Syntax change: streamline ascriptions in match scrutinees

Allow ascriptions in match scrutinees without requiring parentheses.

Author: odersky

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

@@ -1200,14 +1200,15 @@ object Parsers {
      *                      |  ForExpr
      *                      |  [SimpleExpr `.'] id `=' Expr
      *                      |  SimpleExpr1 ArgumentExprs `=' Expr
-     *                      |  PostfixExpr [Ascription]
-     *                      |  [‘inline’] PostfixExpr `match' `{' CaseClauses `}'
+     *                      |  Expr2
+     *                      |  [‘inline’] Expr2 `match' `{' CaseClauses `}'
      *                      |  `implicit' `match' `{' ImplicitCaseClauses `}'
-     *  Bindings          ::= `(' [Binding {`,' Binding}] `)'
-     *  Binding           ::= (id | `_') [`:' Type]
-     *  Ascription        ::= `:' CompoundType
-     *                      | `:' Annotation {Annotation}
-     *                      | `:' `_' `*'
+     *  Bindings          ::=  `(' [Binding {`,' Binding}] `)'
+     *  Binding           ::=  (id | `_') [`:' Type]
+     *  Expr2             ::=  PostfixExpr [Ascription]
+     *  Ascription        ::=  `:' InfixType
+     *                      |  `:' Annotation {Annotation}
+     *                      |  `:' `_' `*'
      */
     val exprInParens: () => Tree = () => expr(Location.InParens)
 
@@ -1324,7 +1325,8 @@ object Parsers {
              t
          }
       case COLON =>
-        ascription(t, location)
+        val t1 = ascription(t, location)
+        if (in.token == MATCH) expr1Rest(t1, location) else t1
       case MATCH =>
         matchExpr(t, startOffset(t), Match)
       case _ =>

docs/docs/internals/syntax.md

@@ -196,9 +196,10 @@ Expr1             ::=  ‘if’ ‘(’ Expr ‘)’ {nl}
                     |  ForExpr
                     |  [SimpleExpr ‘.’] id ‘=’ Expr                             Assign(expr, expr)
                     |  SimpleExpr1 ArgumentExprs ‘=’ Expr                       Assign(expr, expr)
-                    |  PostfixExpr [Ascription]
-                    |  [‘inline’] PostfixExpr ‘match’ ‘{’ CaseClauses ‘}’       Match(expr, cases) -- point on match
+                    |  Expr2
+                    |  [‘inline’] Expr2 ‘match’ ‘{’ CaseClauses ‘}’             Match(expr, cases) -- point on match
                     |  ‘implicit’ ‘match’ ‘{’ ImplicitCaseClauses ‘}’
+Expr2             ::=  PostfixExpr [Ascription]
 Ascription        ::=  ‘:’ InfixType                                            Typed(expr, tp)
                     |  ‘:’ Annotation {Annotation}                              Typed(expr, Annotated(EmptyTree, annot)*)
 Catches           ::=  ‘catch’ Expr
@@ -224,7 +225,7 @@ SimpleExpr1       ::=  Literal
 Quoted            ::=  ‘'’ ‘{’ Block ‘}’
                     |  ‘'’ ‘[’ Type ‘]’
 ExprsInParens     ::=  ExprInParens {‘,’ ExprInParens}
-ExprInParens      ::=  PostfixExpr ‘:’ Type
+ExprInParens      ::=  PostfixExpr ‘:’ Type                                     -- normal Expr allows only RefinedType here
                     |  Expr
 ParArgumentExprs  ::=  ‘(’ ExprsInParens ‘)’                                    exprs
                     |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘:’ ‘_’ ‘*’ ‘)’          exprs :+ Typed(expr, Ident(wildcardStar))

tests/pos-special/fatal-warnings/unchecked-scrutinee.scala

@@ -0,0 +1,5 @@
+object Test {
+  List(1: @unchecked, 2, 3): @unchecked match {
+    case a :: as =>
+  }
+}