Drop vestigial support for parameterless contextual functions

This was dropped from the syntax, so this isn't needed anymore.

Author: smarter

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

@@ -1418,12 +1418,12 @@ object desugar {
    *      def $anonfun(params) = body
    *      Closure($anonfun)
    */
-  def makeClosure(params: List[ValDef], body: Tree, tpt: Tree | Null = null, isContextual: Boolean, span: Span)(using Context): Block =
+  def makeClosure(params: List[ValDef], body: Tree, tpt: Tree | Null = null, span: Span)(using Context): Block =
     Block(
       DefDef(nme.ANON_FUN, params :: Nil, if (tpt == null) TypeTree() else tpt, body)
         .withSpan(span)
         .withMods(synthetic | Artifact),
-      Closure(Nil, Ident(nme.ANON_FUN), if (isContextual) ContextualEmptyTree else EmptyTree))
+      Closure(Nil, Ident(nme.ANON_FUN), EmptyTree))
 
   /** If `nparams` == 1, expand partial function
    *

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

@@ -417,10 +417,7 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
     case Closure(_, meth, _) => true
     case Block(Nil, expr) => isContextualClosure(expr)
     case Block(DefDef(nme.ANON_FUN, params :: _, _, _) :: Nil, cl: Closure) =>
-      if params.isEmpty then
-        cl.tpt.eq(untpd.ContextualEmptyTree) || defn.isContextFunctionType(cl.tpt.typeOpt)
-      else
-        isUsingClause(params)
+      isUsingClause(params)
     case _ => false
   }
 

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

@@ -1192,7 +1192,6 @@ object Trees {
 
     @sharable val EmptyTree: Thicket = genericEmptyTree
     @sharable val EmptyValDef: ValDef = genericEmptyValDef
-    @sharable val ContextualEmptyTree: Thicket = new EmptyTree() // an empty tree marking a contextual closure
 
     // ----- Auxiliary creation methods ------------------
 

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

@@ -236,10 +236,12 @@ object NameOps {
      */
     def isPlainFunction(using Context): Boolean = functionArity >= 0
 
-    /** Is a function name that contains `mustHave` as a substring */
-    private def isSpecificFunction(mustHave: String)(using Context): Boolean =
+    /** Is a function name that contains `mustHave` as a substring
+     *  and has arity `minArity` or greater.
+     */
+    private def isSpecificFunction(mustHave: String, minArity: Int = 0)(using Context): Boolean =
       val suffixStart = functionSuffixStart
-      isFunctionPrefix(suffixStart, mustHave) && funArity(suffixStart) >= 0
+      isFunctionPrefix(suffixStart, mustHave) && funArity(suffixStart) >= minArity
 
     def isContextFunction(using Context): Boolean = isSpecificFunction("Context")
     def isImpureFunction(using Context): Boolean = isSpecificFunction("Impure")

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1511,6 +1511,7 @@ object Parsers {
             TermLambdaTypeTree(params.asInstanceOf[List[ValDef]], resultType)
           else if imods.isOneOf(Given | Impure) || erasedArgs.contains(true) then
             if imods.is(Given) && params.isEmpty then
+              imods &~= Given
               syntaxError(em"context function types require at least one parameter", paramSpan)
             FunctionWithMods(params, resultType, imods, erasedArgs.toList)
           else if !ctx.settings.YkindProjector.isDefault then

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

@@ -1633,7 +1633,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 )
               cpy.ValDef(param)(tpt = paramTpt)
           if isErased then param0.withAddedFlags(Flags.Erased) else param0
-      desugared = desugar.makeClosure(inferredParams, fnBody, resultTpt, isContextual, tree.span)
+      desugared = desugar.makeClosure(inferredParams, fnBody, resultTpt, tree.span)
 
     typed(desugared, pt)
       .showing(i"desugared fun $tree --> $desugared with pt = $pt", typr)
@@ -1669,9 +1669,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                   errorTree(tree,
                     em"""cannot turn method type $mt into closure
                         |because it has internal parameter dependencies""")
-                else if ((tree.tpt `eq` untpd.ContextualEmptyTree) && mt.paramNames.isEmpty)
-                  // Note implicitness of function in target type since there are no method parameters that indicate it.
-                  TypeTree(defn.FunctionOf(Nil, mt.resType, isContextual = true))
                 else if hasCaptureConversionArg(mt.resType) then
                   errorTree(tree,
                     em"""cannot turn method type $mt into closure
@@ -3106,6 +3103,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val ifpt = defn.asContextFunctionType(pt)
           val result =
             if ifpt.exists
+              && defn.functionArity(ifpt) > 0 // ContextFunction0 is only used after ElimByName
               && xtree.isTerm
               && !untpd.isContextualClosure(xtree)
               && !ctx.mode.is(Mode.Pattern)