In debugging, arguments are always shown instead of locals

[dsyme]

Given this:

let inp = seq { 1 .. 3 }
let h3 x =
    for x in inp do
       printfn $"hello, x = {x}"
       printfn "hello"

h3 10

then when you stop inside the loop, the value x shows as 10 even though in the loop it is 1, 2, 3. The locals simply aren't shown at all.

I'm not actually sure there is a fix to this without #2544 knowing how to resolve the names according to F# rules. It looks to me like the C# locals display is simply deciding not to show any locals that conflict with an argument name (which they never do according to C# rules)

Note this may have changed in recent VS, I've not seen the problem before, though it's likely it's just always been there.

[dsyme]

Note that in the "watch" window the value "x" displays correctly, as does hovering over "x". So it's only the debug locals window which is strange.

[baronfel]

Same results in both cases on VSCode for me. I'd expect this, given it's (IIRC) all the same expression evaluator, but just wanted to toss in my results.

[dsyme]

I can confirm this is in VS2019 as well, and is not related to any changes in the F# compiler, it's just always been like this.

There is unfortunately no way to specify scopes for arguments in the .NET PDB format, so we can't workaround this through clever PDB emit, and can't give the "(shadowed)" names to the arguments when they're out of scope.

It's strange that the debugger engine just completely ignores actual locals that conflict with arguments (since it is fine with showing multiple locals with the same name). It may be an easy fix to the debugger engine that doesn't affect C# at all, though it would be much better if we just implemented a debugger engine for F#.