Tighten condition when to do SAM type conversion

 - No conversion from contextual lambdas
 - No conversion after erasure

Fixes scala#23234

[Cherry-picked 5170dca]

Author: odersky

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

@@ -1701,7 +1701,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           case mt: MethodType =>
             pt.findFunctionType match {
               case SAMType(samMeth, samParent)
-              if !defn.isFunctionNType(samParent) && mt <:< samMeth =>
+              if !ctx.erasedTypes && !defn.isFunctionNType(samParent)
+                  && mt <:< samMeth && !mt.isImplicitMethod =>
                 if defn.isContextFunctionType(mt.resultType) then
                   report.error(
                     em"""Implementation restriction: cannot convert this expression to `$samParent`

tests/neg-custom-args/captures/i23234.scala

@@ -0,0 +1,5 @@
+abstract class MyFun:
+  def apply(x: Int): Int
+
+object Test:
+  val myFun: MyFun = (x: Int) ?=> x + 10 // error