[CP] Fix crashes due to unhandled StructuralParameterTypes in UP in the CFE

[chloestefantsova]

Commit(s) to merge

https://dart-review.googlesource.com/c/sdk/+/379800

Target

stable

Prepared changelist for beta/stable

https://dart-review.googlesource.com/c/sdk/+/380240

Issue Description

The algorithm for computing the standard upper bound of two types, also known as UP, was partially missing its implementation for StructuralParameterType objects. In some corner casing the lacking implementation resulted in a crash of the compiler. All platforms using the CFE were affected.

What is the fix

The fix adds the missing part of the UP algorithm.

Why cherry-pick

The crashes due to the lacking implementation of UP in the CFE were reported by Flutter users: flutter/flutter#152984.

Risk

Low. The update is local and minimal and passes our CQ tests. Moreover, the missing part of the algorithm is implemented via abstracting a similar part of the same algorithm for TypeParameterType objects and applying it to StructuralParameterType objects.

Issue link(s)

flutter/flutter#152984

Extra Info

No response

[dart-github-bot]

Summary: The Dart compiler (CFE) crashed in certain scenarios due to an incomplete implementation of the "upper bound" (UP) algorithm for StructuralParameterType objects. The fix completes the UP algorithm for these types, preventing crashes and ensuring proper type inference.