Backport "add tracking of NotNullInfo for Match, Case, Try trees (fix #…

…21380)" to LTS (#22121)

Backports #21389 to the 3.3.5.

PR submitted by the release tooling.
[skip ci]

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

@@ -1856,14 +1856,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         case1
       }
       .asInstanceOf[List[CaseDef]]
-    assignType(cpy.Match(tree)(sel, cases1), sel, cases1).cast(pt)
+    var nni = sel.notNullInfo
+    if cases1.nonEmpty then nni = nni.seq(cases1.map(_.notNullInfo).reduce(_.alt(_)))
+    assignType(cpy.Match(tree)(sel, cases1), sel, cases1).cast(pt).withNotNullInfo(nni)
   }
 
   // Overridden in InlineTyper for inline matches
   def typedMatchFinish(tree: untpd.Match, sel: Tree, wideSelType: Type, cases: List[untpd.CaseDef], pt: Type)(using Context): Tree = {
     val cases1 = harmonic(harmonize, pt)(typedCases(cases, sel, wideSelType, pt.dropIfProto))
       .asInstanceOf[List[CaseDef]]
-    assignType(cpy.Match(tree)(sel, cases1), sel, cases1)
+    var nni = sel.notNullInfo
+    if cases1.nonEmpty then nni = nni.seq(cases1.map(_.notNullInfo).reduce(_.alt(_)))
+    assignType(cpy.Match(tree)(sel, cases1), sel, cases1).withNotNullInfo(nni)
   }
 
   def typedCases(cases: List[untpd.CaseDef], sel: Tree, wideSelType: Type, pt: Type)(using Context): List[CaseDef] =
@@ -1925,17 +1929,22 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       }
       val pat1 = indexPattern(tree).transform(pat)
       val guard1 = typedExpr(tree.guard, defn.BooleanType)
-      var body1 = ensureNoLocalRefs(typedExpr(tree.body, pt1), pt1, ctx.scope.toList)
+      var body1 = ensureNoLocalRefs(
+        typedExpr(tree.body, pt1)(using ctx.addNotNullInfo(guard1.notNullInfoIf(true))),
+        pt1, ctx.scope.toList)
       if ctx.gadt.isNarrowing then
         // Store GADT constraint to later retrieve it (in PostTyper, for now).
         // GADT constraints are necessary to correctly check bounds of type app,
         // see tests/pos/i12226 and issue #12226. It might be possible that this
         // will end up taking too much memory. If it does, we should just limit
         // how much GADT constraints we infer - it's always sound to infer less.
         pat1.putAttachment(InferredGadtConstraints, ctx.gadt)
-      if (pt1.isValueType) // insert a cast if body does not conform to expected type if we disregard gadt bounds
+      if pt1.isValueType then // insert a cast if body does not conform to expected type if we disregard gadt bounds
         body1 = body1.ensureConforms(pt1)(using originalCtx)
-      assignType(cpy.CaseDef(tree)(pat1, guard1, body1), pat1, body1)
+      val nni = pat1.notNullInfo
+        .seq(guard1.notNullInfoIf(true))
+        .seq(body1.notNullInfo)
+      assignType(cpy.CaseDef(tree)(pat1, guard1, body1), pat1, body1).withNotNullInfo(nni)
     }
 
