Introduce harmonization of numeric arguments

Harmonization is Dotty's alternative to Scala 2's notion of weak conformance. It is less powerful but also
less entangled with the core type system. The idea is that in some specific contexts trees that
all have primitive numeric types will be converted as necessary so that they all have the same numeric type.
These tree sets are:

 - the two branches of an if
 - the alternatives of a match
 - the body together with the catch blocks of a try
 - the arguments of a vararg parameter

Examples are in the test file, harmonize.scala.

Author: odersky

src/dotty/tools/dotc/typer/Applications.scala

@@ -127,6 +127,9 @@ trait Applications extends Compatibility { self: Typer =>
      */
     protected def makeVarArg(n: Int, elemFormal: Type): Unit
 
+    /** If all `args` have primitive numeric types, make sure it's the same one */
+    protected def harmonizeArgs(args: List[TypedArg]): List[TypedArg]
+
     /** Signal failure with given message at position of given argument */
     protected def fail(msg: => String, arg: Arg): Unit
 
@@ -334,7 +337,14 @@ trait Applications extends Compatibility { self: Typer =>
                 addTyped(arg, formal)
               case _ =>
                 val elemFormal = formal.widenExpr.argTypesLo.head
-                args foreach (addTyped(_, elemFormal))
+                val origConstraint = ctx.typerState.constraint
+                var typedArgs = args.map(typedArg(_, elemFormal))
+                val harmonizedArgs = harmonizeArgs(typedArgs)
+                if (harmonizedArgs ne typedArgs) {
+                  ctx.typerState.constraint = origConstraint
+                  typedArgs = harmonizedArgs
+                }
+                typedArgs.foreach(addArg(_, elemFormal))
                 makeVarArg(args.length, elemFormal)
             }
           else args match {
@@ -389,6 +399,7 @@ trait Applications extends Compatibility { self: Typer =>
     def argType(arg: Tree, formal: Type): Type = normalize(arg.tpe, formal)
     def treeToArg(arg: Tree): Tree = arg
     def isVarArg(arg: Tree): Boolean = tpd.isWildcardStarArg(arg)
+    def harmonizeArgs(args: List[Tree]) = harmonize(args)
   }
 
   /** Subclass of Application for applicability tests with type arguments and value
@@ -405,6 +416,7 @@ trait Applications extends Compatibility { self: Typer =>
     def argType(arg: Type, formal: Type): Type = arg
     def treeToArg(arg: Tree): Type = arg.tpe
     def isVarArg(arg: Type): Boolean = arg.isRepeatedParam
+    def harmonizeArgs(args: List[Type]) = harmonizeTypes(args)
   }
 
   /** Subclass of Application for type checking an Apply node, where
@@ -430,6 +442,8 @@ trait Applications extends Compatibility { self: Typer =>
       typedArgBuf += seqToRepeated(seqLit)
     }
 
+    def harmonizeArgs(args: List[TypedArg]) = harmonize(args)
+
     override def appPos = app.pos
 
     def fail(msg: => String, arg: Trees.Tree[T]) = {
@@ -1025,25 +1039,34 @@ trait Applications extends Compatibility { self: Typer =>
     }
   }
 
