deprecate synthesis of Manifests

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

@@ -470,7 +470,15 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
     formal.argInfos match
       case arg :: Nil =>
-        synthesize(fullyDefinedType(arg, "Manifest argument", span), kind, topLevel = true)
+        val manifest = synthesize(fullyDefinedType(arg, "Manifest argument", span), kind, topLevel = true)
+        if manifest != EmptyTree then
+          report.deprecationWarning(
+            i"""Compiler synthesis of Manifest and OptManifest is deprecated,
+               |and calls should be replaced by `reflect.classTag[$arg]`.
+               |Alternatively, the using new metaprogramming features of Scala 3
+               |could help avoid the need for runtime type information, see
+               |https://docs.scala-lang.org/scala3/reference/metaprogramming.html""", ctx.source.atSpan(span))
+        manifest
       case _ =>
         EmptyTree
 

tests/neg-custom-args/deprecation/manifest-summoning.check

@@ -0,0 +1,16 @@
+-- Error: tests/neg-custom-args/deprecation/manifest-summoning.scala:1:34 ----------------------------------------------
+1 |val foo = manifest[List[? <: Int]] // error
+  |                                  ^
+  |                                  Compiler synthesis of Manifest and OptManifest is deprecated,
+  |                                  and calls should be replaced by `reflect.classTag[List[? <: Int]]`.
+  |                                  Alternatively, the using new metaprogramming features of Scala 3
+  |                                  could help avoid the need for runtime type information, see
+  |                                  https://docs.scala-lang.org/scala3/reference/metaprogramming.html
+-- Error: tests/neg-custom-args/deprecation/manifest-summoning.scala:2:41 ----------------------------------------------
+2 |val bar = optManifest[Array[? <: String]] // error
+  |                                         ^
+  |                                         Compiler synthesis of Manifest and OptManifest is deprecated,
+  |                                         and calls should be replaced by `reflect.classTag[Array[? <: String]]`.
+  |                                         Alternatively, the using new metaprogramming features of Scala 3
+  |                                         could help avoid the need for runtime type information, see
+  |                                         https://docs.scala-lang.org/scala3/reference/metaprogramming.html

tests/neg-custom-args/deprecation/manifest-summoning.scala

@@ -0,0 +1,2 @@
+val foo = manifest[List[? <: Int]] // error
+val bar = optManifest[Array[? <: String]] // error