[bejaratommy]

Problem

keys() is declared as func(dicts ...map[string]interface{}) []string. Go's text/template does static argument type-checking against the exact declared parameter type before calling a function, so passing any map whose value type isn't precisely interface{} (e.g. map[string]string, map[string]*MyStruct) panics before keys is ever invoked:

panic: template: example:4:28: executing "example" at <keys>: wrong type for value; expected map[string]interface {}; got map[string]string

This makes keys unusable on any dict-like data that doesn't happen to already be map[string]interface{}, e.g. maps produced by strongly-typed Go code passed into a template.

Reported in #455, hit independently by two people with different concrete map types (map[string]string and map[string]*T).

Fix

• dict.go: change keys to func keys(dicts ...interface{}) ([]string, error). Internally use reflect to confirm each argument is a map (dereferencing one level of pointer) and that its keys are strings, then collect them. Returns an error for non-map arguments or maps with non-string keys instead of panicking.
• dict_test.go: added TestKeysNonInterfaceMaps (covers map[string]string and map[string]int) and TestKeysNonMapErrors (non-map argument now returns an error rather than panicking). Existing TestKeys (covering the original map[string]interface{} behavior) still passes unmodified.

Testing

go build ./...
go vet ./...
go test ./... -run Keys -v   # PASS
go test ./...                # PASS (one unrelated pre-existing failure in
                              # TestRound reproduces identically on unmodified
                              # master — a floating point rounding edge case
                              # unrelated to this change)

Fixes #455