Fix #5526: Better type inference for dependent function types

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

@@ -1157,7 +1157,9 @@ class Namer { typer: Typer =>
       case TypeTree() =>
         checkMembersOK(inferredType, mdef.pos)
       case DependentTypeTree(tpFun) =>
-        tpFun(paramss.head)
+        val tpe = tpFun(paramss.head)
+        if (isFullyDefined(tpe, ForceDegree.none)) tpe
+        else typedAheadExpr(mdef.rhs, tpe).tpe
       case TypedSplice(tpt: TypeTree) if !isFullyDefined(tpt.tpe, ForceDegree.none) =>
         val rhsType = typedAheadExpr(mdef.rhs, tpt.tpe).tpe
         mdef match {

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

@@ -801,10 +801,10 @@ class Typer extends Namer
       val typeArgs = params1.map(_.tpt) :+ resTpt
       val tycon = TypeTree(funCls.typeRef)
       val core = assignType(cpy.AppliedTypeTree(tree)(tycon, typeArgs), tycon, typeArgs)
-      val appMeth = ctx.newSymbol(ctx.owner, nme.apply, Synthetic | Deferred, mt)
+      val appMeth = ctx.newSymbol(ctx.owner, nme.apply, Synthetic | Method | Deferred, mt, coord = body.pos)
       val appDef = assignType(
         untpd.DefDef(appMeth.name, Nil, List(params1), resultTpt, EmptyTree),
-        appMeth)
+        appMeth).withPos(body.pos)
       RefinedTypeTree(core, List(appDef), ctx.owner.asClass)
     }
 
@@ -1233,7 +1233,9 @@ class Typer extends Namer
         }
       case _ =>
         tree.withType(
-          if (isFullyDefined(pt, ForceDegree.none)) pt else UnspecifiedErrorType)
+          if (isFullyDefined(pt, ForceDegree.noBottom)) pt
+          else if (ctx.reporter.errorsReported) UnspecifiedErrorType
+          else errorType(i"cannot infer type; expected type $pt is not fully defined", tree.pos))
     }
   }
 

tests/pos/i5526a.scala

@@ -0,0 +1,17 @@
+trait A
+object test1 {
+  def foo[E](f: (a: A) => (a.type, E)): E = {
+    val a = new A {}
+    f(a)._2
+  }
+   val res = foo { a => (a, 42) }
+  val x: Int = res
+}
+ object test2 {
+  trait F[A, -E]
+  def empty[A](value: A): F[A, Any] = ???
+   def hof[R](f: (p: AnyRef) => F[R, p.type]): F[R, Any] = ???
+   hof { p =>
+    empty(42)
+  }
+}

tests/pos/i5526b.scala

@@ -0,0 +1,10 @@
+trait F[A, -E]
+object Test {
+  def empty[A](value: A): F[A, Any] = ???
+
+  def hof[R](f: (p: AnyRef) => F[R, p.type]): F[R, Any] = ???
+
+  hof { p =>
+    empty(42)
+  }
+}