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mir typeck: subtyping cannot deal with bivariance #104409

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lcnr opened this issue Nov 14, 2022 · 2 comments · Fixed by #104411
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mir typeck: subtyping cannot deal with bivariance #104409

lcnr opened this issue Nov 14, 2022 · 2 comments · Fixed by #104411
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A-variance Area: Variance (https://doc.rust-lang.org/nomicon/subtyping.html) C-bug Category: This is a bug. I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ T-types Relevant to the types team, which will review and decide on the PR/issue.

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@lcnr
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lcnr commented Nov 14, 2022

trait Trait {
    type Assoc;
}

struct Foo<T, U>(T)
where
    T: Trait<Assoc = U>;

impl Trait for for<'a> fn(&'a ()) {
    type Assoc = u32;
}
impl Trait for fn(&'static ()) {
    type Assoc = String;
}
    
fn foo(x: Foo<for<'a> fn(&'a ()), u32>) -> Foo<fn(&'static ()), String> {
    x
}

fn main() {}

results in the following ICE:

error: internal compiler error: broken MIR in DefId(0:15 ~ example[9884]::foo) (_0 = move _1): bad assignment (Foo<fn(&()), std::string::String> = Foo<for<'a> fn(&'a ()), u32>): NoSolution
  --> <source>:17:5
   |
17 |     x
   |     ^

thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1561:30
stack backtrace:
----------- SNIP ----------------------
  17:     0x7fe0e8c751b3 - <rustc_errors[1020869c0a53ac34]::HandlerInner>::panic_if_treat_err_as_bug
  18:     0x7fe0e8c73258 - <rustc_errors[1020869c0a53ac34]::HandlerInner>::emit_diagnostic
  19:     0x7fe0ea774481 - <rustc_errors[1020869c0a53ac34]::HandlerInner>::emit_diag_at_span::<rustc_span[efd8b0f1895919e0]::span_encoding::Span>
  20:     0x7fe0ea774603 - <rustc_errors[1020869c0a53ac34]::HandlerInner>::span_bug::<rustc_span[efd8b0f1895919e0]::span_encoding::Span, &str>
  21:     0x7fe0ea773c55 - <rustc_errors[1020869c0a53ac34]::Handler>::delay_span_bug::<rustc_span[efd8b0f1895919e0]::span_encoding::Span, &str>
  22:     0x7fe0e9228157 - <rustc_borrowck[e2ad9be94bf77b7a]::type_check::TypeChecker>::typeck_mir
  23:     0x7fe0e919c351 - rustc_borrowck[e2ad9be94bf77b7a]::type_check::type_check
  24:     0x7fe0e9187179 - rustc_borrowck[e2ad9be94bf77b7a]::nll::compute_regions
  25:     0x7fe0e915c6e8 - rustc_borrowck[e2ad9be94bf77b7a]::do_mir_borrowck
  26:     0x7fe0e9b8f296 - rustc_borrowck[e2ad9be94bf77b7a]::mir_borrowck

godbolt

This should compile

@lcnr lcnr added C-bug Category: This is a bug. A-variance Area: Variance (https://doc.rust-lang.org/nomicon/subtyping.html) T-types Relevant to the types team, which will review and decide on the PR/issue. labels Nov 14, 2022
@Rageking8
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@rustbot label +I-ICE

@rustbot rustbot added the I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ label Nov 14, 2022
@lcnr
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lcnr commented Nov 14, 2022

@rustbot claim

Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue Nov 19, 2022
…rors

nll: correctly deal with bivariance

fixes rust-lang#104409

when in a bivariant context, relating stuff should always trivially succeed. Also changes the mir validator to correctly deal with higher ranked regions.

r? types cc `@RalfJung`
@bors bors closed this as completed in 00876c6 Nov 19, 2022
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Labels
A-variance Area: Variance (https://doc.rust-lang.org/nomicon/subtyping.html) C-bug Category: This is a bug. I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ T-types Relevant to the types team, which will review and decide on the PR/issue.
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