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

Using Swift
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14

You wouldn't have to add explicit type annotations within the body; that would clearly not be consistent with current type-checker behavior.

A one-way constraint is like the status quo for type-checking a variable binding:

let x = 15
return x + UInt(27)

This doesn't type-check because type information only flows "one way" through the variable binding: we decide that x: Int without considering that x is required to be convertible to UInt on the next line.

Another way of understanding the use of one-way constraints in function builders is that they're as if the closure was transformed to assign all the statement results into locals. So given this closure:

Sequential { 
  Conv2D<Float>(...)
  AvgPool2D<Float>(...)
  Flatten<Float>()
  Dense<Float>(...)
  Dense<Float>(...)
}

Instead of thinking of the transform like this:

Sequential {
  LayerBuilder.buildBlock(
    Conv2D<Float>(...),
    AvgPool2D<Float>(...),
    Flatten<Float>(),
    Dense<Float>(...),
    Dense<Float>(...))
}

Think of it more like this:

Sequential { 
  let a = Conv2D<Float>(...)
  let b = AvgPool2D<Float>(...)
  let c = Flatten<Float>()
  let d = Dense<Float>(...)
  let e = Dense<Float>(...)
  return LayerBuilder.buildBlock(a, b, c, d, e)
}
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