Improve warning for wildcard matching only null under the explicit nu…

…lls flag (scala#21577) (#21623)

Improve warning for wildcard matching only null under the explicit nulls. Fix #21577

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -14,6 +14,7 @@ import typer.*, Applications.*, Inferencing.*, ProtoTypes.*
 import util.*
 
 import scala.annotation.internal.sharable
+import scala.annotation.tailrec
 import scala.collection.mutable
 
 import SpaceEngine.*
@@ -696,7 +697,7 @@ object SpaceEngine {
           else NoType
         }.filter(_.exists)
         parts
-
+      case tp: FlexibleType => List(tp.underlying, ConstantType(Constant(null)))
       case _ => ListOfNoType
     end rec
 
@@ -876,6 +877,7 @@ object SpaceEngine {
     case tp: SingletonType                          => toUnderlying(tp.underlying)
     case tp: ExprType                               => toUnderlying(tp.resultType)
     case AnnotatedType(tp, annot)                   => AnnotatedType(toUnderlying(tp), annot)
+    case tp: FlexibleType                           => tp.derivedFlexibleType(toUnderlying(tp.underlying))
     case _                                          => tp
   })
 
@@ -910,52 +912,41 @@ object SpaceEngine {
     && !sel.tpe.widen.isRef(defn.QuotedExprClass)
     && !sel.tpe.widen.isRef(defn.QuotedTypeClass)
 
-  def checkReachability(m: Match)(using Context): Unit = trace(i"checkReachability($m)") {
-    val cases = m.cases.toIndexedSeq
-
+  def checkReachability(m: Match)(using Context): Unit = trace(i"checkReachability($m)"):
     val selTyp = toUnderlying(m.selector.tpe).dealias
-
-    val isNullable = selTyp.classSymbol.isNullableClass
-    val targetSpace = trace(i"targetSpace($selTyp)")(if isNullable
+    val isNullable = selTyp.isInstanceOf[FlexibleType] || selTyp.classSymbol.isNullableClass
+    val targetSpace = trace(i"targetSpace($selTyp)"):
+      if isNullable && !ctx.mode.is(Mode.SafeNulls)
       then project(OrType(selTyp, ConstantType(Constant(null)), soft = false))
       else project(selTyp)
-    )
-
-    var i        = 0
-    val len      = cases.length
-    var prevs    = List.empty[Space]
-    var deferred = List.empty[Tree]
-
-    while (i < len) {
-      val CaseDef(pat, guard, _) = cases(i)
-
-      val curr = trace(i"project($pat)")(project(pat))
-
-      val covered = trace("covered")(simplify(intersect(curr, targetSpace)))
-
-      val prev = trace("prev")(simplify(Or(prevs)))
 
-      if prev == Empty && covered == Empty then // defer until a case is reachable
-        deferred ::= pat
-      else {
-        for (pat <- deferred.reverseIterator)
-          report.warning(MatchCaseUnreachable(), pat.srcPos)
-        if pat != EmptyTree // rethrow case of catch uses EmptyTree
-            && !pat.symbol.isAllOf(SyntheticCase, butNot=Method) // ExpandSAMs default cases use SyntheticCase
-            && isSubspace(covered, prev)
-        then {
-          val nullOnly = isNullable && i == len - 1 && isWildcardArg(pat)
-          val msg = if nullOnly then MatchCaseOnlyNullWarning() else MatchCaseUnreachable()
-          report.warning(msg, pat.srcPos)
-        }
-        deferred = Nil
-      }
-
-      // in redundancy check, take guard as false in order to soundly approximate
-      prevs ::= (if guard.isEmpty then covered else Empty)
-      i += 1
-    }
-  }
+    @tailrec def recur(cases: List[CaseDef], prevs: List[Space], deferred: List[Tree]): Unit =
+      cases match
+        case Nil =>
+        case CaseDef(pat, guard, _) :: rest =>
+          val curr = trace(i"project($pat)")(project(pat))
+          val covered = trace("covered")(simplify(intersect(curr, targetSpace)))
+          val prev = trace("prev")(simplify(Or(prevs)))
+          if prev == Empty && covered == Empty then // defer until a case is reachable
+            recur(rest, prevs, pat :: deferred)
+          else
+            for pat <- deferred.reverseIterator
+            do report.warning(MatchCaseUnreachable(), pat.srcPos)
+
+            if pat != EmptyTree // rethrow case of catch uses EmptyTree
+                && !pat.symbol.isAllOf(SyntheticCase, butNot=Method) // ExpandSAMs default cases use SyntheticCase
+                && isSubspace(covered, prev)
+            then
+              val nullOnly = isNullable && rest.isEmpty && isWildcardArg(pat)
+              val msg = if nullOnly then MatchCaseOnlyNullWarning() else MatchCaseUnreachable()
+              report.warning(msg, pat.srcPos)
+
+            // in redundancy check, take guard as false in order to soundly approximate
+            val newPrev = if guard.isEmpty then covered :: prevs else prevs
+            recur(rest, newPrev, Nil)
+
+    recur(m.cases, Nil, Nil)
+  end checkReachability
 
