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The change in question broke the following pattern, commonly used in name based pattern matching:
```scala
object ProdEmpty {
def _1: Int = ???
def _2: String = ???
def isEmpty = true
def get = this
}
```
This type define both `_1` and `get` + `isEmpty` (but is not <: Product). After #1938, `ProdEmpty` became eligibles for both product and name based pattern. Because "in case of ambiguities, *Product Pattern* is preferred over *Name Based Pattern*", isEmpty wouldn't be used, breaking the scalac semantics.
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Change case class desugaring and decouple Products and name-based-pattern-matching
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Product pattern use to:
- have a `<: Product` requirement
- compute the arity of a pattern by looking at `N` in a `ProductN` superclass.
This commit changes `<: Product`, instead we look for a `_1` member. The arity is determined by inspecting `_1` to `_N` members instead.
---
Here another attempt to formalize Dotty's pattern-matching (base on #1805). This covers the *Extractor Patterns* [section of the spec](https://www.scala-lang.org/files/archive/spec/2.12/08-pattern-matching.html#extractor-patterns). Dotty support 4 different extractor patterns: Boolean Pattern, Product Pattern, Seq Pattern and Name Based Pattern.
Boolean Pattern
- Extractor defines `def unapply(x: T): Boolean`
- Pattern-matching on exactly `0` patterns
Product Pattern
- Extractor defines `def unapply(x: T): U`
- `N > 0` is the maximum number of consecutive (parameterless `def` or `val`) `_1: P1` ... `_N: PN` members in `U`
- Pattern-matching on exactly `N` patterns with types `P1, P2, ..., PN`
Seq Pattern
- Extractor defines `def unapplySeq(x: T): U`
- `U` has (parameterless `def` or `val`) members `isEmpty: Boolean` and `get: S`
- `S <: Seq[V]`
- Pattern-matching on any number of pattern with types `V, V, ..., V`
Name Based Pattern
- Extractor defines `def unapply(x: T): U`
- `U` has (parameterless `def` or `val`) members `isEmpty: Boolean` and `get: S`
- If there is exactly `1` pattern, pattern-matching on `1` pattern with type `S`
- Otherwise fallback to Product Pattern on type `U`
In case of ambiguities, *Product Pattern* is preferred over *Name Based Pattern*.
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MethodTypes have paramTypes whereas PolyTypes have paramBounds.
We now harmonize by alling both paramInfos, and parameterizing
types that will become common to both.
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Remove dead code in pattern matching
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Starting from unreachable case https://github.com/lampepfl/dotty/blob/f75caad00256db52bbd3310f245e51d23c2a76cc/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala#L1408
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Otherwise checks are done also on type projections.
Same pitfall as https://issues.scala-lang.org/browse/SI-7214
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ThisType doesn't have a termSymbol.
And the check is actually too strong,
and not needed.
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This has two advantages:
- We can distinguish BackquotedIdent from Ident, allowing the user to
use a defined "type `&`", see testcase.
- We get better positions for the operators. This is useful in IDEs, for
example to get the type at point.
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This implements the rules laid down in #1805.
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New implementation following the scheme outlined in #1790.
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