Fix #9538: repl crashes when :type and :doc are invoked with an empty expression

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

@@ -376,17 +376,25 @@ class ReplDriver(settings: Array[String],
       }
 
     case TypeOf(expr) =>
-      compiler.typeOf(expr)(newRun(state)).fold(
-        displayErrors,
-        res => out.println(SyntaxHighlighting.highlight(res)(using state.context))
-      )
+      expr match {
+        case "" => out.println(s":type <expression>")
+        case _  =>
+          compiler.typeOf(expr)(newRun(state)).fold(
+            displayErrors,
+            res => out.println(SyntaxHighlighting.highlight(res)(using state.context))
+          )
+      }
       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

@@ -0,0 +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) }
 

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

@@ -21,4 +21,9 @@ class TypeTests extends ReplTest {
       run(":type x")
       assertEquals("Int", storedOutput().trim)
     }
+
+  @Test def typeOfEmpty = fromInitialState { implicit s =>
+    run(":type")
+    assertEquals(":type <expression>", storedOutput().trim)
+  }
 }