wrong error message with if-let return type mismatch

[durka]

This function

fn foo() -> bool { if let Some(()) = Some(()) { } else { } }

Produces E0308 (correct) with the message "if-let arms have incompatible types" (nonsense).

[durka]

cc @jonathandturner @GuillaumeGomez

[nagisa]

This is what is reported for match version of this:

error: match arms have incompatible types [--explain E0308]
 --> <anon>:2:5
2 |>     match Some(()) {
  |>     ^ expected bool, found ()
note: expected type `_`
note:    found type `()`
note: match arm with an incompatible type
 --> <anon>:3:21
3 |>         Some(()) => { }
  |>                     ^^^

I feel like this falls in the same category, and the issue could be fixed by highlighting the branch with conflicting type.

[durka]

Makes sense, since if let is desugared to match pretty early on. But IMO that error message is wrong too. The arms are compatible with each other, but they don't match the return type of the function. Also, it says "expected type _" and I thought we got rid of that.

[nagisa]

I think there’s multiple ways to interpret the message. I guess you’re interpreting it as

match/if-let arms have incompatible types (between branches, i.e. one branch has mismatching type with another)

whereas another entirely valid interpretation is

match/if-let arm(s) have incompatible types (compared to what they ought to be, given the type of the whole expression)

[durka]

Yes, you're right. Let's add those parentheticals as NOTEs in the error
message :)

On Mon, Aug 15, 2016 at 1:25 PM, Simonas Kazlauskas <
[email protected]> wrote:

I think there’s multiple ways to interpret the message. I guess you’re
interpreting it as

match/if-let arms have incompatible types (between branches, i.e. one
branch has mismatching type with another)

whereas another entirely valid interpretation is

match/if-let arm(s) have incompatible types (compared to what they ought
to be given inferred type of the whole expression)

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[Mark-Simulacrum]

Technically, today's message is correct. Ideally we'd error on the whole expression instead of each arm, but I'm going to close since we no longer say anything about incompatible types of arms.

error[E0308]: mismatched types
 --> test.rs:1:47
  |
1 | fn foo() -> bool { if let Some(()) = Some(()) { } else { } }
  |                                               ^^^ expected bool, found ()
  |
  = note: expected type `bool`
             found type `()`

error[E0308]: mismatched types
 --> test.rs:1:56
  |
1 | fn foo() -> bool { if let Some(()) = Some(()) { } else { } }
  |                                                        ^^^ expected bool, found ()
  |
  = note: expected type `bool`
             found type `()`

error: aborting due to 2 previous errors