[pull] swiftwasm-release/5.3 from release/5.3 #1283
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…aints
Under certain circumstances, introducing a concrete same-type or
superclass constraint can re-introduce conformance constraints
which were previously redundant.
For example, consider this code, which we correctly support today:
protocol P {
associatedtype T : Q
}
protocol Q {}
class SomeClass<U : Q> {}
struct Outer<T> where T : P {
func inner<U>(_: U) where T == SomeClass<U>, U : Q {}
}
The constraint 'T == SomeClass<U>' makes the outer constraint
`T : P' redundant, because SomeClass already conforms to P.
It also introduces an implied same-type constraint 'U == T.T'.
However, whereas 'T : P' together with 'U == T.T' make 'U : Q'
redundant, the introduction of the constraint 'T == SomeClass<U>'
removes 'T : P', so we re-introduce an explicit constraint 'U : Q'
in order to get a valid generic signature.
This code path did the right thing for constraints derived via
concrete same-type constraints, but it did not handle superclass
constraints.
As a result, this case was broken:
struct Outer<T> where T : P {
func inner<U>(_: U) where T : SomeClass<U>, U : Q {}
}
This is the same example as above, except T is related via a
superclass constraint to SomeClass<U>, instead of via a concrete
same-type constraint.
The subtlety here is that we must check if the superclass type
actually conforms to the requirement source's protocol, because it
is possible to have a superclass-constrained generic parameter
where some conformances are abstract. Eg, if SomeClass did not
conform to another protocol P2, we could write
func foo<T, U>(_: T, _: U) where T : SomeClass<U>, T : P2 {}
In this case, 'T : P2' is an abstract conformance on the type 'T'.
The common case where this would come up in real code is when you
have a class that conforms to a protocol with an associated type,
and one of the protocol requirements was fulfilled by a default in
a protocol extension, eg:
protocol P {
associatedtype T : Q
func foo()
}
extension P {
func foo() {}
}
class ConformsWithDefault<T : Q> : P {}
The above used to crash; now it will type-check correctly.
Fixes <rdar://problem/44736411>, <https://bugs.swift.org/browse/SR-8814>..
Disallow holes to be inferred as supertype bindings while inferring bindings from other type variables (based on transitivity of subtype conversions), otherwise it would be possible to record a hole without recording associated fix. Resolves: rdar://problem/64368545 (cherry picked from commit 2283208)
AST rewriter needs to make sure that all of the arguments are loaded
since it's currently possible to pass l-value type as an argument to
object literal (`#{file, color, image}Literal`).
Resolves: rdar://problem/62927467
(cherry picked from commit baaff0d)
…ete-with-superclass-5.3 GSB: Teach 'derived via concrete' computation about superclass constraints [5.3]
[5.3][CSApply] Always try to load arguments constructing object literals
[5.3][ConstraintSystem] Don't propagate holes through supertype inference
MaxDesiatov
approved these changes
Jun 23, 2020
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