Restrict outdent in parens for certain region prefixes

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -735,7 +735,14 @@ object Scanners {
             case _ => false
           currentRegion match
             case r: Indented if isEnclosedInParens(r.outer) =>
-              insert(OUTDENT, offset)
+              // For some region prefixes (COLONeol, EQUALS) only OUTDENT if COMMA at EOL
+              if canStartIndentTokens.contains(r.prefix) && !statCtdTokens.contains(r.prefix) then
+                val lookahead = LookaheadScanner()
+                lookahead.nextToken()
+                if lookahead.isAfterLineEnd then
+                  insert(OUTDENT, offset)
+              else
+                insert(OUTDENT, offset)
             case _ =>
               peekAhead()
               if isAfterLineEnd

tests/neg/i22527.scala

@@ -0,0 +1,24 @@
+
+//rule of thumb is COLONeol was at EOL, so COMMA must be at EOL
+def test: Unit =
+  assert(
+    identity:
+      true, "ok" // error end of statement expected but ',' found
+  )
+
+def toss: Unit =
+  assert(
+    throw
+      null, "ok" // error same
+  )
+
+def callme[A](x: => A, msg: String) = try x.toString catch case t: RuntimeException => msg
+
+// not all indented regions require COMMA at EOL for OUTDENT
+def orElse(x: Int): Unit =
+  callme(
+    if x > 0 then
+      class X extends AnyRef, Serializable // error Not found: Serializable - did you mean Specializable?
+      true // error ',' or ')' expected, but 'true' found
+    else
+      false, "fail")

tests/pos/i22527.scala

@@ -0,0 +1,65 @@
+
+def f: Unit =
+  identity(
+    identity:
+      class X extends AnyRef, Serializable
+      42
+  )
+
+def g: Unit =
+  identity(
+    x =
+      class X extends AnyRef, Serializable
+      27
+  )
+
+def test: Unit =
+  assert(
+    identity:
+      true,
+    "ok"
+  )
+
+def toss: Unit =
+  assert(
+    throw
+      null,
+    "ok"
+  )
+
+def callme[A](x: => A, msg: String) = try x.toString catch case t: RuntimeException => msg
+
+def orElse(x: Int): Unit =
+  callme(
+    if x > 0 then
+      true
+    else
+      false, "fail")
+
+def onlyIf(x: Int): Unit =
+  callme(
+    if (x > 0)
+      true, "fail") // warn value discard
+
+def h(xs: List[Int]) =
+  xs.foldLeft(0)
+    (
+      (acc, x) =>
+          acc
+        + x,
+    )
+
+def sum(x: Int, y: Int, z: Int) = x+y+z
+
+def k(xs: List[Int], y: Int, z: Int) =
+  xs.foldLeft(0)
+    (
+      (acc, x) =>
+          sum(
+            x
+          + y
+            + z,
+          y,
+          z,
+          )
+    )