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Get operation on private accessor without getter should throw

See original GitHub issue

Bug Report

I would like to work on a fix!

Current behavior

REPL

Transpiled code doesn’t throw an error in foo() and returns undefined.

Input Code

class C {
  foo() { return this.#bar; }
  set #bar(a) {}
}

new C().foo();

Expected behavior

new C().foo(); should throw a TypeError.

Example from Google Chrome Version 90.0.4393.0 Canary:

VM490:2 Uncaught TypeError: '#bar' was defined without a getter
    at C.foo (<anonymous>:2:11)
    at <anonymous>:6:9

Related test262 test: get-access-of-missing-private-getter.js.

Babel Configuration: See the REPL link

Environment

Babel version(s): v7.12.12
Node/npm version: 10.13
OS: Using REPL
Monorepo: Using REPL
How you are using Babel: Using REPL

Possible Solution

_classPrivateFieldGet helper should not fallback to returning descriptor.value if descriptor.get is false-y. If get property exists but is undefined, it can throw this particular error.

Additional context

See PrivateFieldGet-6.b from the spec:

PrivateFieldGet ( P, O )
    1. Assert: P is a Private Name.
    2. If O is not an object, throw a TypeError exception.
    3. If P.[[Kind]] is "field",
        a. Let entry be PrivateFieldFind(P, O).
        b. If entry is empty, throw a TypeError exception.
        c. Return entry.[[PrivateFieldValue]].
    4. Perform ? PrivateBrandCheck(O, P).
    5. If P.[[Kind]] is "method",
        a. Return P.[[Value]].
    6. Else,
        a. Assert: P.[[Kind]] is "accessor".
        b. If P does not have a [[Get]] field, throw a TypeError exception.
        c. Let getter be P.[[Get]].
        d. Return ? Call(getter, O).