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dc81f6d549
More DefineOpaqueTypes::Yes
This accepts more code on stable. It is now possible to have match arms return a function item `foo::<ConcreteType>` and a function item `foo::<OpaqueTypeInDefiningScope>` in another, and that will constrain `OpaqueTypeInDefiningScope` to have the hidden type `ConcreteType`. So the following function will now compile, but on master it errors with a type mismatch on the second match arm
```rust
// The function item whose generic params we want to merge.
fn foo<T>(t: T) -> T { t }
// Helper ensuring we can constrain `T` on `F` without explicitly specifying it
fn bind<T, F: FnOnce(T) -> T>(_: T, f: F) -> F { f }
fn k() -> impl Sized {
let x = match true {
true => {
// `f` is `FnDef(foo, [infer_var])`
let f = foo;
// Get a value of an opaque type on stable
let t = k();
// this returns `FnDef(foo, [k::return])`
bind(t, f)
}
false => foo::<()>,
};
todo!()
}
```
r? ``@compiler-errors``
cc https://github.com/rust-lang/rust/issues/116652
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