Future migration warning for alphanumeric infix operator (#18908)

Copy of #18835 with a clean branch name.

Author: nicolasstucki

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

@@ -1081,21 +1081,21 @@ trait Checking {
  !name.isOperatorName &&
  !meth.isDeclaredInfix &&
  !meth.maybeOwner.is(Scala2x) &&
- !infixOKSinceFollowedBy(tree.right) &&
- sourceVersion.isAtLeast(future) =>
+ !infixOKSinceFollowedBy(tree.right) =>
  val (kind, alternative) =
  if (ctx.mode.is(Mode.Type))
  ("type", (n: Name) => s"prefix syntax $n[...]")
  else if (ctx.mode.is(Mode.Pattern))
  ("extractor", (n: Name) => s"prefix syntax $n(...)")
  else
  ("method", (n: Name) => s"method syntax .$n(...)")
- def rewriteMsg = Message.rewriteNotice("The latter", options = "-deprecation")
- report.deprecationWarning(
+ def rewriteMsg = Message.rewriteNotice("The latter", version = `future-migration`)
+ report.errorOrMigrationWarning(
  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)
- if (ctx.settings.deprecation.value) {
+ tree.op.srcPos,
+ from = future)
+ if sourceVersion == `future-migration` then {
  patch(Span(tree.op.span.start, tree.op.span.start), "`")
  patch(Span(tree.op.span.end, tree.op.span.end), "`")
  }

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -63,6 +63,7 @@ class CompilationTests {
  compileFile("tests/rewrites/uninitialized-var.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
  compileFile("tests/rewrites/with-type-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
  compileFile("tests/rewrites/private-this.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
+ compileFile("tests/rewrites/alphanumeric-infix-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
  compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
  compileFile("tests/rewrites/refutable-pattern-bindings.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
  compileFile("tests/rewrites/i8982.scala", defaultOptions.and("-indent", "-rewrite")),

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 -deprecation.
+ | The latter can be rewritten automatically under -rewrite -source future-migration.

tests/rewrites/alphanumeric-infix-operator.check

@@ -0,0 +1,3 @@
+extension (x: Int) def foo(y: Int): Int = x + y
+
+def f: Unit = 2 `foo` 4

tests/rewrites/alphanumeric-infix-operator.scala

@@ -0,0 +1,3 @@
+extension (x: Int) def foo(y: Int): Int = x + y
+
+def f: Unit = 2 foo 4