Promote to non-null based on null-checking a conditional member access

[leonsenft]

• Dart SDK 2.12.2
• Reproducible in DartPad on Chrome

Consider the following snippet:

abstract class Identifier {  
  Uri? get moduleUri;
}

void f(Identifier? id) {
  if (id?.moduleUri != null) {
    print(id.moduleUri);
  }
}

Currently the id.moduleUrl within the body of the if statement produces an error:

line 7 • The property 'moduleUri' can't be unconditionally accessed because the receiver can be 'null'.
Try making the access conditional (using '?.') or adding a null check to the target ('!').

The expression id?.moduleUri != null can only be true if id is non-null. Could the compiler/analyzer perform this type flow analysis in order to promote id to a non-null type within the if statement?

[eernstg]

You'd need the getter moduleUri to be stable (a feature proposed in #1518), otherwise the second evaluation of id.moduleUri could return null. But if moduleUri is stable then id.moduleUri could also be a stable expression, and it would work.

[leonsenft]

You'd need the getter moduleUri to be stable (a feature proposed in #1518), otherwise the second evaluation of id.moduleUri could return null.

Right, I guess this isn't a great example since it would still fail to compile even if id were promoted. Consider this change:

abstract class Identifier {  
  Uri? get moduleUri;
}

void f(Identifier? id) {
  if (id?.moduleUri != null) {
    print(id.moduleUri!);
  }
}

This better highlights my point as it still doesn't compile. Again, for the body of the if statement to be entered, id must be non-null, yet you get a compile error that id could be null.

[leafpetersen]

I believe this is a duplicate of #1224 .