GDScript Editor setting "Highlight Type Safe Lines" not working as intended

[michal-gora]

Tested versions

Reproducible in 4.3.stable

System information

Linux Ubuntu (KDE plasma)

Issue description

The setting "highlight type safe code" does not work as intended. When turning on the setting (which it is by default) and changing theme colors ("Line Number Color" green and "Safe Line Number Color" blue) to be better visible, this is the result of my example code. This behaviour seems inconsistent. Expected behaviour: Additionally green highlighting in line 9, 10, 14

I was trying to find the original issue/pull request adding this setting feature to find the intended documented behaviour so I am not sure if I might be misunderstanding this bug report, in that case I will close the issue again.

Steps to reproduce

1. Go to 'Editor > Editor Settings > Text Editor > Appearance > Gutters > Highlight Type Safe Lines' and enable it.
2. Go to 'Editor > Editor Settings > Text Editor > Theme > Highlighting > Line Number Color' and 'Editor > Editor Settings > Text Editor > Theme > Highlighting > Line Number Color' and change colors to something more vibrant.

3. Create following script:

   func hello():
   var basecase: int = 42
   basecase = 3
   
   var unsafe = 12
   unsafe = 4
   unsafe = "lol"
   
   var veryunsafe = 24
   veryunsafe = "hi"
   veryunsafe = 4

Minimal reproduction project (MRP)

bug-report-type-safe-line-highlighting.zip

[dalexeev]

The unsafe lines feature highlight lines that contain unsafe operations, i.e. that may cause type errors at runtime.

• Lines 9 and 14 are highlighted as safe because they are declarations of local untyped variables, you can assign anything to them.
• Line 10 is highlighted as safe because the assigned expression is of the same type as the implicitly inferred type of the variable.
• Lines 11 and 15 are highlighted as unsafe because the assigned expression is of a different type than the implicitly inferred type of the variable.
• Line 16 is highlighted as unsafe because after an unsafe operation with an untyped local variable (line 15), the implicitly inferred type is invalidated.

Actually, in this example, all lines should be highlighted as safe because untyped local variables can take any value. We just don't have good control flow analysis. We could track the current type of a local variable at all points in a function. For an untyped or Variant-typed local variable, only uses can be unsafe, not assignments.