Inferred Contextual params are less nameable

Author: som-snytt

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

@@ -1987,10 +1987,12 @@ object desugar {
       .collect:
         case vd: ValDef => vd
 
-  def makeContextualFunction(formals: List[Tree], paramNamesOrNil: List[TermName], body: Tree, erasedParams: List[Boolean])(using Context): Function =
+  def makeContextualFunction(formals: List[Tree], paramNamesOrNil: List[TermName], body: Tree, erasedParams: List[Boolean], augmenting: Boolean = false)(using Context): Function =
     val paramNames =
-      if paramNamesOrNil.nonEmpty then paramNamesOrNil
-      else formals.map(_ => ContextFunctionParamName.fresh())
+      if paramNamesOrNil.nonEmpty then
+        if augmenting then paramNamesOrNil.map(ContextFunctionParamName.fresh(_))
+        else paramNamesOrNil
+      else List.fill(formals.length)(ContextFunctionParamName.fresh())
     val params = for (tpt, pname) <- formals.zip(paramNames) yield
       ValDef(pname, tpt, EmptyTree).withFlags(Given | Param)
     FunctionWithMods(params, body, Modifiers(Given), erasedParams)

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

@@ -303,10 +303,12 @@ object NameKinds {
   /** The name of an inferred contextual function parameter:
    *
    *      val x: A ?=> B = b
+   *      val f: (x: A) ?=> B = b
    *
    *  becomes:
    *
    *      val x: A ?=> B = (contextual$1: A) ?=> b
+   *      val f: (x: A) ?=> B = (xcontextual$1: A) ?=> b
    */
   val ContextFunctionParamName: UniqueNameKind = new UniqueNameKind("contextual$")
 

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -4,6 +4,7 @@ package printing
 import core.*
 import Texts.*, Types.*, Flags.*, Names.*, Symbols.*, NameOps.*, Constants.*, Denotations.*
 import StdNames.*
+import NameKinds.ContextFunctionParamName
 import Contexts.*
 import Scopes.Scope, Denotations.Denotation, Annotations.Annotation
 import StdNames.nme
@@ -361,9 +362,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: LazyRef =>
         def refTxt =
           try toTextGlobal(tp.ref)
-          catch {
-            case ex: Throwable => Str("...")
-          }
+          catch case _: Throwable => Str("...") // reconsider catching errors
         "LazyRef(" ~ refTxt ~ ")"
       case Range(lo, hi) =>
         toText(lo) ~ ".." ~ toText(hi)
@@ -395,7 +394,13 @@ class PlainPrinter(_ctx: Context) extends Printer {
     Text(lam.paramRefs.map(paramText), ", ")
   }
 
-  protected def ParamRefNameString(name: Name): String = nameString(name)
+  protected def ParamRefNameString(name: Name): String =
+    nameString:
+      if name.is(ContextFunctionParamName) then
+        name match
+        case ContextFunctionParamName(name, _) => name
+        case name => name
+      else name
 
   protected def ParamRefNameString(param: ParamRef): String =
     ParamRefNameString(param.binder.paramNames(param.paramNum))

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

@@ -3912,7 +3912,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ => paramTypes.map(_ => false)
     }
 
-    val ifun = desugar.makeContextualFunction(paramTypes, paramNamesOrNil, tree, erasedParams)
+    val ifun = desugar.makeContextualFunction(paramTypes, paramNamesOrNil, tree, erasedParams, augmenting = true)
     typr.println(i"make contextual function $tree / $pt ---> $ifun")
     typedFunctionValue(ifun, pt)
       .tap:

tests/neg/i24375.scala

@@ -0,0 +1,12 @@
+trait Ord[X]:
+  def compare(x: X, y: X): Int
+  type T
+
+trait Show[X]:
+  def show(x: X): String
+
+val f0: Show[String] ?=> String = summon[Show[String]].show("hello, world") * 2
+
+val f1: (x: Show[String]) ?=> String = x.show("hello, world") * 2 // error
+
+val f2 = (x: Show[String]) ?=> x.show("hello, world") * 2