Changes to default names for context bound witnesses

 1. Use the constrained type's name as a term name only for single context bounds
 2. Apply the same scheme to deferred givens

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -246,13 +246,14 @@ object desugar {
       flags: FlagSet,
       freshName: => TermName)(using Context): Tree = rhs match
     case ContextBounds(tbounds, cxbounds) =>
-      var useParamName = Feature.enabled(Feature.modularity)
       for bound <- cxbounds do
         val evidenceName = bound match
-          case ContextBoundTypeTree(_, _, ownName) if !ownName.isEmpty => ownName
-          case _ if useParamName  => tname.toTermName
-          case _ => freshName
-        useParamName = false
+          case ContextBoundTypeTree(_, _, ownName) if !ownName.isEmpty =>
+            ownName
+          case _ if Feature.enabled(Feature.modularity) && cxbounds.tail.isEmpty  =>
+            tname.toTermName
+          case _ =>
+            freshName
         val evidenceParam = ValDef(evidenceName, bound, EmptyTree).withFlags(flags)
         evidenceParam.pushAttachment(ContextBoundParam, ())
         evidenceBuf += evidenceParam
@@ -498,7 +499,7 @@ object desugar {
     val evidenceBuf = new ListBuffer[ValDef]
     val result = cpy.TypeDef(tdef)(rhs =
       desugarContextBounds(tdef.name, tdef.rhs, evidenceBuf,
-        (tdef.mods.flags.toTermFlags & AccessFlags) | DeferredGivenFlags, EmptyTermName))
+        (tdef.mods.flags.toTermFlags & AccessFlags) | Lazy | DeferredGivenFlags, EmptyTermName))
     if evidenceBuf.isEmpty then result else Thicket(result :: evidenceBuf.toList)
 
   /** The expansion of a class definition. See inline comments for what is involved */

docs/_docs/reference/experimental/typeclasses-syntax.md

@@ -90,13 +90,15 @@ So far, an unnamed context bound for a type parameter gets a synthesized fresh n
     xs.foldLeft(A.unit)(_ `combine` _)
 ```
 
+
+
 The use of a name like `A` above in two variants, both as a type name and as a term name is of course familiar to Scala programmers. We use the same convention for classes and companion objects. In retrospect, the idea of generalizing this to also cover type parameters is obvious. It is surprising that it was not brought up before.
 
 **Proposed Rules**
 
  1. The generated evidence parameter for a context bound `A : C as a` has name `a`
  2. The generated evidence for a context bound `A : C` without an `as` binding has name `A` (seen as a term name). So, `A : C` is equivalent to `A : C as A`.
- 3. If there are more than one context bounds for a type parameter, the generated evidence parameter for every context bound except the first one has a fresh synthesized name, unless the context bound carries an `as` clause, in which case rule (1) applies.
+ 3. If there are multiple context bounds for a type parameter, as in `A : {C_1, ..., C_n}`, the generated evidence parameter for every context bound `C_i` has a fresh synthesized name, unless the context bound carries an `as` clause, in which case rule (1) applies.
 
 The default naming convention reduces the need for named context bounds. But named context bounds are still essential, for at least two reasons:
 
@@ -183,15 +185,22 @@ The compiler expands this to the following implementation:
 ```scala
 trait Sorted:
   type Element
-  given Ord[Element] = compiletime.deferred
+  given Ord[Element] as Element = compiletime.deferred
 
 class SortedSet[A](using A: Ord[A]) extends Sorted:
   type Element = A
-  override given Ord[Element] = A // i.e. the A defined by the using clause
+  override given Ord[Element] as Element = A // i.e. the A defined by the using clause
 ```
 
 The using clause in class `SortedSet` provides an implementation for the deferred given in trait `Sorted`.
 
+If there is a single context bound, as in
+```scala
+  type T : C
+```
+the synthesized deferred given will get the (term-)name of the constrained type `T`. If there are multiple bounds,
+the standard convention for naming anonymous givens applies.
+
 **Benefits:**
 
  - Better orthogonality, type parameters and abstract type members now accept the same kinds of bounds.

docs/_docs/reference/experimental/typeclasses.md

@@ -219,7 +219,7 @@ The use of a name like `A` above in two variants, both as a type name and as a t
 
  1. The generated evidence parameter for a context bound `A : C as a` has name `a`
  2. The generated evidence for a context bound `A : C` without an `as` binding has name `A` (seen as a term name). So, `A : C` is equivalent to `A : C as A`.
- 3. If there are more than one context bounds for a type parameter, the generated evidence parameter for every context bound except the first one has a fresh synthesized name, unless the context bound carries an `as` clause, in which case rule (1) applies.
+ 3. If there are multiple context bounds for a type parameter, as in `A : {C_1, ..., C_n}`, the generated evidence parameter for every context bound `C_i` has a fresh synthesized name, unless the context bound carries an `as` clause, in which case rule (1) applies.
 
 The default naming convention reduces the need for named context bounds. But named context bounds are still essential, for at least two reasons:
 
@@ -306,15 +306,22 @@ The compiler expands this to the following implementation:
 ```scala
 trait Sorted:
   type Element
-  given Ord[Element] = compiletime.deferred
+  given Ord[Element] as Element = compiletime.deferred
 
