Compiler API getTypeAtLocation(node) doesn't return Type Alias

[drudru]

First, thanks for making TypeScript. I'm really enjoying this language.

I opened this issue because I'm experiencing an inconsistency or bug in the compiler API surrounding Type Aliases. I like Type Aliases since they are the only way to allow me to differentiate built-in types like number, string, etc. Let me illustrate the issue below.

If I have a class like:

type MyString = string;

class Test
{
    prop1:MyString;

    method() {
        let w = this.prop1;
        this.prop1 = "foo";
    }
}

and I compile it, I will get an 'AST'. Here is a portion of the AST.

      Block
        VariableStatement
          VariableDeclarationList
            VariableDeclaration
              Identifier
              PropertyAccessExpression
                ThisKeyword
                DotToken
                Identifier
        ExpressionStatement
          BinaryExpression
            PropertyAccessExpression
              ThisKeyword
              DotToken
              Identifier
            FirstAssignment
            StringLiteral

When I query the VariableStatement or the ExpressionStatement nodes for type via the compiler API using:

tc.getTypeAtLocation(node)

I don't get 'MyString'. I get 'string'.

However, the PropertyDeclaration node has a Type node that properly returns 'MyString'.
If I hover over the statements in Visual Studio Code, I also only see the type 'string'.

To me, this appears like either a bug or a mistake in my attempts at trying to get the information.

I can provide a sample project if needed too.

[DanielRosenwasser]

This seems partially related to #6070.

[DanielRosenwasser]

A reference to a type aliases is meant to eagerly resolve to its aliased type, so we'd have to change the way type aliases themselves are understood within the compiler. I think that's more a technical problem than a philosophical one.

[drudru]

@DanielRosenwasser - thanks for your response. I think I'll try a different tact in the meantime. So you can close this one if you need to.

Is there a good way to subclass the built-in number and string, yet allow those instances to be used in binary expressions with '+-*/' operators? I just looked at #2031 and #2365.

[DanielRosenwasser]

I think you want to look at #4372.

[drudru]

@DanielRosenwasser - that is very helpful - thanks again and Happy New Year

[DanielRosenwasser]

You're very welcome, have a happy new year!

[mhegazy]

This is actually by design. the alias is not a separate type. There is only one type, string in this case, MyString is just a pointer to it, and is not propagated.