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Allow prefix operators on the LHS of assignments #13328

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Aug 23, 2021
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Allow prefix operators on the LHS of assignments #13328

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5939849
Allow prefix operators on the LHS of assignments
odersky Aug 13, 2021
6b04f15
Tighten condition when to emit a "unary method cannot have parameters…
odersky Aug 18, 2021
614812b
Also apply syntax changes in Parser comments
odersky Aug 18, 2021
1010692
Move parsing `unary_!_=` test
nicolasstucki Aug 23, 2021
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8 changes: 5 additions & 3 deletions compiler/src/dotty/tools/dotc/parsing/Parsers.scala
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Expand Up @@ -1886,6 +1886,7 @@ object Parsers {
* | `return' [Expr]
* | ForExpr
* | [SimpleExpr `.'] id `=' Expr
* | PrefixOperator SimpleExpr `=' Expr
* | SimpleExpr1 ArgumentExprs `=' Expr
* | PostfixExpr [Ascription]
* | ‘inline’ InfixExpr MatchClause
Expand Down Expand Up @@ -2045,7 +2046,7 @@ object Parsers {
def expr1Rest(t: Tree, location: Location): Tree = in.token match
case EQUALS =>
t match
case Ident(_) | Select(_, _) | Apply(_, _) =>
case Ident(_) | Select(_, _) | Apply(_, _) | PrefixOp(_, _) =>
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@odersky odersky Aug 19, 2021 •
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What must have happened in Scala 2: We had similar logic here, but we did not mark prefix operations specially; so a prefix operation was accepted since it was represented by a Select. Similarly for infix. The following expression is parsed in Scala 2, but will get a type error afterwards:

   1 + 1 = 2

In dotc you get instead a parse error:

-- Error: ../../new/test.scala:4:10 --------------------------------------------
4 |    1 + 1 = 2
  |          ^
  |          end of statement expected but '=' found

That's because we represent infix operations as Infix nodes instead of Apply nodes.

atSpan(startOffset(t), in.skipToken()) {
val loc = if location.inArgs then location else Location.ElseWhere
Assign(t, subPart(() => expr(loc)))
Expand Down Expand Up @@ -2206,8 +2207,9 @@ object Parsers {
isOperator = !(location.inArgs && followingIsVararg()),
maybePostfix = true)

/** PrefixExpr ::= [`-' | `+' | `~' | `!'] SimpleExpr
*/
/** PrefixExpr ::= [PrefixOperator'] SimpleExpr
* PrefixOperator ::= ‘-’ | ‘+’ | ‘~’ | ‘!’
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+, - and ~ are not working.

class Foo(var value: Int):
  def `unary_+_=`(value: Int): Unit = this.value = +value
  def `unary_-_=`(value: Int): Unit = this.value = -value
  def `unary_~_=`(value: Int): Unit = this.value = ~value
end Foo

def test =
  val x = Foo(9)
  +x = 10 // error: value unary_+ is not a member of Foo - did you mean x.unary_+_=?
  -x = 10 // error: value unary_- is not a member of Foo, but could be made available as an extension method.
  ~x = 10 // error: value unary_~ is not a member of Foo - did you mean x.unary_~_=?

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That's because Foo is missing unary_+ and so on. So that's as expected.

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Does that mean that we cannot write !x = 3 if we have unary_!_= but not unary_!?

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Yes, exactly

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Why? Do we typecheck x.unary_! in x.unary_! = 3 before fully desugaring it into x.unarry_!_=(3)?

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Something like that, yes. Using a setter foo_= as x.foo = y is only allowed/attempted when x.foo exists itself. This is the same in Scala 2, and I think it's mentioned in the spec somewhere.

*/
val prefixExpr: Location => Tree = location =>
if isIdent && nme.raw.isUnary(in.name)
&& in.canStartExprTokens.contains(in.lookahead.token)
Expand Down
10 changes: 8 additions & 2 deletions compiler/src/dotty/tools/dotc/typer/RefChecks.scala
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Expand Up @@ -1039,7 +1039,7 @@ object RefChecks {
val what =
if tpe.paramNames.isEmpty then "empty parameter list.\n\nPossible fix: remove the `()` arguments."
else "parameters"
report.warning(s"Unary method cannot take $what", sym.sourcePos)
report.warning(s"unary_<op> method cannot take $what", sym.sourcePos)
case tpe: PolyType =>
checkParameters(tpe.resType)
case _ =>
Expand All @@ -1057,7 +1057,13 @@ object RefChecks {
case tpe: PolyType =>
checkExtensionParameters(tpe.resType)

