OP's code only works because the method goes from T? to T. If it accepted T and returned T?, he would've gotten an error like this: CS0111: Type 'Utils' alr

OP's code only works because the method goes from T? to T. If it accepted T and returned T?, he would've gotten an error like this: CS0111: Type 'Utils' already defines a member called 'Foo' with the same parameter types That's because the "type constraints are part of the signature of the method and there is no ambiguity" statement is wrong. They are not.