checker error when a lambda expression is used as a parameter to a generic function that accepts a fn callback

[spytheman]

V doctor:

V full version: V 0.4.8 4ee948f.e598db1
OS: linux, Ubuntu 20.04.6 LTS
Processor: 2 cpus, 64bit, little endian, Intel(R) Core(TM) i3-3225 CPU @ 3.30GHz

getwd: /space/v/oo
vexe: /space/v/oo/v
vexe mtime: 2024-10-01 13:24:57

vroot: OK, value: /space/v/oo
VMODULES: OK, value: /home/delian/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.46.0
Git vroot status: 0.4.8-14-ge598db12-dirty (4 commit(s) behind V master)
.git/config present: true

CC version: cc (Ubuntu 10.5.0-1ubuntu1~20.04) 10.5.0
thirdparty/tcc status: thirdparty-linux-amd64 0134e9b9

What did you do?
./v -g -o vdbg cmd/v && ./vdbg generic_fn_called_with_lambda.v

import arrays

items := ['item1', 'item2', 'item3']
list := arrays.map_indexed[string, string](items, |i, item| '${i}. ${item}')
println(list)

What did you expect to see?

a compiled program, producing ['0. item1', '1. item2', '2. item3']

What did you see instead?

generic_fn_called_with_lambda.v:4:70: error: no known default format for type `T`
    2 | 
    3 | items := ['item1', 'item2', 'item3']
    4 | list := arrays.map_indexed[string, string](items, |i, item| '${i}. ${item}')
      |                                                                      ~~~~
    5 | println(list)
generic_fn_called_with_lambda.v:4:60: error: cannot use `string` as type `R` in return argument
    2 | 
    3 | items := ['item1', 'item2', 'item3']
    4 | list := arrays.map_indexed[string, string](items, |i, item| '${i}. ${item}')
      |                                                            ~~~~~~~~~~~~~~~~
    5 | println(list)

Note: the full version works:

import arrays
items := ['item1', 'item2', 'item3']
list := arrays.map_indexed[string, string](items, fn(i int, item string) string { return '${i}. ${item}' })
println(list)

Probably related: #22109 .

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