 class SortedSet[A](using A: Ord[A]) extends Sorted:
   type Element = A
-  override given Ord[Element] = A // i.e. the A defined by the using clause
+  override given Ord[Element] as Element = A // i.e. the A defined by the using clause
 ```
 
 The using clause in class `SortedSet` provides an implementation for the deferred given in trait `Sorted`.
 
+If there is a single context bound, as in
+```scala
+  type T : C
+```
+the synthesized deferred given will get the (term-)name of the constrained type `T`. If there are multiple bounds,
+the standard convention for naming anonymous givens applies.
+
 **Benefits:**
 
  - Better orthogonality, type parameters and abstract type members now accept the same kinds of bounds.

tests/neg/FromString.scala

@@ -0,0 +1,12 @@
+//> using options -language:experimental.modularity -source future
+
+trait FromString:
+  type Self
+  def fromString(s: String): Self
+
+given Int forms FromString = _.toInt
+
+given Double forms FromString = _.toDouble
+
+def add[N: {FromString, Numeric as num}](a: String, b: String): N =
+  num.plus(N.fromString(a), N.fromString(b)) // error: Not found: N // error

tests/pos/FromString.scala

@@ -8,5 +8,5 @@ given Int forms FromString = _.toInt
 
 given Double forms FromString = _.toDouble
 
-def add[N: {FromString, Numeric as num}](a: String, b: String): N =
-  num.plus(N.fromString(a), N.fromString(b))
+def add[N: {FromString as fs, Numeric as num}](a: String, b: String): N =
+  num.plus(fs.fromString(a), fs.fromString(b))

tests/pos/deferred-givens.scala

@@ -1,6 +1,18 @@
 //> using options -language:experimental.modularity -source future
 import compiletime.*
 class Ord[Elem]
+given Ord[Double]
+
+trait A:
+  type Elem : Ord
+  def foo = summon[Ord[Elem]]
+
+class AC extends A:
+  type Elem = Double
+  override given Ord[Elem] as Elem = ???
+
+class AD extends A:
+  type Elem = Double
 
 trait B:
   type Elem
@@ -22,3 +34,4 @@ class E(using x: Ord[String]) extends B:
 
 class F[X: Ord] extends B:
   type Elem = X
+

tests/run/for-desugar-strawman.scala

@@ -0,0 +1,96 @@
+
+@main def Test =
+  println:
+    for
+      x <- List(1, 2, 3)
+      y = x + x
+      if x >= 2
+      i <- List.range(0, y)
+      z = i * i
+      if z % 2 == 0
+    yield
+      i * x
+
+  println:
+    val xs = List(1, 2, 3)
+    xs.flatMapDefined: x =>
+      val y = x + x
+      xs.applyFilter(x >= 2):
+        val is = List.range(0, y)
+        is.mapDefined: i =>
+          val z = i * i
+          is.applyFilter(z % 2 == 0):
+            i * x
+
+extension [A](as: List[A])
+
+  def applyFilter[B](p: => Boolean)(b: => B) =
+    if p then Some(b) else None
+
+  def flatMapDefined[B](f: A => Option[IterableOnce[B]]): List[B] =
+    as.flatMap: x =>
+      f(x).getOrElse(Nil)
+
+  def mapDefined[B](f: A => Option[B]): List[B] =
+    as.flatMap(f)
+
+object UNDEFINED
+
+extension [A](as: Vector[A])
+
+  def applyFilter[B](p: => Boolean)(b: => B) =
+    if p then b else UNDEFINED
+
+  def flatMapDefined[B](f: A => IterableOnce[B] | UNDEFINED.type): Vector[B] =
+    as.flatMap: x =>
+      f(x) match
+        case UNDEFINED => Nil
+        case y: IterableOnce[B] => y
+
+  def mapDefined[B](f: A => B | UNDEFINED.type): Vector[B] =
+    as.flatMap: x =>
+      f(x) match
+        case UNDEFINED => Nil
+        case y: B => y :: Nil
+
+/*
+F ::= val x = E; F
+      x <- E; G
+G ::= []
+      val x = E; G
+      if E; G
+      x <- E; G
+
+Translation scheme:
+
+{ for F yield E }c    where c = undefined
+{ for G yield E }c    where c is a reference to the generator preceding the G sequence
+
+{ for [] yield E }c              =  E
+{ for p = Ep; G yield E }c       =  val p = Ep; { for G yield E }c
+{ for if Ep; G yield E}c         =  c.applyFilter(Ep)({ for G yield E }c)
+{ for p <- Ep; G yield E }c      =  val c1 = Ep; c1.BIND{ case p => { for G yield E }c1 }    (c1 fresh)
+
+      where BIND = flatMapDefined if isGen(G), isFilter(G)
+                 = mapDefined     if !isGen(G), isFilter(G)
+                 = flatMap        if isGen(G), !isFilter(G)
+                 = map            if !isGen(G), !isFilter(G)
+
+{ for case p <- Ep; G yield E }c =  { for $x <- Ep; if $x match case p => true case _ => false; p = $x@RuntimeChecked; G yield E }c
+{ for case p = Ep; G yield E }c  =  { for $x = Ep; if $x match case p => true case _ => false; p = $x@RuntimeChecked; G yield E}c
+
+isFilter(if E; S)
+isFilter(val x = E; S)  if  isFilter(S)
+
+isGen(x <- E; S)
+isGen(val x = E; S)     if isGen(S)
+isGen(if E; S)          if isGen(S)
+
+*/
+
+val foo = 1
+
+def main2 =
+  foo
+  ???
+  ??? match { case _ => 0 }