build-optimizer: enum wrapping pass is unsafe in atypical use cases

[alexeagle]

From @clydin on October 23, 2017 14:27

Bug Report or Feature Request (mark with an x)

- [X] bug report -> please search issues before submitting
- [ ] feature request

Area

- [X] devkit
- [ ] schematics

Versions

Repro steps

An enum member can contain an initializer which may have side effects. Currently all enums are wrapping in an IIFE and annotated with PURE which will prevent the side effects from occurring if the enum is unused.

function getValue() {
  console.log('hi');
  return 6;
}

enum Test {
  ValueA,
  ValueB = getValue()
}

Desired functionality

As this will most likely be an extremely rare occurrence, a documentation note may suffice. Additionally a warning or informational message may also be useful when an enum is encountered with a member containing a non-constant expression.

Mention any other details that might be useful

Copied from original issue: angular/devkit#234

[OrangeDog]

@alexeagle looks like you typo'd the workaround label?

[dgp1130]

Just verified this locally and I'm still able to reproduce on v9.1.7. I didn't look into the pure comments for Terser, but running the following code:

function getBar(): number {
  console.log('Getting bar');
  return 2;
}

enum MyEnum {
  foo = 1,
  bar = getBar(),
}

Will print "Getting bar" when run with ng serve and will print nothing with ng serve --prod. Most likely the enum is getting tree shaken because it is unused and assumed to be pure via Terser comments when it actually isn't. I don't see any warning or compile-time error as a result of using a side-effectful function in an enum initializer.

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