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1f46c18921
Previously, we used a very fine-grained representation for individual
constraints: each constraint was _either_ a range constraint, a
not-equivalent constraint, or an incomparable constraint. These three
pieces are enough to represent all of the "real" constraints we need to
create — range constraints and their negation.
However, it meant that we weren't picking up as many chances to simplify
constraint sets as we could. Our simplification logic depends on being
able to look at _pairs_ of constraints or clauses to see if they
simplify relative to each other. With our fine-grained representation,
we could easily encounter situations that we should have been able to
simplify, but that would require looking at three or more individual
constraints.
For instance, negating a range constraint would produce:
```
¬(Base ≤ T ≤ Super) = ((T ≤ Base) ∧ (T ≠ Base)) ∨ (T ≁ Base) ∨
((Super ≤ T) ∧ (T ≠ Super)) ∨ (T ≁ Super)
```
That is, `T` must be (strictly) less than `Base`, (strictly) greater
than `Super`, or incomparable to either.
If we tried to union those back together, we should get `always`, since
`x ∨ ¬x` should always be true, no matter what `x` is. But instead we
would get:
```
(Base ≤ T ≤ Super) ∨ ((T ≤ Base) ∧ (T ≠ Base)) ∨ (T ≁ Base) ∨ ((Super ≤ T) ∧ (T ≠
Super)) ∨ (T ≁ Super)
```
Nothing would simplify relative to each other, because we'd have to look
at all five union elements to see that together they do in fact combine
to `always`.
The fine-grained representation was nice, because it made it easier to
[work out the math](https://dcreager.net/theory/constraints/) for
intersections and unions of each kind of constraint. But being able to
simplify is more important, since the example above comes up immediately
in #20093 when trying to handle constrained typevars.
The fix in this PR is to go back to a more coarse-grained
representation, where each individual constraint consists of a positive
range (which might be `always` / `Never ≤ T ≤ object`), and zero or more
negative ranges. The intuition is to think of a constraint as a region
of the type space (representable as a range) with zero or more "holes"
removed from it.
With this representation, negating a range constraint produces:
```
¬(Base ≤ T ≤ Super) = (always ∧ ¬(Base ≤ T ≤ Super))
```
(That looks trivial, because it is! We just move the positive range to
the negative side.)
The math is not that much harder than before, because there are only
three combinations to consider (each for intersection and union) —
though the fact that there can be multiple holes in a constraint does
require some nested loops. But the mdtest suite gives me confidence that
this is not introducing any new issues, and it definitely removes a
troublesome TODO.
(As an aside, this change also means that we are back to having each
clause contain no more than one individual constraint for any typevar.
This turned out to be important, because part of our simplification
logic was also depending on that!)
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
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| .. | ||
| annotations | ||
| assignment | ||
| binary | ||
| boolean | ||
| boundness_declaredness | ||
| call | ||
| class | ||
| comparison | ||
| comprehensions | ||
| conditional | ||
| dataclasses | ||
| declaration | ||
| diagnostics | ||
| directives | ||
| doc | ||
| exception | ||
| expression | ||
| function | ||
| generics | ||
| ide_support | ||
| import | ||
| literal | ||
| loops | ||
| narrow | ||
| regression | ||
| scopes | ||
| shadowing | ||
| snapshots | ||
| stubs | ||
| subscript | ||
| suppressions | ||
| type_compendium | ||
| type_of | ||
| type_properties | ||
| type_qualifiers | ||
| unary | ||
| with | ||
| .mdformat.toml | ||
| async.md | ||
| attributes.md | ||
| classes.md | ||
| cycle.md | ||
| decorators.md | ||
| del.md | ||
| deprecated.md | ||
| descriptor_protocol.md | ||
| enums.md | ||
| exhaustiveness_checking.md | ||
| final.md | ||
| instance_layout_conflict.md | ||
| intersection_types.md | ||
| invalid_syntax.md | ||
| known_constants.md | ||
| mdtest_config.md | ||
| mdtest_custom_typeshed.md | ||
| metaclass.md | ||
| mro.md | ||
| named_tuple.md | ||
| overloads.md | ||
| pep695_type_aliases.md | ||
| properties.md | ||
| protocols.md | ||
| public_types.md | ||
| statically_known_branches.md | ||
| sys_platform.md | ||
| sys_version_info.md | ||
| t_strings.md | ||
| terminal_statements.md | ||
| ty_extensions.md | ||
| typed_dict.md | ||
| union_types.md | ||
| unpacking.md | ||
| unreachable.md | ||