Fix #9538: doc of empty expression

Author: december32

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -387,10 +387,14 @@ class ReplDriver(settings: Array[String],
       state
 
     case DocOf(expr) =>
-      compiler.docOf(expr)(newRun(state)).fold(
-        displayErrors,
-        res => out.println(res)
-      )
+      expr match {
+        case "" => out.println(s":doc <expression>.")
+        case _  =>
+          compiler.docOf(expr)(newRun(state)).fold(
+            displayErrors,
+            res => out.println(res)
+          )
+      }
       state
 
     case Quit =>

compiler/test-resources/repl/i9538

@@ -1,2 +1,5 @@
 scala>:type
 :type <expression>.
+
+scala>:doc
+:doc <expression>.

compiler/test/dotty/tools/repl/DocTests.scala

@@ -177,6 +177,12 @@ class DocTests extends ReplTest {
       assertEquals("Expansion: some-value", doc("Foo.hello"))
     }
 
+  @Test def docOfEmpty =
+    fromInitialState { implicit s =>
+    run(":doc")
+    assertEquals(":doc <expression>.", storedOutput().trim)
+  }
+
   private def eval(code: String): State =
     fromInitialState { implicit s => run(code) }