SI-8918 Unary ids are plain ids

Allow +,-,!,~ to be used as unprefixed identifiers.

As prefix operators, they must be followed by
a simple expression, so otherwise, consume the
id itself as the start of a simple expression.

Author: som-snytt

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -665,6 +665,15 @@ self =>
     }
     def isLiteral = isLiteralToken(in.token)
 
+    def isSimpleExprIntroToken(token: Token): Boolean = isLiteralToken(token) || (token match {
+      case IDENTIFIER | BACKQUOTED_IDENT |
+           THIS | SUPER | NEW | USCORE |
+           LPAREN | LBRACE | XMLSTART => true
+      case _ => false
+    })
+
+    def isSimpleExprIntro: Boolean = isExprIntroToken(in.token)
+
     def isExprIntroToken(token: Token): Boolean = isLiteralToken(token) || (token match {
       case IDENTIFIER | BACKQUOTED_IDENT |
            THIS | SUPER | IF | FOR | NEW | USCORE | TRY | WHILE |
@@ -1565,11 +1574,14 @@ self =>
     def prefixExpr(): Tree = {
       if (isUnaryOp) {
         atPos(in.offset) {
-          val name = nme.toUnaryName(rawIdent().toTermName)
-          if (name == nme.UNARY_- && isNumericLit)
-            simpleExprRest(literal(isNegated = true), canApply = true)
-          else
-            Select(stripParens(simpleExpr()), name)
+          if (lookingAhead(isSimpleExprIntro)) {
+            val uname = nme.toUnaryName(rawIdent().toTermName)
+            if (uname == nme.UNARY_- && isNumericLit)
+              simpleExprRest(literal(isNegated = true), canApply = true)
+            else
+              Select(stripParens(simpleExpr()), uname)
+          }
+          else simpleExpr()
         }
       }
       else simpleExpr()

test/files/run/t8918-unary-ids.scala

@@ -0,0 +1,49 @@
+
+
+import scala.tools.partest.SessionTest
+
+// Taking unary ids as plain
+object Test extends SessionTest {
+  def session =
+"""Type in expressions to have them evaluated.
+Type :help for more information.
+
+scala> val - = 42
+-: Int = 42
+
+scala> val i = -
+i: Int = 42
+
+scala> - { 42 }
+res0: Int = -42
+
+scala> - if (true) 1 else 2
+<console>:1: error: illegal start of simple expression
+       - if (true) 1 else 2
+         ^
+
+scala> - - 1
+<console>:1: error: ';' expected but integer literal found.
+       - - 1
+           ^
+
+scala> -.-(1)
+res1: Int = 41
+
+scala> -
+res2: Int = 42
+
+scala> - -
+res3: Int = -42
+
+scala> + -
+res4: Int = 42
+
+scala> object X { def -(i: Int) = 42 - i ; def f(g: Int => Int) = g(7) ; def j = f(-) }
+defined object X
+
+scala> X.j
+res5: Int = 35
+
+scala> :quit"""
+}