error TS1206: Decorators are not valid here. in anonymous class

[Redvik80]

function testDecorator(target: any, key: string) {
    console.log(target, key);
}

export const testMixin = (superClass: any = Object) => class extends superClass {
    @testDecorator testField: string;
}

This produces the following compiler error:
error TS1206: Decorators are not valid here.

export const testMixin = (superClass: any = Object) => {
    class Mixin extends superClass {
        @testDecorator testField: string;
    }
    return Mixin;
}

In the second case, there are no problems.

[MartinJohns]

Duplicate of #7342.

[typescript-bot]

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