I tried this code:
fn foo() {}
fn bar() {}
fn main() {
let _ = [foo, if false { bar } else { foo }];
}I expected to see this happen: It compiles, like in if or match.
fn foo() {}
fn bar() {}
fn main() {
let _ = if false {
foo
} else {
if false {
bar
} else {
foo
}
};
let _ = match 1 {
2 => foo,
_ => if false {
bar
} else {
foo
},
};
}Instead, this happened: type mismatch when trying to coerce bar into foo.
The cause: check_expr_array uses a type variable as the expectation during element typecking but the type variable is unified with the first element's type during coercion.
|
let coerce_to = expected |
|
.to_option(self) |
|
.and_then(|uty| self.try_structurally_resolve_type(expr.span, uty).builtin_index()) |
|
.unwrap_or_else(|| self.next_ty_var(expr.span)); |
|
let mut coerce = CoerceMany::with_coercion_sites(coerce_to, args); |
|
assert_eq!(self.diverges.get(), Diverges::Maybe); |
|
for e in args { |
|
let e_ty = self.check_expr_with_hint(e, coerce_to); |
|
let cause = self.misc(e.span); |
|
coerce.coerce(self, &cause, e, e_ty); |
|
} |
It should use
try_structurally_resolve_and_adjust_for_branches on the element expectation as
if and
match do, which turns the expectation into
NoExpectation if the expected_ty is a true type variable.
However, this simple fix can break
some other code.
Recursive tuple coercion can fix this case. But it's
complicated.
Meta
rustc --version --verbose:
1.91.0-nightly
(2025-08-06 7d82b83ed57d188ab3f2)
I tried this code:
I expected to see this happen: It compiles, like in
iformatch.Instead, this happened: type mismatch when trying to coerce
barintofoo.The cause:
check_expr_arrayuses a type variable as the expectation during element typecking but the type variable is unified with the first element's type during coercion.rust/compiler/rustc_hir_typeck/src/expr.rs
Lines 1818 to 1828 in 9b1a30e
It should use
try_structurally_resolve_and_adjust_for_brancheson the element expectation asifandmatchdo, which turns the expectation intoNoExpectationif the expected_ty is a true type variable.However, this simple fix can break some other code.
Recursive tuple coercion can fix this case. But it's complicated.
Meta
rustc --version --verbose: