Compiler warnings about unused variables

I have a simple program

var a = 40;
var b = a;
var c = 20;

Console.WriteLine(a);

var a = 40;
var b = a;
var c = 20;

Console.WriteLine(a);

On my vscode, only

is underlined with this error: The variable 'c' is assigned but its value is never used

Why is

not underlined? I mean it's dimmed but why isn't this a compiler warning as well?

Compiler warnings about unused variables - C#