Fix false positive "no overlap" error when comparing this with subclass instances

[Copilot]

TypeScript incorrectly reported error TS2367 "This comparison appears to be unintentional because the types 'this' and 'BB' have no overlap" when comparing this with an instance of a subclass, even though the comparison is legitimate and works correctly at runtime.

Consider this example:

class AA {
    do1() {
        const b = dd.getB();
        if (this === b) {  // ❌ Error: no overlap between 'this' and 'BB'
            console.log("this === b");
        }
    }
}

class BB extends AA {
    getB(): BB { return this; }
}

let dd = new BB();
dd.do1(); // ✅ Correctly outputs "this === b" at runtime

Root Cause

The isSimpleTypeRelatedTo function in checker.ts did not handle this type parameters specially for the comparableRelation. When comparing this (a type parameter) with a concrete class type, it failed to recognize that the this type is constrained to a base class that could be related to the target type.

Solution

Added special handling in isSimpleTypeRelatedTo for this type parameters when using comparableRelation. The fix "reverts" this types back to their constraint (the class type) for comparison purposes by:

1. Detecting when source or target is a this type parameter (isThisType flag)
2. Getting the constraint of the this type parameter (the class it's bound to)
3. Using isTypeRelatedTo to check if the constraint is comparable to the other type

Testing

• ✅ Original issue fixed: this === subclassInstance no longer errors
• ✅ Legitimate errors preserved: this === unrelatedClass still correctly errors
• ✅ Multi-level inheritance works: this === inheritedInstance allowed
• ✅ Interface comparisons work: this === interfaceImplementation allowed
• ✅ No regressions: All related type comparison tests continue passing

The fix is minimal and surgical - only 11 lines of code added to handle this specific case while preserving all existing behavior.

Fixes #46709.

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[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

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