality-constraint.slang b/tests/autodiff/self-differential-type-equality-constraint.slang
new file mode 100644
index 000000000..0baaa07d6
--- /dev/null
+++ b/tests/autodiff/self-differential-type-equality-constraint.slang
@@ -0,0 +1,35 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=BUFFER):-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=BUFFER):-vk -compute -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=BUFFER):-cuda -compute -shaderobj -output-using-type
+
+interface IV : IDifferentiablePtrType{
+	int get();
+}
+
+struct V : IV
+{
+	typealias Differential = This;
+
+	int get()
+	{
+		return 12;
+	}
+}
+
+int g<T:IV>(DifferentialPtrPair<T> obj)
+	where T.Differential == T
+{
+	return obj.d.get();
+}
+
+//TEST_INPUT:ubuffer(data=[0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+[shader("compute")]
+void computeMain()
+{
+	V v = {};
+	DifferentialPtrPair<V> p = {v, v};
+
+	// BUFFER: 12
+	outputBuffer[0] = g(p);
+}