Allow prefix operators on the LHS of assignments

Fixes #13282

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2045,7 +2045,7 @@ 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(_, _) =>
             atSpan(startOffset(t), in.skipToken()) {
               val loc = if location.inArgs then location else Location.ElseWhere
               Assign(t, subPart(() => expr(loc)))

docs/docs/internals/syntax.md

@@ -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)
@@ -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
                     |  ‘_’
@@ -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

docs/docs/reference/syntax.md

@@ -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
@@ -232,7 +233,8 @@ InfixExpr         ::=  PrefixExpr
                     |  InfixExpr id [nl] InfixExpr
                     |  InfixExpr MatchClause
 MatchClause       ::=  ‘match’ <<< CaseClauses >>>
-PrefixExpr        ::=  [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr
+PrefixExpr        ::=  [PrefixOperator] SimpleExpr
+PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
 SimpleExpr        ::=  SimpleRef
                     |  Literal
                     |  ‘_’

tests/pos/i13282.scala

@@ -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)