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During type-checking, newly created instances share a type checking Context which de-duplicates identical instances. However, when unexpanded types escape the type-checking pass or are created via calls to Instantiate, they lack this shared context. As reported in #52728, this may lead to infinitely many identical but distinct types that are reachable via the API. This CL introduces a new invariant that ensures we don't create such infinitely expanding chains: instances created during expansion share a context with the type that led to their creation. During expansion, the expanding type passes its Context to any newly created instances. This ensures that cycles will eventually terminate with a previously seen instance. For example, if we have an instantiation chain T1[P]->T2[P]->T3[P]->T1[P], by virtue of this Context passing the expansion of T3[P] will find the instantiation T1[P]. In general, storing a Context in a Named type could lead to pinning types in memory unnecessarily, but in this case the Context pins only those types that are reachable from the original instance. This seems like a reasonable compromise between lazy and eager expansion. Our treatment of Context was a little haphazard: Checker.bestContext made it easy to get a context at any point, but made it harder to reason about which context is being used. To fix this, replace bestContext with Checker.context, which returns the type-checking context and panics on a nil receiver. Update all call-sites to verify that the Checker is non-nil when context is called. Also make it a panic to call subst with a nil context. Instead, update subst to explicitly accept a local (=instance) context along with a global context, and require that one of them is non-nil. Thread this through to the call to Checker.instance, and handle context updating there. Fixes #52728 Change-Id: Ib7f26eb8c406290325bc3212fda25421a37a1e8e Reviewed-on: https://go-review.googlesource.com/c/go/+/404885 Reviewed-by: Robert Griesemer <gri@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Robert Findley <rfindley@google.com> |
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