setfield! does not call `convert`

[stevengj]

type Foo; x::Float64; end
foo = Foo(0)
foo.x = 3
foo.(:x) = 4
setfield!(foo, :x, 5)

The last line throws TypeError: setfield!: expected Float64, got Int64.

[yuyichao]

This is expected since convert is inserted by the parser. Calling convert in the actual builtin sounds much more complicated (it needs to find the right one for example).

[stevengj]

Since the foo.(:x) syntax may be going away (#6268 and/or #15032), it seems desirable to beef up setfield! a bit so that it is more usable.

[stevengj]

The relevant code is in builtins.c. Since it knows the type that it wants at that point, it doesn't seem too hard to insert a call to Base.convert.

[vtjnash]

We can change it so that Base exports:

setfield!(x, i, v) = Core.setfield!(x, i, convert(fieldtype(x, i), v))

Otherwise, it's pretty hard to insert a call to convert directly from an Intrinsic, since Core.Intrinsics.convert is probably not what you ever intended to call (it's undefined) :P