diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index 90c26e279d01..1c44d0c34e76 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -1071,12 +1071,19 @@ trait Checking {
   def checkValidInfix(tree: untpd.InfixOp, meth: Symbol)(using Context): Unit = {
     tree.op match {
       case id @ Ident(name: Name) =>
+        def methCompiledBeforeDeprecation =
+          meth.compilationUnitInfo != null && {
+            meth.compilationUnitInfo.nn.tastyVersion match
+              case Some(version) => version.minor < 4 // compiled before 3.4
+              case _ => false // compiled with the current compiler
+          }
         name.toTermName match {
           case name: SimpleName
           if !untpd.isBackquoted(id) &&
              !name.isOperatorName &&
              !meth.isDeclaredInfix &&
              !meth.maybeOwner.is(Scala2x) &&
+             !methCompiledBeforeDeprecation &&
              !infixOKSinceFollowedBy(tree.right) =>
             val (kind, alternative) =
               if (ctx.mode.is(Mode.Type))
@@ -1085,13 +1092,14 @@ trait Checking {
                 ("extractor", (n: Name) => s"prefix syntax $n(...)")
               else
                 ("method", (n: Name) => s"method syntax .$n(...)")
-            def rewriteMsg = Message.rewriteNotice("The latter", version = `future-migration`)
-            report.errorOrMigrationWarning(
+            def rewriteMsg = Message.rewriteNotice("The latter", version = `3.4-migration`)
+            report.gradualErrorOrMigrationWarning(
               em"""Alphanumeric $kind $name is not declared ${hlAsKeyword("infix")}; it should not be used as infix operator.
                   |Instead, use ${alternative(name)} or backticked identifier `$name`.$rewriteMsg""",
               tree.op.srcPos,
-              from = future)
-            if sourceVersion == `future-migration` then {
+              warnFrom = `3.4`,
+              errorFrom = future)
+            if sourceVersion.isMigrating && sourceVersion.isAtLeast(`3.4-migration`) then {
               patch(Span(tree.op.span.start, tree.op.span.start), "`")
               patch(Span(tree.op.span.end, tree.op.span.end), "`")
             }
diff --git a/compiler/test-resources/repl/i1374 b/compiler/test-resources/repl/i1374
index 3d117fdb4ff9..2e0b5be900af 100644
--- a/compiler/test-resources/repl/i1374
+++ b/compiler/test-resources/repl/i1374
@@ -1,4 +1,4 @@
-scala> implicit class Padder(val sb: StringBuilder) extends AnyVal { def pad2(width: Int) = { 1 to width - sb.length foreach { sb append '*' }; sb } }
+scala> implicit class Padder(val sb: StringBuilder) extends AnyVal { infix def pad2(width: Int) = { 1 to width - sb.length foreach { sb append '*' }; sb } }
 // defined class Padder
 def Padder(sb: StringBuilder): Padder
 scala> val greeting = new StringBuilder("Hello, kitteh!")
diff --git a/compiler/test-resources/type-printer/infix b/compiler/test-resources/type-printer/infix
index 2fe2864ad9fe..bedb7071e7f2 100644
--- a/compiler/test-resources/type-printer/infix
+++ b/compiler/test-resources/type-printer/infix
@@ -29,7 +29,7 @@ def foo: (Int &: String) & Boolean
 scala> def foo: Int &: (Boolean & String) = ???
 def foo: Int &: (Boolean & String)
 scala> import scala.annotation.showAsInfix
-scala> @scala.annotation.showAsInfix class Mappy[T,U]
+scala> @scala.annotation.showAsInfix infix class Mappy[T,U]
 // defined class Mappy
 scala> def foo: (Int Mappy Boolean) && String = ???
 def foo: (Int Mappy Boolean) && String
diff --git a/language-server/test/dotty/tools/languageserver/util/embedded/CodeMarker.scala b/language-server/test/dotty/tools/languageserver/util/embedded/CodeMarker.scala
index 416730dc0f4e..33e6a875750a 100644
--- a/language-server/test/dotty/tools/languageserver/util/embedded/CodeMarker.scala
+++ b/language-server/test/dotty/tools/languageserver/util/embedded/CodeMarker.scala
@@ -13,7 +13,7 @@ import PositionContext.PosCtx
 class CodeMarker(val name: String) extends Embedded {
 
   /** A range of positions between this marker and `other`. */
-  def to(other: CodeMarker): CodeRange = CodeRange(this, other)
+  infix def to(other: CodeMarker): CodeRange = CodeRange(this, other)
 
