Add a full structure test for intermittent failure

scalafmt-tests/src/test/scala/org/scalafmt/CustomStructureTest.scala

@@ -0,0 +1,58 @@
+package org.scalafmt
+
+import munit.FunSuite
+
+import scala.meta._
+import scala.meta.internal.parsers.ScalametaParser
+
+class CustomStructureTest extends FunSuite {
+
+  private def check(
+      original: String,
+      expected: Tree,
+      dialect: Dialect
+  )(implicit loc: munit.Location): Unit = {
+    val parser = new ScalametaParser(Input.String(original))(dialect)
+    assertNoDiff(parser.parseStat().structure, expected.structure)
+  }
+
+  // #3634
+  check(
+    """
+      |def foo(a: Foo[_]): Unit = ???
+      """.stripMargin,
+    Defn.Def(
+      Nil,
+      Term.Name("foo"),
+      List(
+        Member.ParamClauseGroup(
+          Type.ParamClause(Nil),
+          List(
+            Term.ParamClause(
+              List(
+                Term.Param(
+                  Nil,
+                  Term.Name("a"),
+                  Some(
+                    Type.Apply(
+                      Type.Name("Foo"),
+                      Type.ArgClause(
+                        List(Type.Wildcard(Type.Bounds(None, None)))
+                      )
+                    )
+                  ),
+                  None
+                )
+              ),
+              None
+            )
+          )
+        )
+      ),
+      Some(Type.Name("Unit")),
+      Term.Name("???")
+    ),
+    dialects.Scala3
+  )
+
+}