fix: Only fallback to the definition of a synthetic valdef if it is zero extent (#22551)

closes scalameta/metals#7038
closes scalameta/metals#7191
fixes the reproduction issue from #22217

Co-authored-by: Katarzyna Marek <[email protected]>

Author: KacperFKorban

Diff for: presentation-compiler/src/main/dotty/tools/pc/completions/CompletionProvider.scala

@@ -99,9 +99,9 @@ class CompletionProvider(
              *  4|     $1$.sliding@@[Int](size, step)
              *
              */
-            if qual.symbol.is(Flags.Synthetic) && qual.symbol.name.isInstanceOf[DerivedName] =>
+            if qual.symbol.is(Flags.Synthetic) && qual.span.isZeroExtent && qual.symbol.name.isInstanceOf[DerivedName] =>
               qual.symbol.defTree match
-                case valdef: ValDef => Select(valdef.rhs, name) :: tail
+                case valdef: ValDef if !valdef.rhs.isEmpty => Select(valdef.rhs, name) :: tail
                 case _ => tpdPath0
           case _ => tpdPath0
 

Diff for: presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -2168,3 +2168,14 @@ class CompletionSuite extends BaseCompletionSuite:
       """|build: Unit
          |""".stripMargin,
     )
+
+  @Test def i7191 =
+    check(
+      """|val x = Some(3).map(_.@@)
+         |""".stripMargin,
+      """|!=(x: Byte): Boolean
+         |!=(x: Char): Boolean
+         |!=(x: Double): Boolean
+         |""".stripMargin,
+      topLines = Some(3)
+    )