Flow analysis. Assignment is not detected in case ... when ... part

[sgrekhov]

The following program fails in both CFE and the analyzer.

main() {
  int i;
  if (true case == true when (i = 42) > 0) {
    print("Ok");
  }
  i; // Error: Non-nullable variable 'i' must be assigned before it can be used.
}

Flow analysis for if-case statements is not yet specified in the specification but intuitively I'd expect that it should detect that i is definitely assigned here.

cc @stereotype441

Dart SDK version: 3.8.0-166.0.dev (dev) (Tue Mar 4 20:02:17 2025 -0800) on "windows_x64"

[eernstg]

Hmmmmmm, perhaps the flow analysis doesn't conclude that == true is guaranteed to match when the scrutinee is true, so i = 42 remains possibly rather than definitely executed when i; is reached?

[stereotype441]

Hmmmmmm, perhaps the flow analysis doesn't conclude that == true is guaranteed to match when the scrutinee is true, so i = 42 remains possibly rather than definitely executed when i; is reached?

Yes, that is correct. Flow analysis only distinguishes true from false when they're used in a conditional context; otherwise, it considers them both just to be arbitrary values of type bool. So when analyzing if (true case == true when ...), flow analysis just sees if (<some boolean> case == true when ...).

[lrhn]

I'd not expect (or want) flow analysis to guess the result of non-constant == checks.

If you write a seemingly trivial if (1 == 1), that's still not the same as if (true). If you meant the latter, write that!

We "know" that it'll be true because we know the behavior of int.==, but I don't want the inference to have to assume anything about any non-constant invocation of a function, other than that it returns its static type.
We could, but the author could also just write true if they meant it.

That is: If it's not a constant, it's not compile-time true.

if (true case == true when (i = 42) > 0) {

is the same as

if (true == true && (i = 42) > 0) {}

and that also doesn't initialize i. And that's fine with me.