Type promotion for non-local variables using the is operator

[Sander0542]

The is operator in dart can check whether an instance is a type of another type.

int a = 1;
Object b = a;

b is int // true

When using local variables of the type Object there is automatic type promotion, but when the instance of the object is anything else there is no promotion and the use of the as operator is needed to cast.

Other languages like C# support casting inside the is operator.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/patterns#declaration-and-type-patterns

object greeting = "Hello, World!";
if (greeting is string message)
{
    Console.WriteLine(message.ToLower());  // output: hello, world!
}

A feature like this would simplify the code and removes unnecessary lines that don't add anything but clutter.

An example implemenation

int a = 1;
Object b = a;

if (b is int c) {
  // within this block c is definced as int
}

[lrhn]

This looks like a duplicate of "Binding Cast and Check" in #1191.
There might be other similar proposals too.

[Levi-Lesches]

Yep. In particular, see:

• field-promotion Issues related to addressing the lack of field promotion
• Stable getters #1518
• Shadowing a non-local variable #1514
• Declaration expressions and declaration promotion #1420
• Binding expressions #1210
• If-variables #1201