This helps us to filter out cases like operator overloads where
`Self` type comes from e.g. default for collection element -
`[1, "hello"].map { $0 + 1 }`. Main problem here is that
collection type couldn't be determined without unification to
`Any` and `+` failing for all numeric overloads is just a consequence.
If there is an argument-to-parameter conversion which is associated with
`inout` parameter, subtyping is now permitted, types have to be identical.
```swift
protocol P {}
struct S : P {}
func foo(_: inout P) {}
var s = S()
foo(&s) // `s` has to be defined as `P` e.g. `var s: P = S()`
// to be used as an argument to `inout P` parameter.
```
Instead of recording `TreatRValueAsLValue` fix directly inside
`matchTypes`, let's move towards recording it specifically for
each possible case in `repairFailures` which makes it a lot
easier to determine what other fixes could be applied (if any).
This loop in the constraint solver won't terminate when given
ill-formed code involving circular inheritance. Make it
terminate. Fixes rdar://problem/54296278.
Introduce callables: values of types that declare `func callAsFunction`
methods can be called like functions. The call syntax is shorthand for
applying `func callAsFunction` methods.
```swift
struct Adder {
var base: Int
func callAsFunction(_ x: Int) -> Int {
return x + base
}
}
var adder = Adder(base: 3)
adder(10) // desugars to `adder.callAsFunction(10)`
```
`func callAsFunction` argument labels are required at call sites.
Multiple `func callAsFunction` methods on a single type are supported.
`mutating func callAsFunction` is supported.
SR-11378 tracks improving `callAsFunction` diagnostics.
A "hole" is a type variable which type couldn't be determined
due to an inference failure e.g. missing member, ambiguous generic
parameter which hasn't been explicitly specified.
It is used to propagate information about failures and avoid
recording fixes which are a consequence of earlier failures e.g.
```swift
func foo<T: BinaryInteger>(_: T) {}
struct S {}
foo(S.bar) // Actual failure here is that `S` doesn't have a member
// `bar` but a consequence of that failure is that generic
// parameter `T` doesn't conform to `BinaryInteger`.
```
`throw` statements are type-checked as having contextual `Error`
type to make sure that thrown type conforms to `Error` protocol.
Let's make sure that's correctly handled by new diagnostics framework.
```swift
func foo() throws {
throw 0 // `Int` doesn't conform to `Error` protocol.
}
```
Detect and diagnose contextual failures originating in an attempt
to convert `nil` to some other non-optional type e.g.
```swift
let _: Int = nil // can't initialize `Int` with `nil`
func foo() -> Int {
return nil // can't return `nil` from `foo`
}
_ = 1 + nil // there is no `+` overload which accepts `Int` and optional
```
This helps us to better diagnose failures related to generic
requirements like `T == [Int]` as well as protocol compositions,
which require deep equality check.
Since this kind of failure is really a conversion failure, let's
inherit from `Contextual{Mismatch, Failure}` which also helps with
storage for from/to types and their resolution.
Also let's use original types involved in conversion to form
this fix, which helps to perserve all of the original sugar.
This commit replaces the `getValue()` and `getValue2()` members on
`ConstraintLocator::PathElement` with specific accessors for each
expected path component kind. IMO this adds some clarity to the call
sites, especially for `getArgIdx()` and `getParamIdx()`.
In addition, this commit adds a private `getValue` member that can
access a value at a given index, which will make it easier to add a
third value in the future.
Such tracking makes it easier to ignore already "fixed" requirements
which have been recorded in the constraint system multiple times e.g.
a call to initializer would open both base type and initializer
method which have shared (if not the same) requirements.
Make `InvalidUseOfAddressOf` a `ContextualFailure`, make `ReturnAddressOf`
a `ContextualMismatch`, and extend this failure to cover using `&` with a
non-inout argument.
One-way constraint expressions, which are the only things that
introduce one-way constraints at this point, want to look through
lvalue types to produce values. Rename OneWayBind to OneWayEqual, map
it down to an Equal constraint when it is simplified (to drop
lvalue-ness), and apply that coercion during constraint application.
Part of rdar://problem/50150793.
Since key path dynamic member depends on information from
`applicable function` constraint associated with invocation
it can't be eagerly solved during constraint generation
phase, so it has to be delayed until actual solving starts.
Subscripts used to always have at least one choice - `keypath application`,
which along with actual valid choices always formed a disjunction.
That is no longer the case since `keypath application` is added
only when it makes sense, which means delayed dynamic member
constraint has to be added to "active" list to get re-attempted.
If the only difference between two functions is `throws` and it
is not a subtype relationship, let's repair the problem by dropping
`throws` attribute and letting solver continue to search for
a solution, which would later be diagnosed.
This makes sure that solver is able to produce specialized
fixes in some cases e.g. existential conversions, mismatched
generic arguments, or force unwraps before fallback to
generic `cannot convert X to Y` failure.
Introduce the notion of "one-way" binding constraints of the form
$T0 one-way bind to $T1
which treats the type variables $T0 and $T1 as independent up until
the point where $T1 simplifies down to a concrete type, at which point
$T0 will be bound to that concrete type. $T0 won't be bound in any
other way, so type information ends up being propagated right-to-left,
only. This allows a constraint system to be broken up in more
components that are solved independently. Specifically, the connected
components algorithm now proceeds as follows:
1. Compute connected components, excluding one-way constraints from
consideration.
2. Compute a directed graph amongst the components using only the
one-way constraints, where an edge A -> B indicates that the type
variables in component A need to be solved before those in component
B.
3. Using the directed graph, compute the set of components that need
to be solved before a given component.
To utilize this, implement a new kind of solver step that handles the
propagation of partial solutions across one-way constraints. This
introduces a new kind of "split" within a connected component, where
we collect each combination of partial solutions for the input
components and (separately) try to solve the constraints in this
component. Any correct solution from any of these attempts will then
be recorded as a (partial) solution for this component.
For example, consider:
let _: Int8 = b ? Builtin.one_way(int8Or16(17)) :
Builtin.one_way(int8Or16(42\
))
where int8Or16 is overloaded with types `(Int8) -> Int8` and
`(Int16) -> Int16`. There are two one-way components (`int8Or16(17)`)
and (`int8Or16(42)`), each of which can produce a value of type `Int8`
or `Int16`. Those two components will be solved independently, and the
partial solutions for each will be fed into the component that
evaluates the ternary operator. There are four ways to attempt that
evaluation:
```
[Int8, Int8]
[Int8, Int16]
[Int16, Int8]
[Int16, Int16]
To test this, introduce a new expression builtin `Builtin.one_way(x)` that
introduces a one-way expression constraint binding the result of the
expression 'x'. The builtin is meant to be used for testing purposes,
and the one-way constraint expression itself can be synthesized by the
type checker to introduce one-way constraints later on.
Of these two, there are only two (partial) solutions that can work at
all, because the types in the ternary operator need a common
supertype:
[Int8, Int8]
[Int16, Int16]
Therefore, these are the partial solutions that will be considered the
results of the component containing the ternary expression. Note that
only one of them meets the final constraint (convertibility to
`Int8`), so the expression is well-formed.
Part of rdar://problem/50150793.