if sym.name.startsWith(nme.UNARY_PREFIX.toString) then
def isUnaryPrefixName(name: Name) = name match
case name: SimpleName =>
name.startsWith("unary_") && nme.raw.isUnary(name.drop(6))
case _ =>
false

if isUnaryPrefixName(sym.name) then
if sym.is(Extension) || sym.name.is(ExtMethName) then
// if is method from `extension` or value class
checkExtensionParameters(sym.info)
Expand Down
10 changes: 6 additions & 4 deletions docs/docs/internals/syntax.md
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Expand Up @@ -223,7 +223,8 @@ Expr1 ::= [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
| ‘return’ [Expr] Return(expr?)
| ForExpr
| [SimpleExpr ‘.’] id ‘=’ Expr Assign(expr, expr)
| SimpleExpr1 ArgumentExprs ‘=’ Expr Assign(expr, expr)
| PrefixOperator SimpleExpr ‘=’ Expr Assign(expr, expr)
| SimpleExpr ArgumentExprs ‘=’ Expr Assign(expr, expr)
| PostfixExpr [Ascription]
| ‘inline’ InfixExpr MatchClause
Ascription ::= ‘:’ InfixType Typed(expr, tp)
Expand All @@ -235,7 +236,8 @@ InfixExpr ::= PrefixExpr
| InfixExpr id ‘:’ IndentedExpr
| InfixExpr MatchClause
MatchClause ::= ‘match’ <<< CaseClauses >>> Match(expr, cases)
PrefixExpr ::= [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr PrefixOp(expr, op)
PrefixExpr ::= [PrefixOperator] SimpleExpr PrefixOp(expr, op)
PrefixOperator ::= ‘-’ | ‘+’ | ‘~’ | ‘!’
SimpleExpr ::= SimpleRef
| Literal
| ‘_’
Expand All @@ -250,8 +252,8 @@ SimpleExpr ::= SimpleRef
| SimpleExpr ‘.’ MatchClause
| SimpleExpr TypeArgs TypeApply(expr, args)
| SimpleExpr ArgumentExprs Apply(expr, args)
| SimpleExpr1 ‘:’ IndentedExpr -- under language.experimental.fewerBraces
| SimpleExpr1 FunParams (‘=>’ | ‘?=>’) IndentedExpr -- under language.experimental.fewerBraces
| SimpleExpr ‘:’ IndentedExpr -- under language.experimental.fewerBraces
| SimpleExpr FunParams (‘=>’ | ‘?=>’) IndentedExpr -- under language.experimental.fewerBraces
| SimpleExpr ‘_’ PostfixOp(expr, _) (to be dropped)
| XmlExpr -- to be dropped
IndentedExpr ::= indent CaseClauses | Block outdent
Expand Down
6 changes: 4 additions & 2 deletions docs/docs/reference/syntax.md
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Expand Up @@ -221,7 +221,8 @@ Expr1 ::= [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
| ‘return’ [Expr]
| ForExpr
| [SimpleExpr ‘.’] id ‘=’ Expr
| SimpleExpr1 ArgumentExprs ‘=’ Expr
| PrefixOperator SimpleExpr ‘=’ Expr
| SimpleExpr ArgumentExprs ‘=’ Expr
| PostfixExpr [Ascription]
| ‘inline’ InfixExpr MatchClause
Ascription ::= ‘:’ InfixType
Expand All @@ -232,7 +233,8 @@ InfixExpr ::= PrefixExpr
| InfixExpr id [nl] InfixExpr
| InfixExpr MatchClause
MatchClause ::= ‘match’ <<< CaseClauses >>>
PrefixExpr ::= [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr
PrefixExpr ::= [PrefixOperator] SimpleExpr
PrefixOperator ::= ‘-’ | ‘+’ | ‘~’ | ‘!’
SimpleExpr ::= SimpleRef
| Literal
| ‘_’
Expand Down
9 changes: 9 additions & 0 deletions tests/pos/i13282.scala
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@@ -0,0 +1,9 @@
class Ptr[T](var value: T):
def `unary_!` : T = value
def `unary_!_=`(value: T): Unit = this.value = value
end Ptr

def test =
val x = Ptr(9)
!x = 10
println(!x)
6 changes: 6 additions & 0 deletions tests/pos/unary-eq.scala
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@@ -0,0 +1,6 @@
final class Baz private (val x: Int) extends AnyVal {
def `unary_!_=`() : Baz = ??? // parses ok, but will not be usable
def `unary_~_=`() : Baz = ??? // parses ok, but will not be usable
def `unary_+_=`() : Baz = ??? // parses ok, but will not be usable
def `unary_-_=`() : Baz = ??? // parses ok, but will not be usable
}