SI-8044 Allow binding backquoted varid in patterns

Previously, a varid could not be backquoted, so that it was not
possible to introduce variables with names such as `type` in a
match expression.

This commit allows backquoted varids in `case x @ _` and
`case x: Int`. In neither position is a stable id accepted,
that is, an id with leading uppercase.

Therefore, this commit merely relaxes the backquoted varid to
be taken as a normal varid in these contexts.

spec/01-lexical-syntax.md

@@ -49,6 +49,8 @@ classes (Unicode general category given in parentheses):
 ```ebnf
 op       ::=  opchar {opchar}
 varid    ::=  lower idrest
+boundvarid ::=  varid
+             | ‘`’ varid ‘`’
 plainid  ::=  upper idrest
            |  varid
            |  op

spec/08-pattern-matching.md

@@ -10,10 +10,10 @@ chapter: 8
 
 ```ebnf
   Pattern         ::=  Pattern1 { ‘|’ Pattern1 }
-  Pattern1        ::=  varid ‘:’ TypePat
+  Pattern1        ::=  boundvarid ‘:’ TypePat
                     |  ‘_’ ‘:’ TypePat
                     |  Pattern2
-  Pattern2        ::=  varid [‘@’ Pattern3]
+  Pattern2        ::=  boundvarid [‘@’ Pattern3]
                     |  Pattern3
   Pattern3        ::=  SimplePattern
                     |  SimplePattern {id [nl] SimplePattern}

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

@@ -1901,29 +1901,30 @@ self =>
       }
 
       /** {{{
-       *  Pattern1    ::= varid `:' TypePat
+       *  Pattern1    ::= boundvarid `:' TypePat
        *                |  `_' `:' TypePat
        *                |  Pattern2
-       *  SeqPattern1 ::= varid `:' TypePat
+       *  SeqPattern1 ::= boundvarid `:' TypePat
        *                |  `_' `:' TypePat
        *                |  [SeqPattern2]
        *  }}}
        */
       def pattern1(): Tree = pattern2() match {
         case p @ Ident(name) if in.token == COLON =>
-          if (treeInfo.isVarPattern(p))
+          if (nme.isVariableName(name)) {
+            p.removeAttachment[BackquotedIdentifierAttachment.type]
             atPos(p.pos.start, in.skipToken())(Typed(p, compoundType()))
-          else {
+          } else {
             syntaxError(in.offset, "Pattern variables must start with a lower-case letter. (SLS 8.1.1.)")
             p
           }
         case p => p
       }
 
       /** {{{
-       *  Pattern2    ::=  varid [ @ Pattern3 ]
+       *  Pattern2    ::=  boundvarid [ @ Pattern3 ]
        *                |   Pattern3
-       *  SeqPattern2 ::=  varid [ @ SeqPattern3 ]
+       *  SeqPattern2 ::=  boundvarid [ @ SeqPattern3 ]
        *                |   SeqPattern3
        *  }}}
        */
@@ -1935,7 +1936,7 @@ self =>
           case Ident(nme.WILDCARD) =>
             in.nextToken()
             pattern3()
-          case Ident(name) if treeInfo.isVarPattern(p) =>
+          case Ident(name) if nme.isVariableName(name) =>
             in.nextToken()
             atPos(p.pos.start) { Bind(name, pattern3()) }
           case _ => p

test/files/neg/t8044.check

@@ -0,0 +1,4 @@
+t8044.scala:3: error: not found: value _
+  def f = 42 match { case `_` : Int => `_` }   // doesn't leak quoted underscore
+                                       ^
+one error found

test/files/neg/t8044.scala

@@ -0,0 +1,4 @@
+
+trait T {
+  def f = 42 match { case `_` : Int => `_` }   // doesn't leak quoted underscore
+}

test/files/pos/t8044.scala

@@ -0,0 +1,7 @@
+
+trait T {
+  def f = 42 match { case `x` @ _ => x }
+  def g = 42 match { case `type` @ _ => `type` }
+  def h = 42 match { case `type` : Int => `type` }
+  def i = (null: Any) match { case _: Int | _: String => 17 }
+}