"expected type parameter but found type parameter" #3404
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Reproduced on 65ed803 |
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Triage bump, still an issue. This is often caused by a mismatch between lifetimes. |
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Confirmed that this is still an issue with Rust 0.9. I ran into it playing around with linked list implementations: pub enum List<T> {
Cons(T, ~List<T>),
Nil,
}
impl<T> List<T> {
fn prepend<T>(mut self, value: T) {
self = Cons(value, ~self);
}
}This causes the compiler to emit two errors, neither of which is very helpful: |
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accepted for P-low. |
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Oct 31, 2014
…atsakis This PR aims to improve the readability of diagnostic messages that involve unresolved type variables. Currently, messages like the following: ```rust mismatched types: expected `core::result::Result<uint,()>`, found `core::option::Option<<generic #1>>` <anon>:6 let a: Result<uint, ()> = None; ^~~~ mismatched types: expected `&mut <generic #2>`, found `uint` <anon>:7 f(42u); ^~~ ``` tend to appear unapproachable to new users. [0] While specific type var IDs are valuable in diagnostics that deal with more than one such variable, in practice many messages only mention one. In those cases, leaving out the specific number makes the messages slightly less terrifying. ```rust mismatched types: expected `core::result::Result<uint, ()>`, found `core::option::Option<_>` <anon>:6 let a: Result<uint, ()> = None; ^~~~ mismatched types: expected `&mut _`, found `uint` <anon>:7 f(42u); ^~~ ``` As you can see, I also tweaked the aesthetics slightly by changing type variables to use the type hole syntax _. For integer variables, the syntax used is: ```rust mismatched types: expected `core::result::Result<uint, ()>`, found `core::option::Option<_#1i>` <anon>:6 let a: Result<uint, ()> = Some(1); ``` and float variables: ```rust mismatched types: expected `core::result::Result<uint, ()>`, found `core::option::Option<_#1f>` <anon>:6 let a: Result<uint, ()> = Some(0.5); ``` [0] https://twitter.com/coda/status/517713085465772032 Closes #2632. Closes #3404. Closes #18426.
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This is a terrible error message. See
compile-fail/issue-2611-5.rs, which I'm about to check in, for an example program that triggers it.The text was updated successfully, but these errors were encountered: