Pattern guard condition type promotion

[rich-j]

Dart 3 introduces patterns with guard conditions that can be used with the new switch expression - nice. However, local variable types checked in the guard condition don't get promoted by the compiler even though the dart analyzer doesn't complain.

In the following example, the string s is checked to be not null in the guard condition and the analyzer doesn't complain:

String f(int x, {String? s}) => switch (x) {
  _ when s != null => s,
  _ => "is null"
};

However compilation fails with:

Error: A value of type 'String?' can't be returned from a function with return type 'String' because 'String?' is nullable and 'String' isn't.

The compile error can be fixed with a non-null assertion _ when s != null => s!,. Now we get an analyzer warning The '!' will have no effect because the receiver can't be null. This warning can then be silenced with // ignore_for_file: unnecessary_non_null_assertion.

The resulting code works. The compiler does correctly promote the type if the same local variable null-check is used in an if condition or ternary expression and we expected the guard condition to behave similarly.

[rich-j]

Realized this should be in the SDK instead dart-lang/sdk#52393