Higher-order argument modes generate contradictory compile errors

[bblum]

error: mismatched types: expected fn@(&&<V1>) but found native fn(str) (expected argument mode ++ but found &&)

This arose when passing a fn(str) as an argument to a function expecting fn@(T). No explicit argument mode annotation was used.

note the contradiction: "expected ..&&.." at first followed by "expected ..++.." later. Patrick seems to think they must be referring to the same thing (the argument of the function being passed) because it wouldn't come up without higher-order arguments.

[catamorphism]

This problem will hopefully go away when argument modes go away, which is hopefully soon.

[bblum]

hopefully!

in the meantime i will also note that it appears to simply have it backwards. changing the fn(str) to fn(-str) produces:

error: mismatched types: expected fn@(&&uint) but found native fn(-<V10>) (expected argument mode - but found &&)

[catamorphism]

Yeah, those "backwards" errors are endemic in typeck -- keep reporting them.

[catamorphism]

(marking as "postponed" since we can probably close this when argument modes go away)

[kud1ing]

Is there a "remove argument mode" issue to link this issue to? Also #2585 is waiting for this.

[nikomatsakis]

The fix is to modify infer to track which value comes "from the user" and which does not.

[nikomatsakis]

I think I opened an issue for this... I'll search and open one if not. Or maybe just fix it. It's just some tedious work.

[nikomatsakis]

closing as a dup of #2806.