But deferring the error means the implementation does not work equally well: it sacrifices safety. When a programmer tries to combine strings X and Y that are i

But deferring the error means the implementation does not work equally well: it sacrifices safety. When a programmer tries to combine strings X and Y that are incompatible, it's because he misunderstands what X or Y actually represents. This misunderstanding is detected when X and Y are brought together, but that doesn't mean it can't also have corrupted upstream code that also uses X or Y, just not together. This upstream code never gets a chance to run if the downstream problem is detected at compile time. But, if you only catch the problem at run time, after the upstream code has already executed, that corrupted upstream code could have launched the missiles. Run time is too late.