   /** The file containing this marker. */
   def file: PosCtx[TestFile] = posCtx.positionOf(this)._1
diff --git a/scaladoc-testcases/src/tests/infixTypes.scala b/scaladoc-testcases/src/tests/infixTypes.scala
index acfc9044e2eb..30fc982f2bca 100644
--- a/scaladoc-testcases/src/tests/infixTypes.scala
+++ b/scaladoc-testcases/src/tests/infixTypes.scala
@@ -4,9 +4,9 @@ package infixTypes
 import annotation.showAsInfix
 
 @showAsInfix
-trait SomeTrait[A, B]
+infix trait SomeTrait[A, B]
 
-trait SomeTrait2[A, B]
+infix trait SomeTrait2[A, B]
 
 def someTrait1[C, D]: C SomeTrait D
   = ???
diff --git a/scaladoc/src/dotty/tools/scaladoc/tasty/comments/MemberLookup.scala b/scaladoc/src/dotty/tools/scaladoc/tasty/comments/MemberLookup.scala
index b2c4e1bdcac4..26c4fb06dfdf 100644
--- a/scaladoc/src/dotty/tools/scaladoc/tasty/comments/MemberLookup.scala
+++ b/scaladoc/src/dotty/tools/scaladoc/tasty/comments/MemberLookup.scala
@@ -236,9 +236,9 @@ object MemberLookup extends MemberLookup {
       // Scaladoc overloading support allows terminal * (and they're meaningless)
       val cleanStr = str.stripSuffix("*")
 
-      if cleanStr endsWith "$" then
+      if cleanStr.endsWith("$") then
         Selector(cleanStr.init, SelectorKind.ForceTerm)
-      else if cleanStr endsWith "!" then
+      else if cleanStr.endsWith("!") then
         Selector(cleanStr.init, SelectorKind.ForceType)
       else
         Selector(cleanStr, SelectorKind.NoForce)
diff --git a/scaladoc/src/dotty/tools/scaladoc/tasty/comments/wiki/Parser.scala b/scaladoc/src/dotty/tools/scaladoc/tasty/comments/wiki/Parser.scala
index dd3187fb5346..125bca102fba 100644
--- a/scaladoc/src/dotty/tools/scaladoc/tasty/comments/wiki/Parser.scala
+++ b/scaladoc/src/dotty/tools/scaladoc/tasty/comments/wiki/Parser.scala
@@ -675,7 +675,7 @@ sealed class CharReader(buffer: String) { reader =>
 
   var offset: Int = 0
   def char: Char =
-    if (offset >= buffer.length) endOfText else buffer charAt offset
+    if (offset >= buffer.length) endOfText else buffer.charAt(offset)
 
   final def nextChar() =
     offset += 1
@@ -712,7 +712,7 @@ sealed class CharReader(buffer: String) { reader =>
     jumpWhitespace()
     val (ok0, chars0) =
       if (chars.charAt(0) == ' ')
-        (offset > poff, chars substring 1)
+        (offset > poff, chars.substring(1))
       else
         (true, chars)
     val ok = ok0 && jump(chars0)
diff --git a/scaladoc/test/dotty/tools/scaladoc/testUtils.scala b/scaladoc/test/dotty/tools/scaladoc/testUtils.scala
index 2ba78c321eab..d8b0e179c5f1 100644
--- a/scaladoc/test/dotty/tools/scaladoc/testUtils.scala
+++ b/scaladoc/test/dotty/tools/scaladoc/testUtils.scala
@@ -71,7 +71,7 @@ def tastyFiles(name: String, allowEmpty: Boolean = false, rootPck: String = "tes
   }
   def collectFiles(dir: File): List[File] = listFilesSafe(dir).toList.flatMap {
       case f if f.isDirectory => collectFiles(f)
-      case f if f.getName endsWith ".tasty" => f :: Nil
+      case f if f.getName.endsWith(".tasty") => f :: Nil
       case _ => Nil
     }
   val outputDir = BuildInfo.test_testcasesOutputDir
diff --git a/tests/init-global/pos/i18628.scala b/tests/init-global/pos/i18628.scala
index 71d9a1ed30a8..e9fd8f359356 100644
--- a/tests/init-global/pos/i18628.scala
+++ b/tests/init-global/pos/i18628.scala
@@ -55,10 +55,10 @@ trait Parsers {
     }
 
