Fix printing of poly function types

Author: odersky

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

@@ -158,14 +158,25 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         argStr ~ " " ~ arrow(isGiven) ~ " " ~ argText(args.last)
       }
 
-    def toTextDependentFunction(appType: MethodType): Text =
-      "("
-      ~ keywordText("erased ").provided(appType.isErasedMethod)
-      ~ paramsText(appType)
-      ~ ") "
-      ~ arrow(appType.isImplicitMethod)
-      ~ " "
-      ~ toText(appType.resultType)
+    def toTextMethodAsFunction(info: Type): Text = info match
+      case info: MethodType =>
+        "("
+        ~ keywordText("erased ").provided(info.isErasedMethod)
+        ~ ( if info.isParamDependent || info.isResultDependent
+            then paramsText(info)
+            else argsText(info.paramInfos)
+          )
+        ~ ") "
+        ~ arrow(info.isImplicitMethod)
+        ~ " "
+        ~ toTextMethodAsFunction(info.resultType)
+      case info: PolyType =>
+        "["
+        ~ paramsText(info)
+        ~ "] => "
+        ~ toTextMethodAsFunction(info.resultType)
+      case _ =>
+        toText(info)
 
     def isInfixType(tp: Type): Boolean = tp match
       case AppliedType(tycon, args) =>
@@ -229,8 +240,10 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       if !printDebug && appliedText(tp.asInstanceOf[HKLambda].resType).isEmpty =>
         // don't eta contract if the application would be printed specially
         toText(tycon)
-      case tp: RefinedType if defn.isFunctionType(tp) && !printDebug =>
-        toTextDependentFunction(tp.refinedInfo.asInstanceOf[MethodType])
+      case tp: RefinedType
+      if (defn.isFunctionType(tp) || (tp.parent.typeSymbol eq defn.PolyFunctionClass))
+          && !printDebug =>
+        toTextMethodAsFunction(tp.refinedInfo)
       case tp: TypeRef =>
         if (tp.symbol.isAnonymousClass && !showUniqueIds)
           toText(tp.info)
@@ -244,6 +257,10 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case ErasedValueType(tycon, underlying) =>
         "ErasedValueType(" ~ toText(tycon) ~ ", " ~ toText(underlying) ~ ")"
       case tp: ClassInfo =>
+        if tp.cls.derivesFrom(defn.PolyFunctionClass) then
+          tp.member(nme.apply).info match
+            case info: PolyType => return toTextMethodAsFunction(info)
+            case _ =>
         toTextParents(tp.parents) ~~ "{...}"
       case JavaArrayType(elemtp) =>
         toText(elemtp) ~ "[]"

tests/neg/polymorphic-functions1.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/polymorphic-functions1.scala:1:53 ---------------------------------------------
+1 |val f: [T] => (x: T) => x.type = [T] => (x: Int) => x // error
+  |                                                     ^
+  |                                                     Found:    [T] => (Int) => Int
+  |                                                     Required: [T] => (x: T) => x.type
+
+longer explanation available when compiling with `-explain`

tests/neg/polymorphic-functions1.scala

@@ -0,0 +1 @@
+val f: [T] => (x: T) => x.type = [T] => (x: Int) => x // error