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Malte UblandClaude Opus 5.5
d84e410981
A base method calling `this.set()` where only a subclass is constructed devirtualizes to the subclass override. With a polymorphic `this` return type, the override's lowered return type is the subclass object while the site keeps the base method's type, and the validator reported an internal compiler error (SC9001: "call %Sub.set type object != return object"). The Next.js extract hit this once its placeholder ResponseCookies instance was gone: MiddlewareResponseCookies became the only constructed class. The direct call now carries the callee's call-site return type and upcasts to the site's class; any other mismatch keeps the virtual call. Calls whose types already matched are emitted exactly as before. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>