Skip extension method params in WUnused (#17178)

Fix #17101

Author: szymon-rd

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -667,7 +667,7 @@ object CheckUnused:
 
     extension (memDef: tpd.MemberDef)
       private def isValidMemberDef(using Context): Boolean =
-        !memDef.symbol.isUnusedAnnot && !memDef.symbol.isAllOf(Flags.AccessorCreationFlags) && !memDef.name.isWildcard
+        !memDef.symbol.isUnusedAnnot && !memDef.symbol.isAllOf(Flags.AccessorCreationFlags) && !memDef.name.isWildcard && !memDef.symbol.owner.is(Extension)
 
       private def isValidParam(using Context): Boolean =
         val sym = memDef.symbol

tests/neg-custom-args/fatal-warnings/i15503g.scala

@@ -12,4 +12,11 @@ def f7(a: Int)(using Int) = summon[Int] + a // OK
 
 /* --- Trivial method check --- */
 def g1(x: Int) = 1 // OK
-def g2(x: Int) = ??? // OK
+def g2(x: Int) = ??? // OK
+
+package foo.test.i17101:
+  type Test[A] = A
+  extension[A] (x: Test[A]) { // OK
+    def value: A = x
+    def causesIssue: Unit = println("oh no")
+  }