Comments (2)
In working on this, I attempted to always promote to an interface{}
when retrieving value. Thus,
Line 87 in 5411ab9
would become something like:
iface := reflect.TypeOf((*interface{})(nil)).Elem()
s.compareAny(reflect.ValueOf(x).Convert(iface), reflect.ValueOf(y).Convert(iface))
However, this does not work because the cmp
package uses assignability to determine applicability of filters and options and interface{}
is not assignable to error
even if the underlying concrete type may be assignable to error
.
My next thought was to dynamically create an interface type T that had the common set of methods for both the x and y values. If the underlying types of x and y are each individually assignable to some interface R, then it is guaranteed that T is also assignable to R. However, this approach requires dynamically creating an interface, which is functionality that reflect
does not currently have. I commented on golang/go#4146 (comment) with this use-case.
On that thread, @Merovius commented:
I might misread this, but the documentation says that
The comparer f must be a function "func(T, T) bool" and is implicitly filtered to input values assignable to T. If T is an interface, it is possible that f is called with two values of different concrete types that both implement T.
That would seem to cover the case you mention? In particular this works.
You are correct that the Comparer
itself allows for "two values of different concrete types that both implement T", but this is not property assumed by the rest of the cmp
. For example, the first rule on cmp.Equal
is:
If two values are not of the same type, then they are never equal and the overall result is false.
This rule prevents Equal
from even attempting to use filters or comparers. Note that we cannot relax this restriction as this assumption is heavily depended upon.
from go-cmp.
Fixed by #112.
from go-cmp.
Related Issues (20)
- Diff is extremely slow HOT 3
- How to ignore specific field by name? HOT 1
- Ideas for a v2 HOT 3
- Default support for the `netip` package HOT 1
- New minor release of the library? HOT 4
- cmp.Diff() switching from property diffs to "new object" diff when object changes significantly HOT 1
- Inconsistent whitespace produced by cmp.Diff HOT 2
- cmpopts: add EquateUnsetFields(want) or FilterFieldsFrom(want) HOT 1
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- Enable a dependency update tool HOT 2
- Package Name Mismatches HOT 4
- unhelpful output for difference in slices HOT 1
- How to implement jests.js's `expect.any(String)` like functionality? HOT 1
- cmpopts.IgnoreFields does not work on structs with an Equal method HOT 1
- Performance Issue Comparing Array Types HOT 1
- Naming clash for new cmp standard package HOT 4
- Comparing two fields with diffirent types that are semantically equivalent HOT 3
- String diff for UUIDs can use strings.Join syntax which makes it more confusing HOT 4
- Reporter truncating important context on diff output but leaving others alone
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from go-cmp.