Fix #9339: Register doc of extension methods

Author: nicolasstucki

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

@@ -593,6 +593,9 @@ class Namer { typer: Typer =>
   def setDocstring(sym: Symbol, tree: Tree)(using Context): Unit = tree match {
     case t: MemberDef if t.rawComment.isDefined =>
       ctx.docCtx.foreach(_.addDocstring(sym, t.rawComment))
+    case t: ExtMethods =>
+      for meth <- t.methods.find(_.span.point == sym.span.point) do
+        setDocstring(sym, meth)
     case _ => ()
   }
 

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

@@ -31,6 +31,23 @@ class DocTests extends ReplTest {
       assertEquals("doc", doc("new Foo"))
     }
 
+  @Test def docOfExtension1 =
+    eval("/** doc */ extension (x: Int) def foo = 0").andThen { implicit s =>
+      assertEquals("doc", doc("extension_foo"))
+    }
+
+  @Test def docOfExtension2 =
+    eval("extension (x: Int) /** doc */ def foo = 0").andThen { implicit s =>
+      assertEquals("doc", doc("extension_foo"))
+    }
+
+  @Test def docOfExtension3 =
+    eval("/** doc0 */ extension (x: Int) { /** doc1 */ def foo = 0; /** doc2 */ def bar = 0; def baz = 0 }").andThen { implicit s =>
+      assertEquals("doc1", doc("extension_foo"))
+      assertEquals("doc2", doc("extension_bar"))
+      assertEquals("doc0", doc("extension_baz"))
+    }
+
   @Test def docOfDefInObject =
     eval("object O { /** doc */ def foo = 0 }").andThen { implicit s =>
       assertEquals("doc", doc("O.foo"))