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go/types, types2: better error message for failing inference for arguments using type parameters #49800

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mpx opened this issue Nov 26, 2021 · 4 comments
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@mpx
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mpx commented Nov 26, 2021

Using compiler from tip:

go version devel +45bae64015 Thu Nov 25 04:02:39 2021 +0000 linux/amd64

Calling a generic function from another using the same type parameters causes inference to fail. The compiler points to the function being called, not the location of the call:

package main

import (
    "fmt"
    "sort"
)

func keys[V any, M ~map[string]V](m M) []string {
    out := make([]string, 0, len(m))
    for k := range m {
        out = append(out, k)
    }
    return out
}

func sortKeys[V any, M ~map[string]V](m M) []string {
    // Fails:
    ks := keys(m)
    // Works:
    //ks := keys[V, M](m)
    sort.Strings(ks)
    return ks
}

func main() {
    m := map[string]int{"foo": 1, "bar": 2}
    fmt.Println(keys(m))
}

Returns:

./keys.go:8:18: M does not match map[string]V

Ideally type checking should be able to infer the type parameters since they are identical, but I'm not sure if this is intended for Go 1.18.

Alternatively, there should to be a clearer error message pointing to the call location, not the callee.

I couldn't see a similar open bug, but this is complicated enough (for me) I might have missed it.

Cc @griesemer @findleyr

@hanchaoqun
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hanchaoqun commented Nov 26, 2021

Seems duplicate with #49575, and the fix CL is https://go-review.googlesource.com/c/go/+/362776

@mpx
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mpx commented Nov 26, 2021

Thanks, yes, appears to be a duplicate of #49575, but changing the inference rules is pushed to Go 1.19.

I think the error message should be fixed in Go 1.18 - it needs to point to the call site and should have a clearer description.

@griesemer griesemer self-assigned this Nov 26, 2021
@griesemer griesemer added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Nov 26, 2021
@griesemer griesemer added this to the Go1.18 milestone Nov 26, 2021
@griesemer griesemer changed the title go/types, types2: inference fails for arguments using type parameters go/types, types2: better error message for failing inference for arguments using type parameters Nov 26, 2021
@hanchaoqun
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Thanks, yes, appears to be a duplicate of #49575, but changing the inference rules is pushed to Go 1.19.

I think the error message should be fixed in Go 1.18 - it needs to point to the call site and should have a clearer description.

@mpx Maybe this CL https://golang.org/cl/362634 can fix the error report on the wrong line.

@gopherbot
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Change https://golang.org/cl/362634 mentions this issue: cmd/compile: fix type error reported on the wrong line

@golang golang locked and limited conversation to collaborators Jun 23, 2023
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