     val pat1 = typedPattern(tree.pat, wideSelType)(using gadtCtx)
@@ -2040,13 +2049,27 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
   def typedTry(tree: untpd.Try, pt: Type)(using Context): Try = {
     val expr2 :: cases2x = harmonic(harmonize, pt) {
-      val cases1 = typedCases(tree.cases, EmptyTree, defn.ThrowableType, pt.dropIfProto)
-      val expr1 = typed(addCanThrowCapabilities(tree.expr, cases1), pt.dropIfProto)
+      // We want to type check tree.expr first to comput NotNullInfo, but `addCanThrowCapabilities`
+      // uses the types of patterns in `tree.cases` to determine the capabilities.
+      // Hence, we create a copy of cases with empty body and type check that first, then type check
+      // the rest of the tree in order.
+      // It may seem that invalid references can be created if the type of the pattern contains
+      // type binds, but this is not a valid `CanThrow` capability (checked by `addCanThrowCapabilities`),
+      // so it is not a problem.
+      val casesEmptyBody1 = tree.cases.mapconserve(cpy.CaseDef(_)(body = EmptyTree))
+      val casesEmptyBody2 = typedCases(casesEmptyBody1, EmptyTree, defn.ThrowableType, WildcardType)
+      val expr1 = typed(addCanThrowCapabilities(tree.expr, casesEmptyBody2), pt.dropIfProto)
+      val casesCtx = ctx.addNotNullInfo(expr1.notNullInfo.retractedInfo)
+      val cases1 = typedCases(tree.cases, EmptyTree, defn.ThrowableType, pt.dropIfProto)(using casesCtx)
       expr1 :: cases1
     }: @unchecked
-    val finalizer1 = typed(tree.finalizer, defn.UnitType)
     val cases2 = cases2x.asInstanceOf[List[CaseDef]]
-    assignType(cpy.Try(tree)(expr2, cases2, finalizer1), expr2, cases2)
+
+    var nni = expr2.notNullInfo.retractedInfo
+    if cases2.nonEmpty then nni = nni.seq(cases2.map(_.notNullInfo.retractedInfo).reduce(_.alt(_)))
+    val finalizer1 = typed(tree.finalizer, defn.UnitType)(using ctx.addNotNullInfo(nni))
+    nni = nni.seq(finalizer1.notNullInfo)
+    assignType(cpy.Try(tree)(expr2, cases2, finalizer1), expr2, cases2).withNotNullInfo(nni)
   }
 
   def typedTry(tree: untpd.ParsedTry, pt: Type)(using Context): Try =

tests/explicit-nulls/neg/i21380.scala

@@ -0,0 +1,19 @@
+@main def test() = {
+  var x: String | Null = null
+  if (false) {
+    x = ""
+
+  } else {
+    x = ""
+  }
+  try {
+    x = ""
+    throw new Exception()
+  }
+  catch {
+    case e: Exception => {
+      x = null
+    }
+  }
+  x.replace("", "") // error
+}

tests/explicit-nulls/neg/i21380b.scala

@@ -0,0 +1,21 @@
+def test1 =
+  var x: String | Null = null
+  x = ""
+  1 match
+    case 1 => x = null
+    case _ => x = x.trim() // error // LTS specific
+  x.replace("", "") // error
+
+def test2(i: Int) =
+  var x: String | Null = null
+  i match
+    case 1 => x = "1"
+    case _ => x = " "
+  x.replace("", "") // error // LTS specific
+
+def test3(i: Int) =
+  var x: String | Null = null
+  i match
+    case 1 if x != null => ()
+    case _ => x = " "
+  x.trim() // error // LTS specific

tests/explicit-nulls/neg/i21380c.scala

@@ -0,0 +1,45 @@
+def test1(i: Int): Int =
+  var x: String | Null = null
+  if i == 0 then x = ""
+  else x = ""
+  try
+    x = x.replace(" ", "") // error // LTS specific
+    throw new Exception()
+  catch
+    case e: Exception =>
+      x = x.replaceAll(" ", "") // error
+      x = null
+  x.length // error
+
+def test2: Int =
+  var x: String | Null = null
+  try throw new Exception()
+  finally x = ""
+  x.length // error // LTS specific
+
+def test3 =
+  var x: String | Null = ""
+  try throw new Exception()
+  catch case e: Exception =>
+    x = (??? : String | Null)
+  finally
+    val l = x.length // error
+
+def test4: Int =
+  var x: String | Null = null
+  try throw new Exception()
+  catch
+    case npe: NullPointerException => x = ""
+    case _ => x = ""
+  x.length // error
+  // Although the catch block here is exhaustive,
+  // it is possible that the exception is thrown and not caught.
+  // Therefore, the code after the try block can only rely on the retracted info.
+
+def test5: Int =
+  var x: String | Null = null
+  try
+    x = ""
+    throw new Exception()
+  catch
+    case npe: NullPointerException => val i: Int = x.length // error