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5df5711b67
In postfix completion, for operator completion, we do:
1. Type check the operand without applying it, but set the resolved
type to the root of the expression.
2. For each possible operators:
i. Build temporary binary/postfix expression
ii. Perform type checking to see whether the operator is applicable
This could be very slow especially if the operand is complex.
* Introduce `ReusePrecheckedType` option to constraint system. With
this option, CSGen respects pre-stored types in expressions and doesn't
take its sub-expressions into account.
* Improve type checking performance because type variables aren't
generated for sub-expressions of LHS (45511835)
* Guarantee that the operand is not modified by the type checker because
expression walkers in `CSGen` doesn't walk into the operand.
* Introduce `TypeChecker::findLHS()` to find LHS for a infix operator from
pre-folded expression. We used to `foldSequence()` temporary
`SequenceExpr` and find 'CodeCompletionExpr' for each attempt.
* No need to flatten folded expression after initial type-checking.
* Save memory of temporary `BinaryExpr` which used to be allocated by
`foldSequence()`.
* Improve accuracy of the completion. `foldSequence()` recovers invalid
combination of operators by `left` associative manner (with
diagnostics). This used to cause false-positive results. For instance,
`a == b <HERE>` used to suggest `==` operator. `findLHS()` returns
`nullptr` for such invalid combination.
rdar://problem/45511835
https://bugs.swift.org/browse/SR-9061