   def checkMatch(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)

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

@@ -213,6 +213,11 @@ class CompilationTests {
     )
   }.checkCompile()
 
+  @Test def explicitNullsWarn: Unit = {
+    implicit val testGroup: TestGroup = TestGroup("explicitNullsWarn")
+    compileFilesInDir("tests/explicit-nulls/warn", explicitNullsOptions)
+  }.checkWarnings()
+
   @Test def explicitNullsRun: Unit = {
     implicit val testGroup: TestGroup = TestGroup("explicitNullsRun")
     compileFilesInDir("tests/explicit-nulls/run", explicitNullsOptions)

tests/explicit-nulls/pos/interop-constructor.scala

@@ -1,4 +1,4 @@
-// Test that constructors have a non-nullab.e return type.
+// Test that constructors have a non-nullable return type.
 
 class Foo {
   val x: java.lang.String = new java.lang.String()

tests/explicit-nulls/warn/i21577.check

@@ -0,0 +1,28 @@
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:5:9 --------------------------------------------
+5 |    case _ => // warn
+  |         ^
+  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:12:9 -------------------------------------------
+12 |    case _ => // warn
+   |         ^
+   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/i21577.scala:20:7 ----------------------------------
+20 |  case _ => // warn
+   |       ^
+   |       Unreachable case
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:29:27 -----------------------------
+29 |def f7(s: String | Null) = s match // warn
+   |                           ^
+   |                           match may not be exhaustive.
+   |
+   |                           It would fail on pattern case: _: Null
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:36:33 -----------------------------
+36 |def f9(s: String | Int | Null) = s match // warn
+   |                                 ^
+   |                                 match may not be exhaustive.
+   |
+   |                                 It would fail on pattern case: _: Int
+   |
+   | longer explanation available when compiling with `-explain`

tests/explicit-nulls/warn/i21577.scala

@@ -0,0 +1,38 @@
+def f(s: String) =
+  val s2 = s.trim()
+  s2 match
+    case s3: String =>
+    case _ => // warn
+
+
+def f2(s: String | Null) =
+  val s2 = s.nn.trim()
+  s2 match
+    case s3: String =>
+    case _ => // warn
+
+def f3(s: String | Null) = s match
+  case s2: String =>
+  case _ =>
+
+def f5(s: String) = s match
+  case _: String =>
+  case _ => // warn
+
+def f6(s: String) = s.trim() match
+  case _: String =>
+  case null =>
+
+def f61(s: String) = s.trim() match
+  case _: String =>
+
+def f7(s: String | Null) = s match // warn
+  case _: String =>
+
+def f8(s: String | Null) = s match
+  case _: String =>
+  case null =>
+
+def f9(s: String | Int | Null) = s match // warn
+  case _: String =>
+  case null =>

tests/explicit-nulls/warn/interop.check

@@ -0,0 +1,8 @@
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/interop/S.scala:8:11 ----------------------------------------
+8 |      case _ => // warn
+  |           ^
+  |           Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/interop/S.scala:9:9 -----------------------------------------
+9 |    case _ => println(2) // warn
+  |         ^
+  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).

tests/explicit-nulls/warn/interop/J.java

@@ -0,0 +1,6 @@
+import java.util.ArrayList;
+
+class J {
+	ArrayList<ArrayList<String>> foo(String x) { return null; }
+	static String fooStatic(String x) { return null; }
+}

tests/explicit-nulls/warn/interop/S.scala

@@ -0,0 +1,10 @@
+import java.util.ArrayList
+def f() =
+  val j = new J()
+  val s2 = j.foo(null)
+  s2 match
+    case s3: ArrayList[ArrayList[String]] => s3.get(0) match
+      case _: ArrayList[_] =>
+      case _ => // warn
+    case _ => println(2) // warn
+