diff --git a/toolchain/check/impl.cpp b/toolchain/check/impl.cpp
index 20d05a2e21b2..1293b68f2131 100644
--- a/toolchain/check/impl.cpp
+++ b/toolchain/check/impl.cpp
@@ -129,9 +129,11 @@ static auto CheckAssociatedFunctionImplementation(
               .generic_id,
           self_type_id, witness_inst_id);
 
-  // TODO: This should be a semantic check rather than a syntactic one. The
-  // functions should be allowed to have different signatures as long as we can
-  // synthesize a suitable thunk.
+  // TODO: The functions should be allowed to have different signatures as long
+  // as we can synthesize a suitable thunk. i.e., when there's an implicit
+  // conversion from the original parameter types to the overriding parameter
+  // types, and from the overriding return type to the original return type.
+  // Also, build that thunk.
   if (!CheckFunctionTypeMatches(
           context, context.functions().Get(impl_function_decl->function_id),
           context.functions().Get(interface_function_type.function_id),