Two errors name the wrong value: a bad + marker reports another parameter, and a double use in a pattern reports another binder

[nohzafk]

Two checker errors name a value that is not the cause. The rules behind them are clear, and the guide documents both. Only the messages are wrong.

1. The + marker on a Type parameter

A + marker makes a value reusable. The guide says that this marker needs a Data value, and that functions, arrays and IO handles are Type. So the checker refuses + on those values. That refusal is correct.

The message does not say this. It reports expected : Data / observed : Type. It never mentions the marker, the copy, or the parameter. When the def has more than one parameter, the Context block names a different parameter. That parameter is correct.

import Base

def two(+name: String, +res: Result<&1, &1, U32 & String, Unit>) -> Unit:
  Unit{}

Output:

Error:
- expected : Data
- observed : Type
Context:
- name : String
Location: two
3>| def two(+name: String, +res: Result<&1, &1, U32 & String, Unit>) -> Unit:

The offender is res. The Context block names name.

With one parameter, the same error gives no Context block at all:

import Base

def hold(+h: File) -> Unit:
  Unit{}

Error:
- expected : Data
- observed : Type
Location: hold

2. A double use inside a 1n+f pattern

This def matches n, then uses n again in the body. That is two uses of n, and n is affine. The checker refuses it.

The message names f instead. f occurs one time, in the recursive call.

import Base

def count(n: Nat, acc: Nat) -> Nat:
  match n
    case 0n:
      acc
    case 1n+f:
      count(f, (n + acc : Nat))

Output:

Error:
- expected : f
- observed : f (consumed more than once)
Location: count
7>|     case 1n+f:
8 |       count(f, (n + acc : Nat))

The line numbers are correct. The name is not.

What would help

1. Name the value that breaks the rule. For the + case, that is the marked parameter. For the pattern case, that is the value used two times.
2. State the rule in the message. For the + case, a sentence such as "+ needs a Data value; this type is Type" would remove the guesswork.

Version

bend 2.0.25   (installed with `bend update`)
bun 1.3.14, Darwin arm64

Related: #957, #964, #965 -- the three bugs this project works around.

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The reproducers were run by me with an AI assistant.