Rather than maintaining a linked list of overload
choices, which must be linearly searched each time
we need to lookup an overload at a given callee
locator, use a MapVector which can be rolled back
at the end of a scope.
Remove ResolvedOverloadSetListItem in favor of
using SelectedOverload, which avoids the need to
convert between them when moving from
ConstraintSystem to Solution.
Some constraint transformations require knowledge about what state
constraint system is currently in e.g. `constraint generation`,
`solving` or `diagnostics` to make a decision whether simplication
is possible. Notable example is `keypath dynamic member lookup`
which requires a presence of `applicable fn` constraint to retrieve
some contextual information.
Currently presence or absence of solver state is used to determine
whether constraint system is in `constraint generation` or `solving`
phase, but it's incorrect in case of `diagnoseFailureForExpr` which
tries to simplify leftover "active" constraints before it can attempt
type-check based diagnostics.
To make this more robust let's introduce (maybe temporarily until
type-check based diagnostics are completely obsoleted) a proper
notion of "phase" to constraint system so it is always clear what
transitions are allowed and what state constraint system is
currently in.
Resolves: rdar://problem/57201781
All of the argument diagnostics have been ported to the new diagnostic
framework, so now is the time to remove `ArgumentMatcher` and the only
place where it was used - `diagnoseSingleCandidateFailures`.
all cases of missing generic parameters.
In `ComponentStep::take` when there are no bindings or disjunctions, use hole
propagation to default remaining free type variables that aren't for generic
parameters and continue solving. Rather than using a defaultable constraint for
holes, assign a fixed type directly when we have no bindings to try.
ProtocolConformanceRef already has an invalid state. Drop all of the
uses of Optional<ProtocolConformanceRef> and just use
ProtocolConformanceRef::forInvalid() to represent it. Mechanically
translate all of the callers and callsites to use this new
representation.
