Deprecate = _ under source 3.1

Author: odersky

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

@@ -3208,9 +3208,14 @@ object Parsers {
       val rhs =
         if tpt.isEmpty || in.token == EQUALS then
           accept(EQUALS)
+          val rhsOffset = in.offset
           subExpr() match
             case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name
                 && !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident]) =>
+              if sourceVersion.isAtLeast(`3.1`) then
+                deprecationWarning(
+                  em"""`= _` has been deprecated; use `= notInitialized` instead.
+                      |`notInitialized` needs to be imported from scala.compiletime.""", rhsOffset)
               placeholderParams = placeholderParams.tail
               atSpan(rhs0.span) { Ident(nme.WILDCARD) }
             case rhs0 => rhs0

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -119,7 +119,7 @@ class CompilationTests {
     aggregateTests(
       compileFilesInDir("tests/neg", defaultOptions),
       compileFilesInDir("tests/neg-tailcall", defaultOptions),
-      compileFilesInDir("tests/neg-strict", defaultOptions.and("-source", "3.1", "-Xfatal-warnings")),
+      compileFilesInDir("tests/neg-strict", defaultOptions.and("-source", "3.1", "-deprecation", "-Xfatal-warnings")),
       compileFilesInDir("tests/neg-no-kind-polymorphism", defaultOptions and "-Yno-kind-polymorphism"),
       compileFilesInDir("tests/neg-custom-args/deprecation", defaultOptions.and("-Xfatal-warnings", "-deprecation")),
       compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")),

tests/neg-strict/i1050.scala

@@ -74,7 +74,7 @@ object Tiark3 {
       def brand(x: Any): p.L = x // error: underlying not concrete
     }
     trait V extends U {
-      type X = B with A
+      type X = B & A
       def p2: X = ???
     }
     val v = new V {}

tests/neg-strict/i11225.scala

@@ -0,0 +1,10 @@
+import compiletime.notInitialized
+
+class Memo[A](x: => A):
+  private var cached: A = _ // error
+  private var known: Boolean = false
+  def force =
+    if !known then
+      known = true
+      cached = x
+    cached

tests/neg-strict/nullless.scala

@@ -29,8 +29,8 @@ object bar {
       type U = UU
     }
     final lazy val nothing: Nothing = nothing
-    final lazy val sub: S2 with Sub = nothing
-    final lazy val box : Box[S2 with Sub] = new Box(nothing)
+    final lazy val sub: S2 & Sub = nothing
+    final lazy val box : Box[S2 & Sub] = new Box(nothing)
     def upcast(t: box.v.M2): box.v.M2 = t // error // error under -strict
   }
   def main(args : Array[String]) : Unit = {