     def flatMap[U](f: T => Parser[U]): Parser[U]
-      = Parser{ in => this(in) flatMapWithNext(f)}
+      = Parser{ in => this(in).flatMapWithNext(f)}
 
     def map[U](f: T => U): Parser[U] //= flatMap{x => success(f(x))}
-      = Parser{ in => this(in) map(f)}
+      = Parser{ in => this(in).map(f)}
 
     def ^^ [U](f: T => U): Parser[U] = map(f)
   }
diff --git a/tests/init/pos/Properties.scala b/tests/init/pos/Properties.scala
index ba92dd54dc83..8dd2fa052c0b 100644
--- a/tests/init/pos/Properties.scala
+++ b/tests/init/pos/Properties.scala
@@ -23,9 +23,9 @@ private[scala] trait PropertiesTrait {
   /** The loaded properties */
   protected lazy val scalaProps: java.util.Properties = {
     val props = new java.util.Properties
-    val stream = pickJarBasedOn getResourceAsStream propFilename
+    val stream = pickJarBasedOn.getResourceAsStream(propFilename)
     if (stream ne null)
-      quietlyDispose(props load stream, stream.close)
+      quietlyDispose(props.load(stream), stream.close)
 
     props
   }
@@ -47,8 +47,8 @@ private[scala] trait PropertiesTrait {
   final def setProp(name: String, value: String)      = System.setProperty(name, value)
   final def clearProp(name: String)                   = System.clearProperty(name)
 
-  final def envOrElse(name: String, alt: String)      = Option(System getenv name) getOrElse alt
-  final def envOrNone(name: String)                   = Option(System getenv name)
+  final def envOrElse(name: String, alt: String)      = Option(System.getenv(name)) getOrElse alt
+  final def envOrNone(name: String)                   = Option(System.getenv(name))
 
   final def envOrSome(name: String, alt: Option[String])       = envOrNone(name) orElse alt
 
@@ -68,7 +68,7 @@ private[scala] trait PropertiesTrait {
   val releaseVersion =
     for {
       v <- scalaPropOrNone("maven.version.number")
-      if !(v endsWith "-SNAPSHOT")
+      if !(v.endsWith("-SNAPSHOT"))
     } yield v
 
   /** The development Scala version, if this is not a final release.
@@ -82,7 +82,7 @@ private[scala] trait PropertiesTrait {
   val developmentVersion =
     for {
       v <- scalaPropOrNone("maven.version.number")
-      if v endsWith "-SNAPSHOT"
+      if v.endsWith("-SNAPSHOT")
       ov <- scalaPropOrNone("version.number")
     } yield ov
 
diff --git a/tests/init/pos/i15465.scala b/tests/init/pos/i15465.scala
index 5b99670e9027..1b0c9e1fead3 100644
--- a/tests/init/pos/i15465.scala
+++ b/tests/init/pos/i15465.scala
@@ -2,7 +2,7 @@ class TestSuite:
   protected val it = new ItWord
 
   protected final class ItWord:
-    def should(string: String) = new ItVerbString("should", string)
+    infix def should(string: String) = new ItVerbString("should", string)
 
   private def registerTestToRun(fun: => Any): Unit = ()
 
diff --git a/tests/neg/alphanumeric-infix-operator-3.4.scala b/tests/neg/alphanumeric-infix-operator-3.4.scala
new file mode 100644
index 000000000000..14d3358127ca
--- /dev/null
+++ b/tests/neg/alphanumeric-infix-operator-3.4.scala
@@ -0,0 +1,11 @@
+//> using options -Werror
+
+import language.`3.4`
+
+class Foo:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
+
+def test(foo: Foo): Unit =
+  foo x 1 // error (because it was compiled with 3.4+)
+  foo y 2 // ok: is marked as infix
diff --git a/tests/neg/alphanumeric-infix-operator.check b/tests/neg/alphanumeric-infix-operator.check
new file mode 100644
index 000000000000..52b08f16b88c
--- /dev/null
+++ b/tests/neg/alphanumeric-infix-operator.check
@@ -0,0 +1,6 @@
+-- Error: tests/neg/alphanumeric-infix-operator.scala:8:6 --------------------------------------------------------------
+8 |  foo x 1 // error (because it was compiled with 3.4+)
+  |      ^
+  |      Alphanumeric method x is not declared infix; it should not be used as infix operator.
+  |      Instead, use method syntax .x(...) or backticked identifier `x`.
+  |      The latter can be rewritten automatically under -rewrite -source 3.4-migration.
diff --git a/tests/neg/alphanumeric-infix-operator.scala b/tests/neg/alphanumeric-infix-operator.scala
new file mode 100644
index 000000000000..1f2233dda6ce
--- /dev/null
+++ b/tests/neg/alphanumeric-infix-operator.scala
@@ -0,0 +1,9 @@
+//> using options -Werror
+
+class Foo:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
+
+def test(foo: Foo): Unit =
+  foo x 1 // error (because it was compiled with 3.4+)
+  foo y 2 // ok: is marked as infix
diff --git a/tests/neg/i10901.scala b/tests/neg/i10901.scala
index 19a2e3023c85..dc1ea6e6eef6 100644
--- a/tests/neg/i10901.scala
+++ b/tests/neg/i10901.scala
@@ -13,23 +13,23 @@ object BugExp4Point2D {
 
