Make sure default exports snapshot synthetic named exports

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
rollup/plugins#658

Description

This is an inconsistency that turned up when looking into rollup/plugins#658 (comment). Basically the following situation:

// main.js
import foo from './dep';
import { update } from './synthetic';

assert.strictEqual(foo, 'original');
update();
// This would fail
assert.strictEqual(foo, 'original');

// dep.js
import { foo } from './synthetic';
// This default export should produce a snapshot of the current value of foo
export default foo;

// synthetic.js
// This should be the synthetic namespace
export const __synthetic = { foo: 'original' };
export const update = () => (__synthetic.foo = 'reassigned');

So in short we have a synthetic named export foo that is "reexported" via an export default declaration which is evaluated before and after the synthetic namespace is updated. This should not change the result as the default export declaration should snapshot. Previously, though, the following output was generated:

const __synthetic = { foo: 'original' };
const update = () => (__synthetic.foo = 'reassigned');
    
assert.strictEqual(__synthetic.foo, 'original');
update();
assert.strictEqual(__synthetic.foo, 'original');

which did in fact live-bind where it should not. This is fixed now:

const __synthetic = { foo: 'original' };
const update = () => (__synthetic.foo = 'reassigned');

// This is the new snapshot
var foo = __synthetic.foo;
    
assert.strictEqual(foo, 'original');
update();
assert.strictEqual(foo, 'original');

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#default-export-synthetic-named-export

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14160

[codecov]

Codecov Report

Merging #3946 (bce9d86) into master (683cc6a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3946      +/-   ##
==========================================
- Coverage   97.18%   97.18%   -0.01%     
==========================================
  Files         188      188              
  Lines        6685     6681       -4     
  Branches     1947     1946       -1     
==========================================
- Hits         6497     6493       -4     
  Misses         99       99              
  Partials       89       89              

Impacted Files	Coverage Δ
src/ast/variables/ExportDefaultVariable.ts	100.00% <ø> (ø)
src/ast/variables/SyntheticNamedExportVariable.ts	96.15% <ø> (-0.52%)	⬇️
src/Module.ts	100.00% <100.00%> (ø)

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