Upper/lower bound computations in Dart give surprising results

[leafpetersen]

Tracking issue for a longstanding problem that didn't get well-resolved in Dart 2. Fixing this will be breaking.

For language team reference, some non-public discussion here.

[natebosch]

Does this issue cover the discussion at dart-lang/sdk#25821 (comment) about removing the LUB computation when there is enough context to type check the two values individually?

[natebosch]

Also, does it cover the discussion at dart-lang/sdk#19425 (comment) ?

[eernstg]

I believe it's fair to say that dart-lang/sdk#19425 is covered: It deals with a case which is now handled by the UP(C<T0, ..., Tn>, C<S0, ..., Sn>) rule.

[eernstg]

Note also the following issues on very similar topics:

• https://github.com/dart-lang/sdk/issues/47445, asking why code using if (b) return e1; else return e2; cannot be replaced by return b ? e1 : e2;.
• Least upper bound bug? Analyzer and CFE infer "Function" #525, proposing an adjustment of LUB to be more tight on some function types.
• Consider allowing context type to provide a hint to "least upper bound" algorithm #1618, proposing that we use use the context type during computation of the type of ?: expressions.
• Better inference of conditional expressions. #1877, also proposing to use the context type for ?: expressions.

I created label least-upper-bound to make these proposals visible as a group.

[lrhn]

Just to keep coming up with issues, and reminding that it's not just about conditional expressons. All LUBs are dangerous. Consider:

class Foo {}
class _BarBase extends Foo {}

class Bar extends _BarBase {}

class _BazBase extends Foo {}

class Baz extends _BazBase {}

void main () {
  var list = [Bar(), Baz()];
  list.add(Foo());
  print(list.length); 
}

All is well, the LUB of Bar and Baz is Foo.

Then I recognize that _BarBase and _BazBase can be combined and change it to:

class Foo {}
class _BarBazBase extends Foo {}

class Bar extends _BarBazBase {}

class Baz extends _BarBazBase {}

void main () {
  var list = [Bar(), Baz()];
  list.add(Foo()); // Error, cant pass `Foo` to argument expecting `_BarBazBase`.
  print(list.length); 
}

(LUB delenda est!)