Parse unary operators as regular identifiers when backquoted and followed by square bracket.

Author: adampauls

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

@@ -2223,7 +2223,7 @@ object Parsers {
      *  PrefixOperator   ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
      */
     val prefixExpr: Location => Tree = location =>
-      if isIdent && nme.raw.isUnary(in.name)
+      if in.isNonBackquotedIdent && nme.raw.isUnary(in.name)
          && in.canStartExprTokens.contains(in.lookahead.token)
       then
         val start = in.offset

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

@@ -70,6 +70,7 @@ object Scanners {
     def isNewLine = token == NEWLINE || token == NEWLINES
     def isStatSep = isNewLine || token == SEMI
     def isIdent = token == IDENTIFIER || token == BACKQUOTED_IDENT
+    def isNonBackquotedIdent = token == IDENTIFIER
     def isIdent(name: Name) = token == IDENTIFIER && this.name == name
 
     def isNestedStart = token == LBRACE || token == INDENT

tests/neg/unary-with-type-params.scala

@@ -0,0 +1,4 @@
+object Test {
+  def +[T](x: T): String = "x"
+  +[Int](6): String // error: expression expected but '[' found
+}

tests/pos/unary-with-type-params.scala

@@ -0,0 +1,6 @@
+object Test {
+  def +[T](x: T): String = "x"
+  `+`[Int](6): String // Parser can treat + as identifier when backquoted and followed by a type argument
+  `+`(6): String // Parser can treat + as identifier when backquoted and followed by a type argument
+  +(6): Int // Parser prioritizes + as unary when possible
+}