Fixes #5995

compiler/semstmts.nim

@@ -1129,7 +1129,7 @@ proc semProcAnnotation(c: PContext, prc: PNode;
       x.add(it.sons[1])
     x.add(prc)
     # recursion assures that this works for multiple macro annotations too:
-    result = semStmt(c, x)
+    result = semExpr(c, x)
     # since a proc annotation can set pragmas, we process these here again.
     # This is required for SqueakNim-like export pragmas.
     if result.kind in procDefs and result[namePos].kind == nkSym and

lib/pure/asyncmacro.nim

@@ -301,7 +301,7 @@ proc verifyReturnType(typeName: string) {.compileTime.} =
 proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
   ## This macro transforms a single procedure into a closure iterator.
   ## The ``async`` macro supports a stmtList holding multiple async procedures.
-  if prc.kind notin {nnkProcDef, nnkLambda, nnkMethodDef}:
+  if prc.kind notin {nnkProcDef, nnkLambda, nnkMethodDef, nnkDo}:
       error("Cannot transform this node kind into an async proc." &
             " proc/method definition or lambda node expected.")
 

tests/async/tlambda.nim

@@ -51,5 +51,8 @@ proc main() =
 
   var builder = newBuilder()
 
+  # Test {.async.} pragma with do notation: #5995
+  builder.client = newClient("builder") do(client: Client, msg: JsonNode) {.async.}:
+    await onMessage(builder, msg)
 
 main()