Deprecation warnings for old syntax: alphanumeric infix operators

This is the first part of scala#18870

Author: nicolasstucki

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

@@ -1081,6 +1081,7 @@ trait Checking {
              !name.isOperatorName &&
              !meth.isDeclaredInfix &&
              !meth.maybeOwner.is(Scala2x) &&
+             // TODO check is meth was compiled with 3.4+ compiler
              !infixOKSinceFollowedBy(tree.right) =>
             val (kind, alternative) =
               if (ctx.mode.is(Mode.Type))
@@ -1089,13 +1090,14 @@ 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), "`")
             }

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

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.

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

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

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

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

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

tests/pos/alphanumeric-infix-operator-compat/Test3.4_2.scala

@@ -0,0 +1,16 @@
+//> 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

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