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cmd/compile: impossible type assertion reports wrong signature for methods that don't match in case #50816

dominikh opened this issue Jan 26, 2022 · 2 comments


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@dominikh dominikh commented Jan 26, 2022

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

$ go version
go version devel go1.18-6eb58cdffa Tue Jan 25 22:56:45 2022 +0000 linux/amd64

Does this issue reproduce with the latest release?


What did you do?

What did you expect to see?

./foo.go:17:7: impossible type assertion:
	T1 does not implement I (missing Foo method)
		have foo()
		want Foo()
./foo.go:18:7: impossible type assertion:
	T2 does not implement I (missing Foo method)
		have foo() string
		want Foo()

What did you see instead?

./foo.go:17:7: impossible type assertion: i.(T1)
	T1 does not implement I (missing Foo method)
		have foo() at ./foo.go:9:11
		want Foo()
./foo.go:18:7: impossible type assertion: i.(T2)
	T2 does not implement I (wrong type for Foo method)
		have foo() string at ./foo.go:13:11
		want Foo()

The compiler complains about the wrong signature even though the method names don't match, which I think is the more important error to report. Bisects to 429d1e0.

/cc @danscales

@ianlancetaylor ianlancetaylor added the NeedsInvestigation label Jan 26, 2022
@ianlancetaylor ianlancetaylor added this to the Go1.18 milestone Jan 26, 2022
@danscales danscales self-assigned this Jan 26, 2022
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@danscales danscales commented Jan 26, 2022

Thanks for the example. The error message should be fixed in this case, which will be like what it is in Go 1.17. I have a simple fix, but it may wait until after beta-2.

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@gopherbot gopherbot commented Jan 28, 2022

Change mentions this issue: cmd/compile: tweak missingMethodReason logic to improve message

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