Type inference cannot propagate through `map` combinator

[Kobzol]

Hi! I was surprised to see that in the following code, explicit return type (or map turbofish) annotation has to be specified for the map combinator for the code to compile:

struct A;
struct B;

trait Trait {}

impl Trait for A {}
impl Trait for B {}

fn foo(arg: bool) {
    // Compiles
    let _: Option<Box<dyn Trait>> = match arg {
        true => Some(Box::new(A)),
        false => Some(Box::new(B)),
    };

    // Doesn't compile
    /*let _: Option<Box<dyn Trait>> = match arg {
        true => Some(()).map(|v| Box::new(A)),
        false => Some(()).map(|v| Box::new(B)),
    };*/

    // Compiles
    let _: Option<Box<dyn Trait>> = match arg {
        true => Some(()).map(|v| -> Box<dyn Trait> { Box::new(A) }), // or Some(()).map::<Box<dyn Trait>, _>(...)
        false => Some(()).map(|v| -> Box<dyn Trait> { Box::new(B) }),
    };
}

The inference can work it out if I construct the Option directly, but once map is added, it cannot see that both of the maps should coerce to Box<dyn Trait>. Is this a limitation of the current type inference or should this work?

[veera-sivarajan]

A minimized example:

struct A;

trait Trait {}

impl Trait for A {}

fn main() {
    // compiles
    // let closure = || -> Option<Box<dyn Trait>> { Some(Box::new(A)) };
    let closure = || { Some(Box::new(A)) };
    let _: Option<Box<dyn Trait>> = closure();
}

[veera-sivarajan]

I spent some time looking through the debug messages and the bug boils down to coerce_unsized()'s inability to coerce nested types without type annotation.

For example: coerce_unsized() works fine when coercing from &[T; 32] to &[T] or from Arc<T> to Arc<dyn Trait> where T: Trait.

But, it fails to coerce nested types like Option<&[T; 32]> to Option<&[T]>. It hits the Err(traits::Unimplemented) arm:

rust/compiler/rustc_hir_typeck/src/coercion.rs

Lines 651 to 654 in 9c8a58f

	Err(traits::Unimplemented) => {
	debug!("coerce_unsized: early return - can't prove obligation");
	return Err(TypeError::Mismatch);
	}

@rustbot label +A-coercions