Conditional type has different result when inline generics

[elektromodulator]

TypeScript Version:
2.8.3
Environment:
Visual Studio Code 1.22.2
Search Terms:
Conditional types
Code

// This example compiles
type Diff<T, U> = T extends U ? never : T;
type ActionData<TAction extends Action<TActionType>, TActionType extends string> = 
    Pick<TAction, Diff<keyof TAction, keyof Action<TActionType>>>;

interface Action<TActionType extends string> {
    readonly type: TActionType;
}

interface TestAction extends Action<'test'> {
    readonly type: 'test';
    readonly testText: string;
    readonly testNum: number;
    readonly testBoolean: boolean;
}

let data: ActionData<TestAction, 'test'> = {
    testText: 'd',
    testNum: 3,
    testBoolean: true
};

// This example does not compile, ActionData has 'type' property
type ActionData<TAction extends Action<TActionType>, TActionType extends string> = 
    Pick<TAction, keyof TAction extends keyof Action<TActionType> ? never : keyof TAction>;

interface Action<TActionType extends string> {
    readonly type: TActionType;
}

interface TestAction extends Action<'test'> {
    readonly type: 'test';
    readonly testText: string;
    readonly testNum: number;
    readonly testBoolean: boolean;
}

let data: ActionData<TestAction, 'test'> = {
    testText: 'd',
    testNum: 3,
    testBoolean: true
};

Expected behavior:
I want to remove the 'type' property of the action type to express the data part. If you look at the working example, the diff expression is defined in a separate type definition. In this case the 'type' property is not part of the ActionData type.
Actual behavior:
I would expect that when I inline the Diff type expression into the ActionData type definition, the result is the same. That is not the case though. In the second case 'type' property is part of ActionData type, so the code does not compile, because the compiler expects a 'type' property to be there in the object literal for the "data" variable.
Playground Link:
Compiling code
https://www.typescriptlang.org/play/#src=type%20Diff%3CT%2C%20U%3E%20%3D%20T%20extends%20U%20%3F%20never%20%3A%20T%3B%0D%0Aexport%20type%20ActionData%3CTAction%20extends%20Action%3CTActionType%3E%2C%20TActionType%20extends%20string%3E%20%3D%20%0D%0A%20%20%20%20Pick%3CTAction%2C%20Diff%3Ckeyof%20TAction%2C%20keyof%20Action%3CTActionType%3E%3E%3E%3B%0D%0A%0D%0Aexport%20interface%20Action%3CTActionType%20extends%20string%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20TActionType%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20TestAction%20extends%20Action%3C'test'%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20'test'%3B%0D%0A%20%20%20%20readonly%20testText%3A%20string%3B%0D%0A%20%20%20%20readonly%20testNum%3A%20number%3B%0D%0A%20%20%20%20readonly%20testBoolean%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Alet%20data%3A%20ActionData%3CTestAction%2C%20'test'%3E%20%3D%20%7B%0D%0A%20%20%20%20testText%3A%20'd'%2C%0D%0A%20%20%20%20testNum%3A%203%2C%0D%0A%20%20%20%20testBoolean%3A%20true%0D%0A%7D%3B
Non compiling code
https://www.typescriptlang.org/play/#src=export%20type%20ActionData%3CTAction%20extends%20Action%3CTActionType%3E%2C%20TActionType%20extends%20string%3E%20%3D%20%0D%0A%20%20%20%20Pick%3CTAction%2C%20keyof%20TAction%20extends%20keyof%20Action%3CTActionType%3E%20%3F%20never%20%3A%20keyof%20TAction%3E%3B%0D%0A%0D%0Aexport%20interface%20Action%3CTActionType%20extends%20string%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20TActionType%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20TestAction%20extends%20Action%3C'test'%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20'test'%3B%0D%0A%20%20%20%20readonly%20testText%3A%20string%3B%0D%0A%20%20%20%20readonly%20testNum%3A%20number%3B%0D%0A%20%20%20%20readonly%20testBoolean%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Alet%20data%3A%20ActionData%3CTestAction%2C%20'test'%3E%20%3D%20%7B%0D%0A%20%20%20%20testText%3A%20'd'%2C%0D%0A%20%20%20%20testNum%3A%203%2C%0D%0A%20%20%20%20testBoolean%3A%20true%0D%0A%7D%3B

[elektromodulator]

Using parentheses does not help:

export type ActionData<TAction extends Action<TActionType>, TActionType extends string> = 
    Pick<TAction, ((keyof TAction) extends (keyof Action<TActionType>) ? never : (keyof TAction))>;

export interface Action<TActionType extends string> {
    readonly type: TActionType;
}

interface TestAction extends Action<'test'> {
    readonly type: 'test';
    readonly testText: string;
    readonly testNum: number;
    readonly testBoolean: boolean;
}

let data: ActionData<TestAction, 'test'> = {
    testText: 'd',
    testNum: 3,
    testBoolean: true
};

https://www.typescriptlang.org/play/#src=export%20type%20ActionData%3CTAction%20extends%20Action%3CTActionType%3E%2C%20TActionType%20extends%20string%3E%20%3D%20%0D%0A%20%20%20%20Pick%3CTAction%2C%20((keyof%20TAction)%20extends%20(keyof%20Action%3CTActionType%3E)%20%3F%20never%20%3A%20(keyof%20TAction))%3E%3B%0D%0A%0D%0Aexport%20interface%20Action%3CTActionType%20extends%20string%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20TActionType%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20TestAction%20extends%20Action%3C'test'%3E%20%7B%0D%0A%20%20%20%20readonly%20type%3A%20'test'%3B%0D%0A%20%20%20%20readonly%20testText%3A%20string%3B%0D%0A%20%20%20%20readonly%20testNum%3A%20number%3B%0D%0A%20%20%20%20readonly%20testBoolean%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Alet%20data%3A%20ActionData%3CTestAction%2C%20'test'%3E%20%3D%20%7B%0D%0A%20%20%20%20testText%3A%20'd'%2C%0D%0A%20%20%20%20testNum%3A%203%2C%0D%0A%20%20%20%20testBoolean%3A%20true%0D%0A%7D%3B

[mhegazy]

Duplicate of #23046.

Please see explanation in #23022 (comment)

And more in https://github.com/Microsoft/TypeScript/wiki/What's-new-in-TypeScript#distributive-conditional-types

[elektromodulator]

Oooh, I see. This is quite a tricky thing. Thanks for the answer. 😄