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reflect: MakeFunc can't handle return values which are the zero Value #28748

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randall77 opened this issue Nov 12, 2018 · 2 comments
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reflect: MakeFunc can't handle return values which are the zero Value #28748

randall77 opened this issue Nov 12, 2018 · 2 comments
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Go1.13

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@randall77
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package main

import "reflect"

func main() {
	r := reflect.MakeFunc(reflect.TypeOf(func() error { return nil }),
		func(args []reflect.Value) []reflect.Value {
			var x [1]reflect.Value
			return x[:]
		}).Interface().(func() error)
	r()
}

This code will cause the r() call to panic with a nil pointer dereference. We should really panic with something sensible instead.

The code is trying to provide a sensible panic, but panics when trying to build that panic message. It gets to here:

			if v.typ != typ {
				panic("reflect: function created by MakeFunc using " + funcName(f) +
					" returned wrong type: have " +
					out[i].typ.String() + " for " + typ.String())
			}

But v.typ is nil, so out[i].typ.String() (v and out[i] are the same) panics inside (*rtype).String.

This has been the case since at least go 1.2. Marking for 1.13.
@randall77 randall77 added this to the Go1.13 milestone Nov 12, 2018
@randall77
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See also #6871.

@gopherbot
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Change https://golang.org/cl/175440 mentions this issue: reflect: special-case panic message when returning Value{} from MakeFunc function

@golang golang locked and limited conversation to collaborators May 5, 2020
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