Infer existential element/key/value types for collection literals as a fallback #4061
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(cherry picked from commit 87a5e4d)
(cherry picked from commit 82ba4df)
…r possibilities. Fixes rdar://problem/17444930. (cherry picked from commit d073375)
Stub out a simplistic 'meet' operation for types that currently only handles finding the meet of two class types. Use it to optimize the constraint solver ever so slightly: when we see a type variable whose constraints state that it is a supertype of two different concrete types, we attempt to compute their meet and, if we find it, only produce one potential binding that is the meet itself. Note that this is an extremely poor implementation of this concept that is meant to address a specific regression being introduced by the defaulting of collection literal element types. A real implementation would, at the very least: * Implement a proper 'meet' that covers all subtyping in the language. * Distinguish between "I don't know if there is a meet" and "I am absolutely certain that there is no meet". * Collapse the constraints themselves to reduce the number of constraints in the system, rather than just the number of type variable bindings we attempt. (cherry picked from commit 2bcf555)
…ny project. This is NFC because nothing sets the bit yet, it is just to aid an ongoing investigation. (cherry picked from commit 1d21b4e)
(cherry picked from commit 8c2d0c5)
…d 'AnyHashable' for dictionary keys.
The id-as-Any work regressed cases where Swift code could specify
heterogeneous collection literals, e.g.,
var states: [String: Any] = [
"California": [
"population": 37_000_000,
"cities": ["Los Angeles", "San Diego", "San Jose"],
],
"Oregon": [
"population": 4_000_000,
"cities": ["Portland", "Salem", "Eugene"],
]
]
Prior to this, the code worked (when Foundation was imported) because
we'd end up with literals of type [NSObject : AnyObject].
The new defaulting rule says that the element type of an array literal
and the key/value types of a dictionary literal can be defaulted if no
stronger type can be inferred. The default type is:
Any, for the element type of an array literal or the value type of a
dictionary literal, or
AnyHashable, for the key type of a dictionary literal.
The latter is intended to compose with implicit conversions to
AnyHashable, so the most-general inferred dictionary type is
[AnyHashable : Any] and will work for any plausible dictionary
literal.
To prevent this inference from diluting types too greatly, we don't
allow this inference in "top-level" expressions, e.g.,
let d = ["a" : 1, "b" : "two"]
will produce an error because it's a heterogeneous dictionary literal
at the top level. One should annotate this with, e.g.,
let d = ["a" : 1, "b" : "two"] as [String : Any]
However, we do permit heterogeneous collections in nested positions,
to support cases like the original motivating example.
Fixes rdar://problem/27661580.
(cherry picked from commit 22287dd)
…nt expression kinds. The constraint generator's optimization to eagerly merge type variables for different keys in a dictionary literal was too eager, merging the type variables for (e.g.) a string literal and an integer literal. This prevented us from properly inferring AnyHashable key types in dictionary literals. Fixes the rest of rdar://problem/27661580. (cherry picked from commit 4a569f3)
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@jckarter please review |
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@swift-ci please test |
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@swift-ci please test |
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actually, maybe @rudkx would be a better candidate to review these type checker changes |
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@swift-ci please test linux |
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LGTM, merge away! |
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@DougGregor Is the defaulting independent for key and value types? If we have homogeneous keys but heterogeneous values, would we still infer e.g. |
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@jckarter We infer ["hi" : 1, "bye" : "two"] as [String : Any] and ["hi" : 1, 2 : "two"] as [AnyHashable : Any] I'll note that given the not-at-top-level rule, we force the user to write the explicit type in both of these cases. |
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Makes sense, @rudkx, that's what I'd expect. Thanks. |
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What's in this pull request?
The id-as-Any work regressed cases where Swift code could specify
heterogeneous collection literals, e.g.,:
Prior to this, the code worked (when Foundation was imported) because
we'd end up with literals of type
[NSObject : AnyObject].The new defaulting rule says that the element type of an array literal
and the key/value types of a dictionary literal can be defaulted if no
stronger type can be inferred. The default type is:
Any, for the element type of an array literal or the value type of a dictionary literal, orAnyHashable, for the key type of a dictionary literal.The latter is intended to compose with implicit conversions to
AnyHashable, so the most-general inferred dictionary type is[AnyHashable : Any]and will work for any plausible dictionaryliteral.
To prevent this inference from diluting types too greatly, we don't
allow this inference in "top-level" expressions, e.g.,:
let d = ["a" : 1, "b" : "two"]
will produce an error because it's a heterogeneous dictionary literal
at the top level. One should annotate this with, e.g.,:
let d = ["a" : 1, "b" : "two"] as [String : Any]
However, we do permit heterogeneous collections in nested positions,
to support cases like the original motivating example.
Resolved bug number: (rdar://problem/27661580)
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