golang/go

cmd/compile: unhelpful error message if type assertion has mismatching pointer/non-pointer type

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#43,673 opened on Jan 13, 2021

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NeedsFixcompiler/runtimehelp wanted

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Description

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

n/a

What did you do?

This code:

type FooError interface {
  Something() int
  Error() string
}

err := someCall();
err.(*FooError)

Complains:

impossible type assertion:
	*FooError does not implement error (missing Error method)

The issue is that you need to write err.(FooError).

What did you expect to see?

It'd be much more helpful if it'd call out that FooError does implement error, so the problem is the added pointer indirection.

This mistake is likely because usually the concrete struct types implement error via the pointer reference, so if you change code from the concrete struct error to an interface type, you're likely to miss a * in there.

What did you see instead?

The error message above.

Contributor guide

Research direction
Examine the type assertion error generation code in the Go compiler (likely in src/cmd/compile/internal/typecheck/typecheck.go) and modify the error message to distinguish between a type that does not implement an interface and a pointer to a type that does implement it. The error should mention that the pointer indirection is the issue when the pointer type does not implement the interface but the underlying type does.
Tech stack
go
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Go
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70