Fix bug super ref check doesn’t honor spec evaluation order

[buunguyen]

Q	A
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	yes
Tests Added/Pass?	yes
Fixed Tickets	Fixes #5800
License	MIT
Doc PR	no
Dependency Changes	no

Per spec, the following is valid code but Babel throws 'super.*' is not allowed before super() error.

class Foo extends Bar {
  constructor() {
    super[super().method]();
  }
}

Fixed by performing validation in exit handler of MemberExpression whose object is super. Also added a related test for the case of super.foo(super()); (should throw).

[mention-bot]

@buunguyen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @existentialism, @hzoo and @loganfsmyth to be potential reviewers.

[jridgewell]

Is this is valid, why are we asserting it throws?

[buunguyen]

I think it's not valid because super.foo is being accessed before the call to super(). (See rule SuperProperty:super.IdentifierName in spec).

That's consistent with behavior in Firefox. This will throw:

class Bar {
  foo() {
    console.log('out');
  }
}
class Foo extends Bar {
  constructor() {
    super.foo(super());
  }
}
new Foo()

=> ReferenceError: |this| used uninitialized in Foo class constructor

But the following would run and print "out" in Firefox.

class Bar {
  foo() {
    console.log('out');
  }
  get method() {
    return 'foo';
  }
}
class Foo extends Bar {
  constructor() {
    super[super().method]();
  }
}
new Foo()

[jridgewell]

Lol, I got this confused with the valid case below. Ignore me.

[codecov]

Codecov Report

Merging #5801 into 7.0 will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              7.0    #5801      +/-   ##
==========================================
- Coverage   84.91%   84.87%   -0.04%     
==========================================
  Files         284      284              
  Lines        9928     9927       -1     
  Branches     2788     2786       -2     
==========================================
- Hits         8430     8426       -4     
- Misses        988      991       +3     
  Partials      510      510

Impacted Files	Coverage Δ
...bel-plugin-transform-es2015-classes/src/vanilla.js	90.59% <100%> (-0.05%)	⬇️
packages/babel-traverse/src/path/context.js	84.48% <0%> (-1.73%)	⬇️
packages/babel-traverse/src/visitors.js	85.71% <0%> (-0.96%)	⬇️

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[buunguyen]

Just resolved conflict from #5802 that was just merged.

[buunguyen]

Any further feedback on this? Thanks!