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Dana JansensandRichard Smith
6041e9aa9d
The type structure is built for the impl lookup query for the
combination of the self type and the interface being queried. Then it is
built for each `impl` definition that is a potential candidate.
The type structures are compared to ensure they have a compatible
structure, and the `impl` declaration is not considered if they do not.
Finally, the type structures are used as a sorting key for the candidate
`impl` declarations, with the most-specified type structures (the ones
with the furthest distance to the first symbolic value) coming first in
the ordering.
See
https://docs.carbon-lang.dev/docs/design/generics/overview.html#parameterized-impl-declarations
for the design of the type structure and related ordering.
Most of the commits in this PR landed in #5158 (11ae0e27ab) by
mistake, but this includes the final changes since that PR was written.
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Co-authored-by: Richard Smith <richard@metafoo.co.uk>