Fix implicit type used for conversions, as seen from usage

Using the minimisation of the specs2 failure that occurred in CI, the
conversion desired is `Conversion[String, Bar.this.Data]`.  But if we
use the given's symbol info, we end up with the type `Conversion[String,
Foo.this.Data]` which the generated tree will fail to adapt to.

Using the widening of the implicit candidate's TermRef will yield a
method type, as seen from the right prefix - `Bar.this.Data`.

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

@@ -1138,7 +1138,7 @@ trait Implicits:
               case _ => pt
             case _ => NoType
           def tryConversion(using Context) = {
-            val restpeConv = if ref.symbol.is(Given) then conversionResultType(ref.symbol.info) else NoType
+            val restpeConv = if ref.symbol.is(Given) then conversionResultType(ref.widenTermRefExpr) else NoType
             val untpdConv =
               if restpeConv.exists then
                 untpd.Select(

tests/pos/i15867.specs2.scala

@@ -0,0 +1,9 @@
+class Foo:
+  given Conversion[String, Data] with
+    def apply(str: String): Data = new Data(str)
+
+  class Data(str: String):
+    def |(str: String) = new Data(this.str + str)
+
+class Bar extends Foo:
+  "str" | "ing"