Fix #4125: Properly type erased dependent function types

Author: smarter

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

@@ -777,7 +777,7 @@ class Typer extends Namer
       completeParams(params)
       val params1 = params.map(typedExpr(_).asInstanceOf[ValDef])
       val resultTpt = typed(body)
-      val companion = if (isImplicit) ImplicitMethodType else MethodType
+      val companion = MethodType.maker(isImplicit = isImplicit, isErased = isErased)
       val mt = companion.fromSymbols(params1.map(_.symbol), resultTpt.tpe)
       if (mt.isParamDependent)
         ctx.error(i"$mt is an illegal function type because it has inter-parameter dependencies", tree.pos)

compiler/test/dotty/tools/dotc/FromTastyTests.scala

@@ -87,6 +87,7 @@ class FromTastyTests extends ParallelTesting {
 
          // Closure type miss match
          "eff-dependent.scala",
+         "i4125.scala",
 
          // Issue unpickling universes
          "phantom-decls-1.scala",

tests/pos/i4125.scala

@@ -0,0 +1,4 @@
+object Test {
+  def foo: (erased (x: Int, y: Int) => Int) = erased (x, y) => 1
+  def bar: (erased implicit (x: Int, y: Int) => Int) = erased implicit (x, y) => 1
+}