Implement the method

Replaces `infer`, has the Shapeless semantics. This required
moving `DottyPredef` to src-bootstrapped since Scala 2.12 can't
express the semantics of `the` in a straightforward way.

Author: odersky

docs/docs/reference/contextual/derivation.md

@@ -317,7 +317,7 @@ calling the `error` method defined in `scala.compiletime`.
 ```scala
 implied [T] with (elemEq: Eq[T]) for Eq[Tree[T]] {
   def eql(x: Tree[T], y: Tree[T]): Boolean = {
-    val ev = infer[Generic[Tree[T]]]
+    val ev = the[Generic[Tree[T]]]
     val mx = ev.reflect(x)
     val my = ev.reflect(y)
     mx.ordinal == my.ordinal && {

docs/docs/reference/contextual/inferable-by-name-parameters.md

@@ -19,7 +19,7 @@ implied optionCodec[T] given (ev: => Codec[T]) for Codec[Option[T]] {
   }
 }
 
-val s = infer[Codec[Option[Int]]]
+val s = the[Codec[Option[Int]]]
 
 s.write(Some(33))
 s.write(None)
@@ -54,7 +54,7 @@ The precise steps for constructing an inferable argument for a by-name parameter
 In the example above, the definition of `s` would be expanded as follows.
 
 ```scala
-val s = infer[Test.Codec[Option[Int]]](
+val s = the[Test.Codec[Option[Int]]](
   optionCodec[Int](intCodec))
 ```
 

docs/docs/reference/contextual/inferable-params.md

@@ -85,16 +85,16 @@ f("abc") given ctx
 
 ## Querying Implied Instances
 
-A method `infer` in `Predef` creates an implied instance of a given type. For example,
+A method `the` in `Predef` creates an implied instance of a given type. For example,
 the implied instance of `Ord[List[Int]]` is generated by
 ```scala
-infer[Ord[List[Int]]]  // reduces to ListOrd given IntOrd
+the[Ord[List[Int]]]  // reduces to ListOrd given IntOrd
 ```
-The `infer` method is simply defined as the identity function over an inferable parameter.
+The `the` method is simply defined as the (non-widening) identity function over an inferable parameter.
 ```scala
-def infer[T] given (x: T) = x
+def the[T] given (x: T): x.type = x
 ```
-Functions like `infer` that have only inferable parameters are also called _context queries_.
+Functions like `the` that have only inferable parameters are also called _context queries_.
 
 ## Syntax
 

docs/docs/reference/contextual/relationship-implicits.md

@@ -75,7 +75,11 @@ Explicit arguments to inferable parameters _must_ be written using `given`,
 mirroring the definition syntax. E.g, `max(2, 3) given IntOrd`.
 Scala 2 uses normal applications `max(2, 3)(IntOrd)` instead. The Scala 2 syntax has some inherent ambiguities and restrictions which are overcome by the new syntax. For instance, multiple implicit parameter lists are not available in the old syntax, even though they can be simulated using auxiliary objects in the "Aux" pattern.
 
-The `infer` method corresponds to `implicitly` in Scala 2.
+The `the` method corresponds to `implicitly` in Scala 2.
+It is precisely the same as the `the` method in Shapeless.
+The difference between `the` (in both versions) and `implicitly` is
+that `the` can return a more precise type than the type that was
+asked for.
 
 ### Context Bounds
 

docs/docs/reference/contextual/typeclasses.md

@@ -17,7 +17,7 @@ trait Monoid[T] extends SemiGroup[T] {
   def unit: T
 }
 object Monoid {
-  def apply[T] = infer[Monoid[T]]
+  def apply[T] = the[Monoid[T]]
 }
 
 implied for Monoid[String] {

library/src-bootstrapped/dotty/DottyPredef.scala

@@ -0,0 +1,38 @@
+package dotty
+
+object DottyPredef {
+
+  @forceInline final def assert(assertion: => Boolean, message: => Any): Unit = {
+    if (!assertion)
+      assertFail(message)
+  }
+
+  @forceInline final def assert(assertion: => Boolean): Unit = {
+    if (!assertion)
+      assertFail()
+  }
+
+  def assertFail(): Unit = throw new java.lang.AssertionError("assertion failed")
+  def assertFail(message: => Any): Unit = throw new java.lang.AssertionError("assertion failed: " + message)
+
+  @forceInline final def implicitly[T](implicit ev: T): T = ev
+
+  @forceInline def locally[T](body: => T): T = body
+
+  /**
+   * Retrieve the single value of a type with a unique inhabitant.
+   *
+   * @example {{{
+   * object Foo
+   * val foo = valueOf[Foo.type]
+   * // foo is Foo.type = Foo
+   *
+   * val bar = valueOf[23]
+   * // bar is 23.type = 23
+   * }}}
+   * @group utilities
+   */
+  @forceInline def valueOf[T](implicit vt: ValueOf[T]): T = vt.value
+
+  inline def the[T](implicit x: T): x.type = x
+}

library/src/dotty/DottyPredef.scala renamed to library/src-non-bootstrapped/dotty/DottyPredef.scala

@@ -4,7 +4,6 @@ import scala.forceInline
 
 object DottyPredef {
 
-
   @forceInline final def assert(assertion: => Boolean, message: => Any): Unit = {
     if (!assertion)
       assertFail(message)
@@ -36,6 +35,4 @@ object DottyPredef {
    * @group utilities
    */
   @forceInline def valueOf[T](implicit vt: ValueOf[T]): T = vt.value
-
-  @forceInline def infer[T](implicit x: T) = x
 }

tests/pos/reference/instances.scala

@@ -120,20 +120,20 @@ object Instances extends Common {
     def f() = {
       locally {
         implied for Context = this.ctx
-        println(infer[Context].value)
+        println(the[Context].value)
       }
       locally {
         lazy val ctx1 = this.ctx
         implied for Context = ctx1
-        println(infer[Context].value)
+        println(the[Context].value)
       }
       locally {
         implied d[T] for D[T]
-        println(infer[D[Int]])
+        println(the[D[Int]])
       }
       locally {
         implied given Context for D[Int]
-        println(infer[D[Int]])
+        println(the[D[Int]])
       }
     }
   }
@@ -205,7 +205,7 @@ object AnonymousInstances extends Common {
   }
 
   def sum[T: Monoid](xs: List[T]): T =
-      xs.foldLeft(infer[Monoid[T]].unit)(_.combine(_))
+      xs.foldLeft(the[Monoid[T]].unit)(_.combine(_))
 }
 
 object Implicits extends Common {

tests/pos/the.scala

@@ -0,0 +1,12 @@
+object Test {
+
+  trait Foo { type T; val x: T }
+
+  implied intFoo for Foo {
+    type T = Int
+    val x = 3
+  }
+
+  val y: Int = the[Foo].x
+
+}

tests/run/implicit-disambiguation.scala

@@ -9,8 +9,8 @@ class C extends A {
 }
 object M {
   def f given B, C : String = {
-    implied a for A = infer[B]
-    infer[A].show
+    implied a for A = the[B]
+    the[A].show
   }
 }
 object Test extends App {