Eagerly ensure that user-supplied providers always return the expected type. Historically this was implicitly validated upon injection into whatever used the provider (via reflection magic). However, with the shift to MethodHandles, that particular signature check becomes harder to isolate/replicate without adding a bunch of unnecessary overhead. Thus, we instead change this to validate user-supplied providers when they provide the value (which is the soonest it can go wrong).

Eagerly ensure that user-supplied providers always return the expected type. Historically this was implicitly validated upon injection into whatever used the provider (via reflection magic). However, with the shift to MethodHandles, that particular signature check becomes harder to isolate/replicate without adding a bunch of unnecessary overhead. Thus, we instead change this to validate user-supplied providers when they provide the value (which is the soonest it can go wrong). #866