       // N - N
       @targetName("point2DConstant")
-      def º(y: T2): Point2D[T1,T2] = ???
+      infix def º(y: T2): Point2D[T1,T2] = ???
 
 
       // N - C
       @targetName("point2DConstantData")
-      def º(y: ColumnType[T2]): Point2D[T1,T2] = ???
+      infix def º(y: ColumnType[T2]): Point2D[T1,T2] = ???
 
 
 
     extension [T1:Numeric, T2:Numeric](x: ColumnType[T1])
       // C - C
       @targetName("point2DData")
-      def º(y: ColumnType[T2]): Point2D[T1,T2] = ???
+      infix def º(y: ColumnType[T2]): Point2D[T1,T2] = ???
 
       // C - N
       @targetName("point2DDataConstant")
-      def º(y: T2): Point2D[T1,T2] = ???
+      infix def º(y: T2): Point2D[T1,T2] = ???
 
 
   }
diff --git a/tests/neg/i2033.check b/tests/neg/i2033.check
index 5751d91f4f3a..7737bba96a5e 100644
--- a/tests/neg/i2033.check
+++ b/tests/neg/i2033.check
@@ -13,3 +13,9 @@
 6 |    val out = new ObjectOutputStream(println)
   |                                     ^^^^^^^
   |method println is eta-expanded even though java.io.OutputStream does not have the @FunctionalInterface annotation.
+-- Warning: tests/neg/i2033.scala:7:18 ---------------------------------------------------------------------------------
+7 |    val arr = bos toByteArray ()  // error
+  |                  ^^^^^^^^^^^
+  |                  Alphanumeric method toByteArray is not declared infix; it should not be used as infix operator.
+  |                  Instead, use method syntax .toByteArray(...) or backticked identifier `toByteArray`.
+  |                  The latter can be rewritten automatically under -rewrite -source 3.4-migration.
diff --git a/tests/neg/rewrite-messages.check b/tests/neg/rewrite-messages.check
index f368c2dc8997..b062ab2bc732 100644
--- a/tests/neg/rewrite-messages.check
+++ b/tests/neg/rewrite-messages.check
@@ -8,4 +8,4 @@
   |    ^^^
   |    Alphanumeric method foo is not declared infix; it should not be used as infix operator.
   |    Instead, use method syntax .foo(...) or backticked identifier `foo`.
-  |    The latter can be rewritten automatically under -rewrite -source future-migration.
+  |    The latter can be rewritten automatically under -rewrite -source 3.4-migration.
diff --git a/tests/neg/syntax-error-recovery.check b/tests/neg/syntax-error-recovery.check
index 0cf550c74d09..d93a939b0c72 100644
--- a/tests/neg/syntax-error-recovery.check
+++ b/tests/neg/syntax-error-recovery.check
@@ -136,3 +136,9 @@
    |       Discarded non-Unit value of type Null. You may want to use `()`.
    |
    | longer explanation available when compiling with `-explain`
+-- Warning: tests/neg/syntax-error-recovery.scala:61:2 -----------------------------------------------------------------
+61 |  println(bam)
+   |  ^^^^^^^
+   |  Alphanumeric method println is not declared infix; it should not be used as infix operator.
+   |  Instead, use method syntax .println(...) or backticked identifier `println`.
+   |  The latter can be rewritten automatically under -rewrite -source 3.4-migration.
diff --git a/tests/pos/alphanumeric-infix-operator-compat/A_1_c3.0.0.scala b/tests/pos/alphanumeric-infix-operator-compat/A_1_c3.0.0.scala
new file mode 100644
index 000000000000..51ea7ec45de6
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/A_1_c3.0.0.scala
@@ -0,0 +1,3 @@
+class A:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
