Error not generated for nested generic array

🔎 Search Terms

error, nested, array, order

🕗 Version & Regression Information

• This changed between versions 4.4.4 and 4.5.5 (from playground testing - all later version exhibit this behaviour).

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#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

💻 Code

type TypeA = number;
type TypeB = boolean;
type DataTypeTwo<T extends TypeA | TypeB> = {
  one: Array<{ two:  Array<T> }>;
};
type DataTypeThree<T extends TypeA | TypeB> = {
  one: Array<{ two: Array<{ three: Array<T> }> }>;
};

let testingThreeErr: DataTypeThree<TypeA> = {
  one: [{ two: [ {three: [ true ] } ]}] // errors as expected
};
let testingTwoErr: DataTypeTwo<TypeA> = {
  one: [{ two: [ false ] }] // errors as expected
};

// Uncomment these lines to see all errors
// let testingTwoPre: DataTypeTwo<TypeA> = {
//   one: [{ two: [ 5 ] }]
// };
// let t2aPre: DataTypeTwo<TypeB> = testingTwoPre;

let testingThree: DataTypeThree<TypeA> = {
  one: [{ two: [ {three: [ 5 ] } ]}]
};
// Comment out this line to see error for t2a assignment
let t3a: DataTypeThree<TypeB> = testingThree; // Should error, but does not

let testingTwo: DataTypeTwo<TypeA> = {
  one: [{ two: [ 5 ] }]
};
let t2a: DataTypeTwo<TypeB> = testingTwo; // errors only if section above commented out

let check: number = "asdf"; // always errors, as it should

// Comment out L27 to see correct error on L32
// Uncomment L27 and lines 18-21 to see all errors correctly

// Works as expected in v4.4.4, but not any later versions

🙁 Actual behavior

No errors on lines 27 and 32 (assigning an incompatible type).

🙂 Expected behavior

Should error on those lines.

Additional information about the issue

The code works (i.e. gives correct errors) in version 4.4.4 (pg).

I assume this is to with depth limits, as it works fine for the two level case (DataTypeTwo) - though if the bad assignment comes after the three level case then even this error is not shown. (i.e. need to comment out line 27 in the example for the bad assignment to the DataTypeTwo variable to correctly error).

Even more oddly, if a bad assignment to a DataTypeTwo variable is made before the bad assignment to a DataTypeThree, the DataTypeThree assignment begins to error correctly. (Uncomment lines 18-21 in the example).

Possibly related to #56291.

Start with the linked TypeScript Playground and compare the reproduction in 4.4.4 with 5.6.3 or later. Investigate why the nested DataTypeTwo and DataTypeThree assignments affect whether incompatible assignments are reported, using the examples and the related issue #56291 as context. Done means the assignments on lines 27 and 32 reliably produce errors without depending on code order, with regression coverage added in the compiler tests.