Function builder cannot infer generic parameters even though direct call to `buildBlock` can

[dan-zheng]

(moving bugs.swift.org discussion here, upon request)

Thanks for the technical explanations John! The "lack of type inference" is expected behavior given how function builder syntax is expanded into variable bindings.

I'm still curious about answers to @anandabits's use-case-motivated questions from above:

@anandabits

Understood. What isn't clear to me though is why this is necessary in function builders if an analogous fluent builder API doesn't run into the same issues. Is there a non-obvious difference in type checking consequences between the two approaches to syntax? Or is the intent to discourage libraries that would rely on this kind of inference altogether?

Should libraries give up on ever using function builders for this style of type inference? Are libraries encouraged to use explicit generic parameters or a different pattern (e.g. fluent method-chaining builders) instead?

// Will this `<Float>` generic parameter type inference never be possible?
let model = Sequential {
    Conv2D<Float>(...) // first layer, feeding output to:
    AvgPool2D(...) // AvgPool2D<Float>, feeding output to...
    Flatten() // Flatten<Float>
    Dense(...) // Dense<Float>
    Dense(...) // Dense<Float>
}