Richard Eisenberg authored Dec 24, 2020 and Marge Bot committed Dec 25, 2020

Previously, we let-bound an identifier to use to carry
the erroring evidence for an out-of-scope variable. But
this failed for levity-polymorphic out-of-scope variables,
leading to a panic (#17812). The new plan is to use
a mutable update to just write the erroring expression directly
where it needs to go.

Close #17812.

Test case: typecheck/should_compile/T17812