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