-  def harmonize(trees: List[Tree])(implicit ctx: Context): List[Tree] = {
-    def numericClasses(trees: List[Tree], acc: Set[Symbol]): Set[Symbol] = trees match {
-      case tree :: trees1 =>
-        val sym = tree.tpe.typeSymbol
-        if (sym.isNumericValueClass && tree.tpe.isRef(sym))
-          numericClasses(trees1, acc + sym)
-        else
-          Set()
+  private def harmonizeWith[T <: AnyRef](ts: List[T])(tpe: T => Type, adapt: (T, Type) => T)(implicit ctx: Context): List[T] = {
+    def numericClasses(ts: List[T], acc: Set[Symbol]): Set[Symbol] = ts match {
+      case t :: ts1 =>
+        val sym = tpe(t).widen.classSymbol
+        if (sym.isNumericValueClass) numericClasses(ts1, acc + sym)
+        else Set()
       case Nil =>
         acc
     }
-    val clss = numericClasses(trees, Set())
+    val clss = numericClasses(ts, Set())
     if (clss.size > 1) {
       val lub = defn.ScalaNumericValueClassList.find(lubCls =>
         clss.forall(defn.isValueSubClass(_, lubCls))).get.typeRef
-      trees.mapConserve(tree => adaptInterpolated(tree, lub, tree))
+      ts.mapConserve(adapt(_, lub))
     }
-    else trees
+    else ts
   }
+
+  def harmonize(trees: List[Tree])(implicit ctx: Context): List[Tree] = {
+    def adapt(tree: Tree, pt: Type): Tree = tree match {
+      case cdef: CaseDef => tpd.cpy.CaseDef(cdef)(body = adapt(cdef.body, pt))
+      case _ => adaptInterpolated(tree, pt, tree)
+    }
+    harmonizeWith(trees)(_.tpe, adapt)
+  }
+
+  def harmonizeTypes(tpes: List[Type])(implicit ctx: Context): List[Type] =
+    harmonizeWith(tpes)(identity, (tp, pt) => pt)
 }
 
 /*

src/dotty/tools/dotc/typer/Typer.scala

@@ -493,7 +493,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     val cond1 = typed(tree.cond, defn.BooleanType)
     val thenp1 = typed(tree.thenp, pt)
     val elsep1 = typed(tree.elsep orElse untpd.unitLiteral withPos tree.pos, pt)
-    assignType(cpy.If(tree)(cond1, thenp1, elsep1), thenp1, elsep1)
+    val thenp2 :: elsep2 :: Nil = harmonize(thenp1 :: elsep1 :: Nil)
+    assignType(cpy.If(tree)(cond1, thenp2, elsep2), thenp2, elsep2)
   }
 
   def typedFunction(tree: untpd.Function, pt: Type)(implicit ctx: Context) = track("typedFunction") {
@@ -629,7 +630,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
             fullyDefinedType(sel1.tpe, "pattern selector", tree.pos))
 
         val cases1 = typedCases(tree.cases, selType, pt)
-        assignType(cpy.Match(tree)(sel1, cases1), cases1)
+        val cases2 = harmonize(cases1).asInstanceOf[List[CaseDef]]
+        assignType(cpy.Match(tree)(sel1, cases2), cases2)
     }
   }
 
@@ -737,7 +739,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     val expr1 = typed(tree.expr, pt)
     val cases1 = typedCases(tree.cases, defn.ThrowableType, pt)
     val finalizer1 = typed(tree.finalizer, defn.UnitType)
-    assignType(cpy.Try(tree)(expr1, cases1, finalizer1), expr1, cases1)
+    val expr2 :: cases2x = harmonize(expr1 :: cases1)
+    val cases2 = cases2x.asInstanceOf[List[CaseDef]]
+    assignType(cpy.Try(tree)(expr2, cases2, finalizer1), expr2, cases2)
   }
 
   def typedThrow(tree: untpd.Throw)(implicit ctx: Context): Tree = track("typedThrow") {

tests/pos/harmonize.scala

@@ -0,0 +1,28 @@
+object Test {
+
+  def main(args: Array[String]) = {
+    val x = true
+    val n = 1
+/*    val y = if (x) 'A' else n
+    val z: Int = y
+
+    val yy = n match {
+      case 1 => 'A'
+      case 2 => n
+      case 3 => 1.0
+    }
+    val zz: Double = yy
+
+    val a = try {
+      'A'
+    } catch {
+      case ex: Exception => n
+      case ex: Error => 3L
+    }
+    val b: Long = a
+*/
+    val xs = List(1.0, n, 'c')
+    val ys: List[Double] = xs
+  }
+
+}