[BUG] While loop logic causes seg fault.

[johnsoez4]

Bug description

This issue started a few days ago with Mojo nightly version 2024.5.1002.
Edit 2024-05-28:

• mojo 2024.5.822 (2cfed0ad) = OK
• mojo 2024.5.1002 (83c7e1be) = KO (issues started)

In the code shown below, function app_run causes a seg fault, but app_close alone compiles and works properly. The logic to exit each while loop is essentially identical but inverted. I presume that both functions should compile and work properly.
Important: I just confirmed that this issue does NOT exist with Mojo stable version 24.3.0 (9882e19d).
stack_dump.txt

Steps to reproduce

fn main() raises:
    app_close()
    app_run()


fn app_close():
    # This compiles and works.
    var app_close = False
    var loops = 10
    while not app_close:
        loops -= 1
        if loops == 0:
            app_close = True
    print("app_close: Success!")


fn app_run():
    # This does not compile (seg fault).
    var app_run = True
    var loops = 10
    while app_run:
        loops -= 1
        if loops == 0:
            app_run = False
    print("app_run: Success!")
    ```
 
### System information
 
```shell
Ubuntu: 24.04 LTS
Python: 3.12.3
Mojo Nightly: 2024.5.1619 (bd52708e)
Modular: 0.7.4 (df7a9e8b)

[johnsoez4]

No problem (confirmed OK on 2024-05-28)

• mojo 2024.5.822 (2cfed0ad) OK

Problem confirmed:

• mojo 2024.5.1002 (83c7e1be)
• mojo 2024.5.1805 (341542be)
• mojo 2024.5.1905 (46b7e7ee)
• mojo 2024.5.2012 (fb60fe4f)
• mojo 2024.5.2119 (79ef45f6)
• mojo 2024.5.2205 (d3cacc7e)
• mojo 2024.5.2305 (1e5d508e)
• mojo 2024.5.2414 (39359abc)
• mojo 2024.5.2605 (9c328b12)
• mojo 2024.5.2805 (87564503)
• mojo 2024.5.2912 (6861a424)
• mojo 2024.5.3005 (0c465b6a)
• mojo 2024.5.3112 (e60dc71f)
• mojo 2024.6.105 (3cf18231)

[JoeLoser]

@ematejska @jopamer also asked on discord at https://discord.com/channels/1087530497313357884/1224434323193594059/1242260289060077589

[johnsoez4]

@ematejska, @JoeLoser , @mzaks, @jopamer, @rd4com, @jdiggins

Here's an interesting clue to the mystery. It works when the line while app_run: is changed to while not not app_run:.

fn app_run():
    # This does not compile (seg fault).
    var app_run = True
    var loops = 10

    while not not app_run:  # This works.
    # while app_run:  # This does not work.
        loops -= 1
        if loops == 0:
            app_run = False
    print("app_run: Success!")
    ```

[JoeLoser]

This is a compiler bug from one of our passes. It's hitting an assertion in our compiler stack inside LLVM:

Assertion failed: (detail::isPresent(Val) && "dyn_cast on a non-existent value"), function dyn_cast, file Casting.h, line 662.FYI  

FYI @jeff

[ematejska]

Thanks for filing! We'll take a look.

[ematejska]

Duplicate filed here: #2821

Reduced use-case from there:

fn main():
    var loop = True
    while loop:
        loop=False