Change:
I realize it's kinda nonsensical, but here:
Why didn't WF catch this?
That's because only checks statics/constants.
Still reproducible with .
This ought to be a mostly easy fix. The problem is that the well formedness check, which ought to be rejected this code, is ignoring things in an : They fall through to this `` case: We would need to add support for the HIR item: which in turn has a number of items: which are defined here: presumably we want the to check that the types of are well-formed, roughly analogous to how we current check normal static items: (We probably want similar checks on foreign item fns, though?) One tricky part might be compatibility, we may need to turn this into a compatibility lint in case there is lots of broken code in the wild.
I am trying to fix this bug, and need help moving forward. function is modified to accept a and a , yet it doesn't report any additional errors and the ICE prevails. Test file: Commit: Should I create a pull request or is it too early?
Looks like suggestions are incomplete ( does not require ). Also, I left a comment on your commit about how you can avoid adding . I suspect the only reason regular s are required to be is they have an initializer (and that, not the type, requires ), i.e. for we have these errors:
Oddly, the PR that were previously posted to fix this (PR , following on PR ) was subsequently closed as inactive ... but the bug itself seems like it may have been resolved in the meantime?
I'm going to assume this was fixed. Tagging as E-needstest, but if someone wants to take the time, it'd be even better to bisect to where this was fixed, and see if a regression test for this was at that time.
Compiler no longer panics, tested with

Discussion:
<ins> fn main() { extern { static symbol: [usize]; //~ ERROR: the size for values of type } println!("{}", symbol[0]); } </ins>

<ins> error[E0277]: the size for values of type `[usize]` cannot be known at compilation time --> $DIR/issue-36122-accessing-externed-dst.rs:3:24 | LL |         static symbol: [usize]; |                        ^^^^^^^ doesn't have a size known at compile-time | = help: the trait `std::marker::Sized` is not implemented for `[usize]` = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait> error: aborting due to previous error For more information about this error, try `rustc --explain E0277`. </ins>

<ins> // Fixed in #66054. // ignore-tidy-trailing-newlines // error-pattern: aborting due to 2 previous errors #[Ѕ </ins>  No newline at end of file

<ins> error: this file contains an un-closed delimiter --> $DIR/issue-58094-missing-right-square-bracket.rs:4:4 | LL | #[Ѕ |  - ^ |  | |  un-closed delimiter error: expected item after attributes --> $DIR/issue-58094-missing-right-square-bracket.rs:4:4 | LL | #[Ѕ |    ^ error: aborting due to 2 previous errors </ins>

<del> #![feature(fn_traits, unboxed_closures)] fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {} struct Compose<F,G>(F,G); impl<T,F,G> FnOnce<(T,)> for Compose<F,G> where F: FnOnce<(T,)>, G: FnOnce<(F::Output,)> { type Output = G::Output; extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output { (self.1)((self.0)(x)) } } struct Str<'a>(&'a str); fn mk_str<'a>(s: &'a str) -> Str<'a> { Str(s) } fn main() { let _: for<'a> fn(&'a str) -> Str<'a> = mk_str; // expected concrete lifetime, found bound lifetime parameter 'a let _: for<'a> fn(&'a str) -> Str<'a> = Str; //~^ ERROR: mismatched types test(|_: &str| {}); test(mk_str); // expected concrete lifetime, found bound lifetime parameter 'x test(Str); //~ ERROR: type mismatch in function arguments test(Compose(|_: &str| {}, |_| {})); test(Compose(mk_str, |_| {})); // internal compiler error: cannot relate bound region: //   ReLateBound(DebruijnIndex { depth: 2 }, //     BrNamed(DefId { krate: 0, node: DefIndex(6) => test::'x }, 'x(65))) //<= ReSkolemized(0, //     BrNamed(DefId { krate: 0, node: DefIndex(6) => test::'x }, 'x(65))) test(Compose(Str, |_| {})); } </del>

<del> error[E0308]: mismatched types --> $DIR/issue-30904.rs:20:45 | LL |     let _: for<'a> fn(&'a str) -> Str<'a> = Str; |                                             ^^^ expected concrete lifetime, found bound lifetime parameter 'a | = note: expected type `for<'a> fn(&'a str) -> Str<'a>` found type `fn(&str) -> Str<'_> {Str::<'_>}` error[E0631]: type mismatch in function arguments --> $DIR/issue-30904.rs:26:10 | LL | fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {} |    ----    -------------------------- required by this bound in `test` ... LL | struct Str<'a>(&'a str); | ------------------------ found signature of `fn(&str) -> _` ... LL |     test(Str); |          ^^^ expected signature of `for<'x> fn(&'x str) -> _` error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0308`. </del>

<ins> error: higher-ranked subtype error --> $DIR/issue-30906.rs:15:5 | LL |     test(Compose(f, |_| {})); |     ^^^^^^^^^^^^^^^^^^^^^^^^ error: aborting due to previous error </ins>

<ins> #![feature(fn_traits, unboxed_closures)] fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {} struct Compose<F,G>(F,G); impl<T,F,G> FnOnce<(T,)> for Compose<F,G> where F: FnOnce<(T,)>, G: FnOnce<(F::Output,)> { type Output = G::Output; extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output { (self.1)((self.0)(x)) } } fn bad<T>(f: fn(&'static str) -> T) { test(Compose(f, |_| {})); //~ ERROR: mismatched types } fn main() {} </ins>

<ins> error[E0308]: mismatched types --> $DIR/issue-30906.rs:15:5 | LL |     test(Compose(f, |_| {})); |     ^^^^ one type is more general than the other | = note: expected type `std::ops::FnOnce<(&'x str,)>` found type `std::ops::FnOnce<(&str,)>` error: aborting due to previous error For more information about this error, try `rustc --explain E0308`. </ins>