Front end: Confusing error message with Null typed condition

[eernstg]

Consider the following program:

void main() {
  if (null) {}
}

At commit 28c4c6c (Apr 8), we get the following response from dart with null-safety:

> dart --enable-experiment=non-nullable n010.dart
lang/dart/scratch/202004(master %)$ head_test_dart --enable-experiment=non-nullable n010.dart
n010.dart:2:7: Error: A value of type 'Null?' can't be assigned to a variable of type 'bool'.
  if (null) {}
      ^

The type Null? seems to arise because its nullability is 'nullable', but the notation is confusing because there is no ? on Null in any relevant location in the program. The reference to a variable is misleading because there are no variables.

A similar error message is emitted if we introduce Null n = null and use if (n) {}.

[chloestefantsova]

That textual representation originates from Null being represented as an InterfaceType in Kernel. We have plans to use a dedicated Kernel node type for Null which would receive its own textual representation. The GitHub issue for that upcoming change is #40122.

[eernstg]

OK! @stefantsov, I'll leave it to you to close this issue if you consider it to be redundant, given #40122.