Allow single-case lambda after colon

Author: odersky

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

@@ -1127,6 +1127,8 @@ object Parsers {
       else if lookahead.token == LPAREN || lookahead.token == LBRACKET then
         lookahead.skipParens()
         isArrowIndent()
+      else if lookahead.token == CASE then
+        Some(() => singleCaseMatch())
       else
         None
 
@@ -2383,7 +2385,7 @@ object Parsers {
 
     /** Expr              ::=  [`implicit'] FunParams (‘=>’ | ‘?=>’) Expr
      *                      |  TypTypeParamClause ‘=>’ Expr
-     *                      |  ExprCaseClause
+     *                      |  ExprCaseClause                              -- under experimental.relaxedLambdaSyntax
      *                      |  Expr1
      *  FunParams         ::=  Bindings
      *                      |  id
@@ -2799,6 +2801,7 @@ object Parsers {
      *  ColonArgument ::= colon [LambdaStart]
      *                    indent (CaseClauses | Block) outdent
      *                 |  colon LambdaStart expr ENDlambda        -- under experimental.relaxedLambdaSyntax
+     *                 |  colon ExprCaseClause                    -- under experimental.relaxedLambdaSyntax
      *  LambdaStart   ::= FunParams (‘=>’ | ‘?=>’)
      *                 |  TypTypeParamClause ‘=>’
      *  ColonArgBody  ::= indent (CaseClauses | Block) outdent

docs/_docs/internals/syntax.md

@@ -298,6 +298,7 @@ ColonArgument     ::=  colon [LambdaStart]
                        indent (CaseClauses | Block) outdent
                     |  colon LambdaStart expr ENDlambda                         -- ENDlambda is inserted for each production at next EOL
                                                                                 -- does not apply if enclosed in parens
+                    |  colon ExprCaseClause
 LambdaStart       ::=  FunParams (‘=>’ | ‘?=>’)
                     |  TypTypeParamClause ‘=>’
 Quoted            ::=  ‘'’ ‘{’ Block ‘}’

tests/pos/closure-args.scala

@@ -13,3 +13,8 @@ val d2: String = xs
   .filter: z => !z.isEmpty
   (0)
 
+val d3: String = xs
+  .map: x => x.toString + xs.collect: case y if y > 0 => y
+  .filter: z => !z.isEmpty
+  (0)
+

tests/run/single-case-expr.scala

@@ -8,6 +8,10 @@ case class Foo(x: Int, y: Int)
   val foo = Foo(1, 2)
   assert(g(foo) == 1)
 
+  val h1: Foo => Int = identity: case Foo(a, b) => a
+  val h2: Foo => Int = identity: case Foo(a, b) =>
+    a
+
   val a1 = Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a)
   assert(a1 == Seq(3))
 
@@ -18,4 +22,9 @@ case class Foo(x: Int, y: Int)
   )
   assert(a2 == Seq(1, 3))
 
+  val a3 = Seq((1, 2), (3, 4)).collect: case (a, b) if b > 2 => a
+  assert(a3 == Seq(3))
+
   val partial: PartialFunction[(Int, Int), Int] = case (a, b) if b > 2 => a
+
+