[afro88]

What you're describing is more or less fuzzing [1], at the unit level. I can't remember the names, but there are tools around that work like this at runtime (ie you define a test that executes functions from the test library that run tests based on input/output types and other user defined constraints).

There's almost always more business logic to what a unit should do than it's types though. Depending on the language, the type system can only encode so much of that logic.

Consider the opposite: can't the compiler generate implementations from types and interfaces? In most cases, no. LLMs are filling some of that gap though because they can use some surrounding context to return the high probability implementation (completion) from the interface or type definition.

[1] https://en.m.wikipedia.org/wiki/Fuzzing