Invalid warning from compiler with smart casting

[islonely]

Describe the bug

Using a variable for if mut var is Type { ... } produces the unused variable V warning even though it is being used. if var is Type { ... } does not have the same warning. Ideally it should instead let me know to use the if statement without the mut in it.

Reproduction Steps

struct Foo {
	value int = 0xf00
}

struct Bar {}

type FooBar = Bar | Foo

fn main() {
	mut foobar := FooBar(Bar{})
	if mut foobar is Foo {
		println('foo')
	} else {
		println('bar')
	}
}

Expected Behavior

output:

bar

Current Behavior

output:

C:/Users/imado/Documents/test.v:10:6: warning: unused variable: `foobar`
    8 | 
    9 | fn main() {
   10 |     mut foobar := FooBar(Bar{})
      |         ~~~~~~
   11 |     if mut foobar is Foo {
   12 |         println('foo')
bar

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.2 37e5616

Environment details (OS name and version, etc.)

V full version: V 0.4.2 37e5616
OS: windows, Microsoft Windows 11 Pro v22621 64-bit
Processor: 16 cpus, 64bit, little endian, 

getwd: C:\Users\imado\Documents\cyberian_tiger
vexe: C:\Users\imado\v\v.exe
vexe mtime: 2023-10-09 22:29:31

vroot: OK, value: C:\Users\imado\v
VMODULES: OK, value: C:\Users\imado\.vmodules
VTMP: OK, value: C:\Users\imado\AppData\Local\Temp\v_0

Git version: git version 2.33.1.windows.1
Git vroot status: weekly.2023.40.1-50-g37e5616b
.git/config present: true

CC version: Error: 'cc' is not recognized as an internal or external command,
operable program or batch file.

thirdparty/tcc status: thirdparty-windows-amd64 e90c2620

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