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Disallow Self in type param defaults of ADTs #64842

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Merged
bors merged 3 commits into from
Oct 4, 2019
Merged

Disallow Self in type param defaults of ADTs #64842

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3a4921c
The crux of the bug fix.
pnkfelix Sep 27, 2019
a18d424
Allocate a new diagnostic for defaulted type parameters cannot use `S…
pnkfelix Sep 27, 2019
e443e1b
Regression tests.
pnkfelix Sep 27, 2019
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The crux of the bug fix. 
Update: review feedback
Update: placate tidy
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@pnkfelix
pnkfelix committed Oct 3, 2019
commit 3a4921cde162af1ffbb2fbff4b395408eab1deab
13 changes: 13 additions & 0 deletions src/librustc_resolve/late.rs
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Expand Up @@ -468,6 +468,19 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
}
}));

// rust-lang/rust#61631: The type `Self` is essentially
// another type parameter. For ADTs, we consider it
// well-defined only after all of the ADT type parameters have
// been provided. Therefore, we do not allow use of `Self`
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@eddyb eddyb Oct 4, 2019

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I disagree with the phrasing (although the conclusion is similar) - see #61631 (comment)

In short, Self expands to/aliases Adt<A, B, C, ...>, referring to all the ADTs' parameters, and we shouldn't allow writing Self where we wouldn't allow Adt<A, B, C, ...>.

// anywhere in ADT type parameter defaults.
//
// (We however cannot ban `Self` for defaults on *all* generic
// lists; e.g. trait generics can usefully refer to `Self`,
// such as in the case of `trait Add<Rhs = Self>`.)
if self.current_self_item.is_some() { // (`Some` if + only if we are in ADT's generics.)
default_ban_rib.bindings.insert(Ident::with_dummy_span(kw::SelfUpper), Res::Err);
}

// We also ban access to type parameters for use as the types of const parameters.
let mut const_ty_param_ban_rib = Rib::new(TyParamAsConstParamTy);
const_ty_param_ban_rib.bindings.extend(generics.params.iter()
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