Handle FlexibleTypes in asContextFunctionType

Fixes #23845

Author: HarrisL2

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

@@ -1996,6 +1996,8 @@ class Definitions {
         asContextFunctionType(TypeComparer.bounds(tp1).hiBound)
       case tp1 @ PolyFunctionOf(mt: MethodType) if mt.isContextualMethod =>
         tp1
+      case tp1: FlexibleType =>
+        asContextFunctionType(tp1.underlying)
       case tp1 =>
         if tp1.typeSymbol.name.isContextFunction && isFunctionNType(tp1) then tp1
         else NoType

tests/explicit-nulls/flexible-unpickle/Flexible_2.scala

@@ -1,7 +1,8 @@
 import unsafeNulls.Foo.*
 import unsafeNulls.Unsafe_1
-import unsafeNulls.{A, B, C, F, G, H, I, J, L, M}
+import unsafeNulls.{A, B, C, F, G, H, I, J, L, M, S, T, U, expects}
 import scala.reflect.Selectable.reflectiveSelectable
+import scala.quoted.*
 
 class Inherit_1 extends Unsafe_1 {
   override def foo(s: String): String = s
@@ -34,6 +35,8 @@ case class cc()
 class K(val b: String) extends J(b) {
 }
 
+def typeNameMacro[A: Type](using Quotes) = Expr(Type.show[A])
+
 @main
 def Flexible_2() =
   val s2: String | Null = "foo"
@@ -97,3 +100,12 @@ def Flexible_2() =
 
   val m: String = M.test(null)
 
+  // i23845
+  transparent inline def typeName[A]: String = ${typeNameMacro[A]}
+
+  implicit val givenT: T = ???
+  def alphaTypeNameMacro[A: S](using T) = U(S.show[A])
+  def res[A] = {
+    implicit val givenS: S[A] = ???
+    expects(alphaTypeNameMacro[A])
+  }

tests/explicit-nulls/flexible-unpickle/Unsafe_1.scala

@@ -73,3 +73,9 @@ object M {
   def test(input: => String): String = "foo " + input
 }
 
+class S[X]
+object S {  def show[X] = "dummyStr" }
+class T
+class U[Y](a: Y)
+def expects(take: (T) ?=> U[String]) = ???
+