diff --git a/tests/pos/alphanumeric-infix-operator-compat/B_1_c3.1.0.scala b/tests/pos/alphanumeric-infix-operator-compat/B_1_c3.1.0.scala
new file mode 100644
index 000000000000..2377b01f7fec
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/B_1_c3.1.0.scala
@@ -0,0 +1,3 @@
+class B:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
diff --git a/tests/pos/alphanumeric-infix-operator-compat/C_1_c3.2.0.scala b/tests/pos/alphanumeric-infix-operator-compat/C_1_c3.2.0.scala
new file mode 100644
index 000000000000..c1e11d2547ee
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/C_1_c3.2.0.scala
@@ -0,0 +1,3 @@
+class C:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
diff --git a/tests/pos/alphanumeric-infix-operator-compat/D_1_c3.3.0.scala b/tests/pos/alphanumeric-infix-operator-compat/D_1_c3.3.0.scala
new file mode 100644
index 000000000000..0e05a819694c
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/D_1_c3.3.0.scala
@@ -0,0 +1,3 @@
+class D:
+  def x(i: Int) = i
+  infix def y(i: Int) = i
diff --git a/tests/pos/alphanumeric-infix-operator-compat/Test3.4_2.scala b/tests/pos/alphanumeric-infix-operator-compat/Test3.4_2.scala
new file mode 100644
index 000000000000..dd6367518fb4
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/Test3.4_2.scala
@@ -0,0 +1,15 @@
+//> using options -Werror
+
+import language.`3.4`
+
+def test1(a: A, b: B, c: C, d: D): Unit =
+  a x 1 // ok: was compiled with 3.0
+  b x 1 // ok: was compiled with 3.1
+  c x 1 // ok: was compiled with 3.2
+  d x 1 // ok: was compiled with 3.3
+
+  // ok: is marked as infix
+  a y 2
+  b y 2
+  c y 2
+  d y 2
diff --git a/tests/pos/alphanumeric-infix-operator-compat/TestFuture_2.scala b/tests/pos/alphanumeric-infix-operator-compat/TestFuture_2.scala
new file mode 100644
index 000000000000..cf6a9d0adfc2
--- /dev/null
+++ b/tests/pos/alphanumeric-infix-operator-compat/TestFuture_2.scala
@@ -0,0 +1,13 @@
+import language.future
+
+def test2(a: A, b: B, c: C, d: D): Unit =
+  a x 1 // ok: was compiled with 3.0
+  b x 1 // ok: was compiled with 3.1
+  c x 1 // ok: was compiled with 3.2
+  d x 1 // ok: was compiled with 3.3
+
+  // ok: is marked as infix
+  a y 2
+  b y 2
+  c y 2
+  d y 2
diff --git a/tests/pos/i7424c.scala b/tests/pos/i7424c.scala
index 9a02328f82a2..b40ef00a48dd 100644
--- a/tests/pos/i7424c.scala
+++ b/tests/pos/i7424c.scala
@@ -1,6 +1,6 @@
 //> using options -Werror
 object Main extends App:
-   enum Extends[A, B]:
+   infix enum Extends[A, B]:
      case Ev[B, A <: B]() extends (A Extends B)
 
      def cast(a: A): B = this match {
diff --git a/tests/semanticdb/metac.expect b/tests/semanticdb/metac.expect
index 717bfb0505fb..2fd8eca47a7b 100644
--- a/tests/semanticdb/metac.expect
+++ b/tests/semanticdb/metac.expect
@@ -3460,6 +3460,7 @@ Text => empty
 Language => Scala
 Symbols => 12 entries
 Occurrences => 33 entries
+Diagnostics => 1 entries
 Synthetics => 4 entries
 
 Symbols:
@@ -3511,6 +3512,11 @@ Occurrences:
 [20:4..20:13): scalameta -> scala/reflect/Selectable#selectDynamic().
 [21:4..21:19): StructuralTypes -> example/StructuralTypes.
 
+Diagnostics:
+[14:20..14:23): [warning] Alphanumeric method foo is not declared infix; it should not be used as infix operator.
+Instead, use method syntax .foo(...) or backticked identifier `foo`.
+The latter can be rewritten automatically under -rewrite -source 3.4-migration.
+
 Synthetics:
 [12:2..12:6):user => reflectiveSelectable(*)
 [13:2..13:6):user => reflectiveSelectable(*)