sys.env lookup falls back on System.getenv(_)

The latter is case-insensitive on certain platforms

src/library/scala/sys/package.scala

@@ -12,8 +12,9 @@
 
 package scala
 
-import scala.collection.immutable
+import scala.collection.immutable.ArraySeq
 import scala.jdk.CollectionConverters._
+import java.util.NoSuchElementException
 
 /** The package object `scala.sys` contains methods for reading
  *  and altering core aspects of the virtual machine as well as the
@@ -58,10 +59,17 @@ package object sys {
   //  def prop(p: String) = Option(System.getProperty(p))
 
   /** An immutable Map representing the current system environment.
+   *
+   *  If lookup fails, use `System.getenv(_)` for case-insensitive lookup
+   *  on a certain platform. If that also fails, throw `NoSuchElementException`.
    *
    *  @return   a Map containing the system environment variables.
    */
-  def env: immutable.Map[String, String] = immutable.Map.from(System.getenv().asScala)
+  def env: Map[String, String] = Map.from(System.getenv().asScala).withDefault { v =>
+    val s = System.getenv(v)
+    if (s == null) throw new NoSuchElementException(v)
+    s
+  }
 
   /** Register a shutdown hook to be run when the VM exits.
    *  The hook is automatically registered: the returned value can be ignored,
@@ -85,6 +93,6 @@ package object sys {
     val tarray = new Array[Thread](num)
     val got    = Thread.enumerate(tarray)
 
-    immutable.ArraySeq.unsafeWrapArray(tarray).take(got)
+    ArraySeq.unsafeWrapArray(tarray).take(got)
   }
 }

test/junit/scala/sys/env.scala

@@ -0,0 +1,18 @@
+package scala.sys
+
+import org.junit.Test
+import org.junit.Assert._
+
+import java.util.NoSuchElementException
+
+import scala.util.Properties.isWin
+import scala.tools.testkit.AssertUtil.assertThrows
+
+class EnvTest {
+  @Test def `env is case insensitive`(): Unit = {
+    assertTrue(sys.env("PATH") != null)
+    if (isWin) assertTrue(sys.env("path") != null)
+  }
+  @Test def `env throws if not found`(): Unit =
+    assertThrows[NoSuchElementException](sys.env("junk"), _ == "junk")
+}