From fd2c24c3159cefa583889a176f31d1e2325fe7e6 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sat, 3 Dec 2016 12:53:10 +0100
Subject: Add syntax for implicit functions

---
 compiler/src/dotty/tools/dotc/ast/untpd.scala      |  4 +++
 .../src/dotty/tools/dotc/parsing/Parsers.scala     | 16 ++++++++----
 compiler/src/dotty/tools/dotc/typer/Typer.scala    |  8 ++++--
 docs/syntax-summary.txt                            |  4 +--
 tests/pos/implicitFuns.scala                       | 30 ++++++++++++++++++++++
 5 files changed, 53 insertions(+), 9 deletions(-)
 create mode 100644 tests/pos/implicitFuns.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/untpd.scala b/compiler/src/dotty/tools/dotc/ast/untpd.scala
index 6c5210287..25b69b1f5 100644
--- a/compiler/src/dotty/tools/dotc/ast/untpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/untpd.scala
@@ -53,6 +53,10 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     override def isTerm = body.isTerm
     override def isType = body.isType
   }
+
+  /** An implicit function type */
+  class ImplicitFunction(args: List[Tree], body: Tree) extends Function(args, body)
+
   /** A function created from a wildcard expression
    *  @param  placeHolderParams  a list of definitions of synthetic parameters
    *  @param  body               the function body where wildcards are replaced by
diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 704f399ca..dabd8d2b0 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -681,7 +681,7 @@ object Parsers {
       }
     }
 
-    /** Type        ::=  FunArgTypes `=>' Type
+    /** Type        ::=  [`implicit'] FunArgTypes `=>' Type
      *                |  HkTypeParamClause `->' Type
      *                |  InfixType
      *  FunArgTypes ::=  InfixType
@@ -689,20 +689,26 @@ object Parsers {
      */
     def typ(): Tree = {
       val start = in.offset
+      val isImplicit = in.token == IMPLICIT
+      if (isImplicit) in.nextToken()
+      def functionRest(params: List[Tree]): Tree =
+        atPos(start, accept(ARROW)) {
+          val t = typ()
+          if (isImplicit) new ImplicitFunction(params, t) else Function(params, t)
+        }
       val t =
         if (in.token == LPAREN) {
           in.nextToken()
           if (in.token == RPAREN) {
             in.nextToken()
-            atPos(start, accept(ARROW)) { Function(Nil, typ()) }
+            functionRest(Nil)
           }
           else {
             openParens.change(LPAREN, 1)
             val ts = commaSeparated(funArgType)
             openParens.change(LPAREN, -1)
             accept(RPAREN)
-            if (in.token == ARROW)
-              atPos(start, in.skipToken()) { Function(ts, typ()) }
+            if (isImplicit || in.token == ARROW) functionRest(ts)
             else {
               for (t <- ts)
                 if (t.isInstanceOf[ByNameTypeTree])
@@ -722,7 +728,7 @@ object Parsers {
         else infixType()
 
       in.token match {
-        case ARROW => atPos(start, in.skipToken()) { Function(List(t), typ()) }
+        case ARROW => functionRest(t :: Nil)
         case FORSOME => syntaxError("existential types no longer supported; use a wildcard type or dependent type instead"); t
         case _ => t
       }
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index eec3859f9..c8deda4bc 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -660,9 +660,13 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
   def typedFunction(tree: untpd.Function, pt: Type)(implicit ctx: Context) = track("typedFunction") {
     val untpd.Function(args, body) = tree
-    if (ctx.mode is Mode.Type)
+    if (ctx.mode is Mode.Type) {
+      val funCls =
+        if (tree.isInstanceOf[untpd.ImplicitFunction]) defn.ImplicitFunctionClass(args.length)
+        else defn.FunctionClass(args.length)
       typed(cpy.AppliedTypeTree(tree)(
-        untpd.TypeTree(defn.FunctionClass(args.length).typeRef), args :+ body), pt)
+        untpd.TypeTree(funCls.typeRef), args :+ body), pt)
+    }
     else {
       val params = args.asInstanceOf[List[untpd.ValDef]]
 
diff --git a/docs/syntax-summary.txt b/docs/syntax-summary.txt
index 04e149de6..fe0ebc89c 100644
--- a/docs/syntax-summary.txt
+++ b/docs/syntax-summary.txt
@@ -95,8 +95,8 @@ grammar.
                       |  [id '.'] `super' [ClassQualifier] `.' id
   ClassQualifier    ::=  `[' id `]'
 
-  Type              ::=  FunArgTypes `=>' Type                                  Function(ts, t)
-                      |  HkTypeParamClause `=>' Type						    TypeLambda(ps, t)
+  Type              ::=  [`implicit'] FunArgTypes `=>' Type                     Function(ts, t)
+                      |  HkTypeParamClause `=>' Type	                        TypeLambda(ps, t)
                       |  InfixType
   FunArgTypes       ::=  InfixType
                       | `(' [ FunArgType {`,' FunArgType } ] `)'
diff --git a/tests/pos/implicitFuns.scala b/tests/pos/implicitFuns.scala
new file mode 100644
index 000000000..e62682546
--- /dev/null
+++ b/tests/pos/implicitFuns.scala
@@ -0,0 +1,30 @@
+object Test {
+
+  val x: ImplicitFunction1[String, Boolean] = ???
+
+  val y: String => Boolean = x
+
+  val b = x("hello")
+
+  val b1: Boolean = b
+
+}
+object Test2 {
+
+  val x: implicit String => Boolean = ???
+
+  val xx: implicit (String, Int) => Int = ???
+
+  val y: String => Boolean = x
+
+  val yy: (String, Int) => Any = xx
+
+  val b = x("hello")
+
+  val b1: Boolean = b
+
+  val c = xx("hh", 22)
+
+  val c1: Int = c
+
+}
-- 
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