Simplify use of toFunctionType (#18429)

Make `isJava` by default `false` and remove `dropLast` argument.

Part of #18305. Note that the use of `toFunctionType` will increase, and
most will not be java functions.

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -698,7 +698,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
     private def toDepFun(args: List[Type], resultType: Type, isContextual: Boolean)(using Context): Type =
       MethodType.companion(isContextual = isContextual)(args, resultType)
-        .toFunctionType(isJava = false, alwaysDependent = true)
+        .toFunctionType(alwaysDependent = true)
 
     /** Turn `expected` into a dependent function when `actual` is dependent. */
     private def alignDependentFunction(expected: Type, actual: Type)(using Context): Type =
@@ -850,7 +850,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               adaptFun(actual, rinfo.paramInfos, rinfo.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
                   rinfo.derivedLambdaType(paramInfos = aargs1, resType = ares1)
-                    .toFunctionType(isJava = false, alwaysDependent = true))
+                    .toFunctionType(alwaysDependent = true))
             case actual: MethodType =>
               adaptFun(actual, actual.paramInfos, actual.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -40,7 +40,7 @@ extends tpd.TreeTraverser:
     MethodType.companion(
         isContextual = defn.isContextFunctionClass(tycon.classSymbol),
       )(argTypes, resType)
-      .toFunctionType(isJava = false, alwaysDependent = true)
+      .toFunctionType(alwaysDependent = true)
 
   /** If `tp` is an unboxed capturing type or a function returning an unboxed capturing type,
    *  convert it to be boxed.
@@ -57,7 +57,7 @@ extends tpd.TreeTraverser:
       case tp1 @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(tp1) =>
         val boxedRinfo = recur(rinfo)
         if boxedRinfo eq rinfo then tp
-        else boxedRinfo.toFunctionType(isJava = false, alwaysDependent = true)
+        else boxedRinfo.toFunctionType(alwaysDependent = true)
       case tp1: MethodOrPoly =>
         val res = tp1.resType
         val boxedRes = recur(res)
@@ -151,7 +151,7 @@ extends tpd.TreeTraverser:
             tp.derivedAppliedType(tycon1, args.mapConserve(arg => this(arg)))
         case tp @ RefinedType(core, rname, rinfo: MethodType) if defn.isFunctionType(tp) =>
           val rinfo1 = apply(rinfo)
-          if rinfo1 ne rinfo then rinfo1.toFunctionType(isJava = false, alwaysDependent = true)
+          if rinfo1 ne rinfo then rinfo1.toFunctionType(alwaysDependent = true)
           else tp
         case tp: MethodType =>
           tp.derivedLambdaType(

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1878,22 +1878,20 @@ object Types {
 
     /** Turn type into a function type.
      *  @pre this is a method type without parameter dependencies.
-     *  @param dropLast        the number of trailing parameters that should be dropped
-     *                         when forming the function type.
+     *  @param isJava          translate repeated params as as java `Array`s?
      *  @param alwaysDependent if true, always create a dependent function type.
      */
-    def toFunctionType(isJava: Boolean, dropLast: Int = 0, alwaysDependent: Boolean = false)(using Context): Type = this match {
+    def toFunctionType(isJava: Boolean = false, alwaysDependent: Boolean = false)(using Context): Type = this match {
       case mt: MethodType =>
         assert(!mt.isParamDependent)
         def nonDependentFunType =
-          val formals1 = if (dropLast == 0) mt.paramInfos else mt.paramInfos dropRight dropLast
           val isContextual = mt.isContextualMethod && !ctx.erasedTypes
           val result1 = mt.nonDependentResultApprox match {
             case res: MethodType => res.toFunctionType(isJava)
             case res => res
           }
           defn.FunctionOf(
-            formals1 mapConserve (_.translateFromRepeated(toArray = isJava)),
+            mt.paramInfos.mapConserve(_.translateFromRepeated(toArray = isJava)),
             result1, isContextual)
         if mt.hasErasedParams then
           defn.PolyFunctionOf(mt)

compiler/src/dotty/tools/dotc/transform/Recheck.scala

@@ -333,7 +333,7 @@ abstract class Recheck extends Phase, SymTransformer:
 
     def recheckClosure(tree: Closure, pt: Type)(using Context): Type =
       if tree.tpt.isEmpty then
-        tree.meth.tpe.widen.toFunctionType(tree.meth.symbol.is(JavaDefined))
+        tree.meth.tpe.widen.toFunctionType(isJava = tree.meth.symbol.is(JavaDefined))
       else
         recheck(tree.tpt)
 

compiler/src/dotty/tools/dotc/transform/Splicing.scala

@@ -197,7 +197,7 @@ class Splicing extends MacroTransform:
           if tree.isTerm then
             if isCaptured(tree.symbol) then
               val tpe = tree.tpe.widenTermRefExpr match {
-                case tpw: MethodicType => tpw.toFunctionType(isJava = false)
+                case tpw: MethodicType => tpw.toFunctionType()
                 case tpw => tpw
               }
               spliced(tpe)(capturedTerm(tree))
@@ -291,7 +291,7 @@ class Splicing extends MacroTransform:
 
     private def capturedTerm(tree: Tree)(using Context): Tree =
       val tpe = tree.tpe.widenTermRefExpr match
-        case tpw: MethodicType => tpw.toFunctionType(isJava = false)
+        case tpw: MethodicType => tpw.toFunctionType()
         case tpw => tpw
       capturedTerm(tree, tpe)
 

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -405,7 +405,17 @@ trait TypeAssigner {
 
   def assignType(tree: untpd.Closure, meth: Tree, target: Tree)(using Context): Closure =
     tree.withType(
-      if (target.isEmpty) meth.tpe.widen.toFunctionType(isJava = meth.symbol.is(JavaDefined), tree.env.length)
+      if target.isEmpty then
+        def methTypeWithoutEnv(info: Type): Type = info match
+          case mt: MethodType =>
+            val dropLast = tree.env.length
+            val paramNames = mt.paramNames.dropRight(dropLast)
+            val paramInfos = mt.paramInfos.dropRight(dropLast)
+            mt.derivedLambdaType(paramNames, paramInfos)
+          case pt: PolyType =>
+            pt.derivedLambdaType(resType = methTypeWithoutEnv(pt.resType))
+        val methodicType = if tree.env.isEmpty then meth.tpe.widen else methTypeWithoutEnv(meth.tpe.widen)
+        methodicType.toFunctionType(isJava = meth.symbol.is(JavaDefined))
       else target.tpe)
 
   def assignType(tree: untpd.CaseDef, pat: Tree, body: Tree)(using Context): CaseDef = {