Missing excess property error for discriminated union

[OliverJAsh]

TypeScript Version: 3.9.5

Search Terms:

Code

type Params = ({ shouldApplyStyles?: true } | { shouldApplyStyles: false }) &
    (
        | {
              internal: false;
          }
        | {
              internal: true;
              internalProp: string;
          }
    );

declare const fn: (params: Params) => void;

fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: true,
});
fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: false,
});
fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: undefined,
});
fn({
    internal: false,
    internalProp: 'foo', // Expected error but got none ❌
});

Expected behavior: See code comments

Actual behavior: See code comments

Playground Link: Link

Related Issues:

• Property not checked in union of objects #30631
• Unions without discriminant properties do not perform excess property checking #20863
• https://stackoverflow.com/questions/52677576/typescript-discriminated-union-allows-invalid-state/52678379#52678379

I looked at #20863 but I don't think this is exactly the same, because in this case we have a tagged aka discriminated union, whereas that issue describes behaviour for unions without tags aka discriminant properties.

[RyanCavanaugh]

Formatted to apply my own mental model:

type Params =
    ({ shouldApplyStyles: false } | { shouldApplyStyles?: true })
    &
    ({ internal: false; } | { internal: true; internalProp: string; });

declare const fn: (params: Params) => void;

fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: true,
});
fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: false,
});
fn({
    internal: false,
    internalProp: 'foo', // ✅ Errors as expected
    shouldApplyStyles: undefined,
});
fn({
    internal: false,
    internalProp: 'foo', // Expected error but got none ❌
});