prefer_const_literals_to_create_immutables false positive with bool.fromEnvironment

[DanTup]

Originally raised at https://github.com/Dart-Code/Dart-Code/issues/4922 by @vatlnetwork.

The following code triggers prefer_const_literals_to_create_immutables:

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

class TopBar {
  AppBar appBar(BuildContext context) {
    return AppBar(
      actions: [
        if (kIsWeb) _button(context),
      ],
    );
  }

  Widget _button(BuildContext context) {
    return const Placeholder();
  }
}

However adding const here (using the quick-fix) results in an error trying to run the application:

lib/main.dart(8,21): error G4020727C: Not a constant expression. [C:\Dev\Test Projects\dartcode_4922\build\windows\x64\flutter\flutter_assemble.vcxproj]
lib/main.dart(8,29): error G4020727C: Not a constant expression. [C:\Dev\Test Projects\dartcode_4922\build\windows\x64\flutter\flutter_assemble.vcxproj]
lib/main.dart(8,21): error G38BEFAAB: Method invocation is not a constant expression. [C:\Dev\Test Projects\dartcode_4922\build\windows\x64\flutter\flutter_assemble.vcxproj]

The issue only seems to happen when the if condition uses a const bool that uses bool.fromEnvironment.

[bwilkerson]

I suspect the bug is in LintContext.canBeConst. It probably doesn't correctly detect that if and for elements can't be in a const literal.

[srawlins]

Actually, an if-element can be in a const literal. This runs:

void main() {
  const x = true;
  const y = [
    if (x) 7,
  ];
  print(y);
}

Even a constant from bool.FromEnvironment is allowed:

import 'package:flutter/foundation.dart';

void main() {
  const y = [
    if (kIsWeb) 3,
  ];
  print(y);
}

There is a bug in evaluating if-expressions with unknown const conditions.

[srawlins]

Fixed with https://github.com/dart-lang/sdk/commit/47123f7e2d8a8d8fcea1fcff3b87be8698670a95