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

Consider this example:

classAA{do1(){constb=dd.getB();if(this===b){// ❌ Error: no overlap between 'this' and 'BB'console.log("this === b");}}}classBBextendsAA{getB():BB{returnthis;}}letdd=newBB();dd.do1();// ✅ Correctly outputs "this === b" at runtime

Root Cause

TheisSimpleTypeRelatedTo function inchecker.ts did not handlethis type parameters specially for thecomparableRelation. When comparingthis (a type parameter) with a concrete class type, it failed to recognize that thethis type is constrained to a base class that could be related to the target type.

Solution

Added special handling inisSimpleTypeRelatedTo forthis type parameters when usingcomparableRelation. The fix "reverts"this types back to their constraint (the class type) for comparison purposes by:

1. Detecting when source or target is athis type parameter (isThisType flag)
2. Getting the constraint of thethis type parameter (the class it's bound to)
3. UsingisTypeRelatedTo 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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