Strong mode analyzer ignores function types in conditional operator

[lexaknyazev]

Dart VM version: 1.14.0-dev.4.0.
This is OK:

typedef int ProcessFunction(int v);
int foo(int a) => a * 2;
int bar(int b) => b * 3;

ProcessFunction process;

void main() {
  var t = true;
  if (t) process = foo;
  else process = bar;
}

However this

typedef int ProcessFunction(int v);
int foo(int a) => a * 2;
int bar(int b) => b * 3;

ProcessFunction process;

void main() {
  var t = true;
  process = t ? foo : bar;
}

results in WARNING: t ? foo : bar (Function) will need runtime check to cast to type (int) → int.

[leafpetersen]

There are two related issues here. The first is that the analyzer (normal mode) does not currently implement least upper bound for function types, see: #23381 and #23663 .

The second is that we shouldn't really be using the dart spec version of least upper bound, since it produces very bad results in lots of places.

dart-archive/dev_compiler#288
#19425

Getting a DDC lub implemented is on my list of things to do fairly soon.

[jmesserly]

@leafpetersen is an improved LUB something we want to tackle in Q1?

[leafpetersen]

Yes. It doesn't have to be complete, but the function case comes up surprisingly often, in addition to the EfficientLength kind of issue.

[jmesserly]

I've merged dart-archive/dev_compiler#288 into this issue. There's some useful discussion on that bug, but the code is part of analyzer now.

[jmesserly]

I think @munificent fixed the first part (LUB in normal mode) ... I think we still need a strong-mode LUB, as the spec version is not sound.

[jmesserly]

merging this into #25802