cmd/compile: compiler error for calling function, which is a field reference of a typeparam (which has a structural type) #50690
Comments
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Thanks @danscales. Why does this example work though? package main
import (
"fmt"
)
type Printer[T ~string] struct {
PrintFn func(T)
}
func Print[T ~string](s T) {
fmt.Println(s)
}
func PrintWithPrinter[T ~string, S struct{ PrintFn func(T) }](message T, obj S) {
obj.PrintFn(message)
}
func main() {
PrintWithPrinter(
"Hello, world.",
struct{ PrintFn func(string) }{PrintFn: Print[string]},
)
} |
griesemer
added
the
NeedsInvestigation
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This issue is happening because, given structural types (i.e. you can now have field references on type params), we now have to do more work to distinguish a method call on a type param (what we call a bound call, calling a method that is allow by the constraint) from a call to a function-valued field of a type param (that has a single structural type). I don't know yet why the second case works and the first does not, but we definitely have to put in more checks to distinguish the cases. |
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@danscales Thanks, makes sense. If I had to guess, the case that works only has a |
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Huh, this still works as well (Golang playground): package main
import (
"fmt"
)
type Printer[T ~string] struct {
PrintFn func(T)
}
func Print[T ~string](s T) {
fmt.Println(s)
}
func PrintWithPrinter[T ~string, S struct {
ID T
PrintFn func(T)
}](message T, obj S) {
obj.PrintFn(message)
}
func main() {
PrintWithPrinter(
"Hello, world.",
struct {
ID string
PrintFn func(string)
}{ID: "fake", PrintFn: Print[string]},
)
}I am going to try it when using a non-anon struct since that is what was used in the original example. If that does not work I will introduce a second generic type since that was also in the original use case. |
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Okay, this fails: package main
import (
"fmt"
)
type Printer[T ~string] struct {
PrintFn func(T)
}
func Print[T ~string](s T) {
fmt.Println(s)
}
func PrintWithPrinter[T ~string, S ~struct {
ID T
PrintFn func(T)
}](message T, obj S) {
obj.PrintFn(message)
}
type PrintShop[T ~string] struct {
ID T
PrintFn func(T)
}
func main() {
PrintWithPrinter(
"Hello, world.",
PrintShop[string]{
ID: "fake",
PrintFn: Print[string],
},
)
}It was the use of |
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Change https://golang.org/cl/379674 mentions this issue: |
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For reasons discussed in #51576, field accesses through type parameters will be disabled for Go 1.18. |
Given we have support for field access to type params with a single structural type, we need to distinguish between methods calls and field access when we have an OXDOT node on an expression which is a typeparam (or correspondingly a shape). We were missing checks in getInstInfo, which figures out the dictionary format, which then caused problems when we generate the dictionaries. We don't need/want dictionary entries for field access, only for bound method calls. Added a new function isBoundMethod() to distinguish OXDOT nodes which are bound calls vs. field accesses on a shape. Removed isShapeDeref() - we can't have field access or method call on a pointer to variable of type param type. Fixes golang#50690 Change-Id: Id692f65e6f427f28cd2cfe474dd30e53c71877a7 Reviewed-on: https://go-review.googlesource.com/c/go/+/379674 Trust: Dan Scales <danscales@google.com> Reviewed-by: Keith Randall <khr@golang.org>
New issue from @akutz for Go 1.18 tip:
This example fails to run with the following error (Golang playground):
The above example compiles as long as I do not call
l.SumFn, even though it is passed into the struct (Golang playground):Originally posted by @akutz in #50417 (comment)
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