fix #6608 (#14963)

* fix #6608

compiler/semexprs.nim

@@ -955,8 +955,17 @@ proc semIndirectOp(c: PContext, n: PNode, flags: TExprFlags): PNode =
             hasErrorType = true
             break
         if not hasErrorType:
+          let typ = n[0].typ
           msg.add(">\nbut expected one of: \n" &
-              typeToString(n[0].typ))
+              typeToString(typ))
+          # prefer notin preferToResolveSymbols
+          # t.sym != nil
+          # sfAnon notin t.sym.flags
+          # t.kind != tySequence(It is tyProc)
+          if typ.sym != nil and sfAnon notin typ.sym.flags and 
+                                typ.kind == tyProc:
+            msg.add(" = " & 
+                typeToString(typ, preferDesc))
           localError(c.config, n.info, msg)
         return errorNode(c, n)
       result = nil

tests/errmsgs/t6608.nim

@@ -0,0 +1,16 @@
+discard """
+  cmd: "nim c --hints:off $file"
+  errormsg: "type mismatch: got <>"
+  nimout: '''t6608.nim(14, 4) Error: type mismatch: got <>
+but expected one of:
+AcceptCB = proc (s: string){.closure.}'''
+  line: 14
+"""
+
+type
+  AcceptCB = proc (s: string)
+
+proc x(x: AcceptCB) =
+  x()
+
+x()