From 0000d23e7530cded8df5637f7aa295fa5fd89302 Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Mon, 29 Jul 2024 15:27:06 +0200
Subject: [PATCH] Only replace *new* errors by warnings under `-migration`

This makes `errorOrMigrationWarning` monotonic
from the sourceVersion onto ok < warn < error

For example, ForComprehensionPatternWithoutCase:
- became an error in 3.4,
- was hence a warning in 3.4-migration,
- but it should still be an error in 3.5-migration.
---
 compiler/src/dotty/tools/dotc/report.scala | 2 +-
 tests/neg/migrate-once.scala               | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/migrate-once.scala

diff --git a/compiler/src/dotty/tools/dotc/report.scala b/compiler/src/dotty/tools/dotc/report.scala
index 1d8ca5f208fa..c77d4eb2fc7e 100644
--- a/compiler/src/dotty/tools/dotc/report.scala
+++ b/compiler/src/dotty/tools/dotc/report.scala
@@ -99,7 +99,7 @@ object report:
 
   def errorOrMigrationWarning(msg: Message, pos: SrcPos, migrationVersion: MigrationVersion)(using Context): Unit =
     if sourceVersion.isAtLeast(migrationVersion.errorFrom) then
-      if !sourceVersion.isMigrating then error(msg, pos)
+      if sourceVersion != migrationVersion.errorFrom.prevMigrating then error(msg, pos)
       else if ctx.settings.rewrite.value.isEmpty then migrationWarning(msg, pos)
     else if sourceVersion.isAtLeast(migrationVersion.warnFrom) then warning(msg, pos)
 
diff --git a/tests/neg/migrate-once.scala b/tests/neg/migrate-once.scala
new file mode 100644
index 000000000000..da5b76e4fb8c
--- /dev/null
+++ b/tests/neg/migrate-once.scala
@@ -0,0 +1,5 @@
+//> using options -source:3.5-migration
+
+object Test:
+  for Some(x) <- Seq(Option(1)) yield x // error
+  // was warn before changes, but should warn only until 3.4-migration