Analyzer: Incorrect function literal inference when in a legacy context

[leafpetersen]

Consider the following program:

out_of_order.dart

// @dart = 2.7

void foo(int Function(int) x) {}

in_order.dart

import 'out_of_order.dart';

void test() {
  int? s = null;
  foo((x) => s);
}

I believe that as specified, we should be inferring the argument to foo in a context where the return type is int* (that is, legacy int). The inferred return type of the body is int?, which is a subtype of int*, and so by my read we should be choosing int? as the inferred return type for the function literal. As a result, the function literal should have inferred type int? Function(int) which is assignable to int* Function(int*), and the body of the function should have no errors, since the return value and the return type are compatible.

The analyzer currently seems to be inferring the non-nullable type int as the return type for the function literal:

leafp-macbookpro:sdk leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseX64/dart-sdk/bin/dartanalyzer --enable-experiment=non-nullable ~/tmp/opted_out.dart
Analyzing /Users/leafp/tmp/opted_out.dart...
  error • The return type 'int?' isn't a 'int', as required by the closure's context. • /Users/leafp/tmp/opted_in.dart:5:14 • return_of_invalid_type_from_closure

cc @yjbanov @scheglov

[scheglov]

This might be because I incorrectly interpreted this. As a result, we use nonNullifyType on int* Function(int*), and use int Function(int) as the context type for the closure.

From your description it seems that we should apply nonNullifyType separately to each parameter of the closure, and let return type inference see int* as the context type?

[leafpetersen]

@scheglov yes, sorry, that's a little unclear. My intent was that inference uses whatever type it is given, but that at the point that you actually have to "write down" an inferred type in an opted in library, you erase it. It's possible that I have incoherent expectations here, but it looks like the CFE is doing what I expect on this, so in any case we have an inconsistency to sort out.

I'll try to make some time to write a few tests to poke corners of this. I'll have to think about where else it's observable.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/151000