Preserve defaults while decoding nested structs

[Al2Klimov]

---
a: A
#b: DEFAULT # already works
c:
  d: D
  #e: DEFAULT # was not DEFAULT, but "", now works

[Al2Klimov]

The new tests are failing, but due to the implementation:

$ go test ./...
--- FAIL: TestDecoder_DefaultValues (0.00s)
    decode_test.go:1781: v.J.K should be `defaultKValue`, got ``
FAIL
FAIL    github.com/goccy/go-yaml        0.150s
?       github.com/goccy/go-yaml/ast    [no test files]
?       github.com/goccy/go-yaml/cmd/ycat       [no test files]
?       github.com/goccy/go-yaml/internal/errors        [no test files]
ok      github.com/goccy/go-yaml/lexer  (cached)
ok      github.com/goccy/go-yaml/parser (cached)
ok      github.com/goccy/go-yaml/printer        (cached)
?       github.com/goccy/go-yaml/scanner        [no test files]
ok      github.com/goccy/go-yaml/token  (cached)
FAIL

With gopkg.in/yaml.v3 ...

$ git diff -U1
diff --git a/decode_test.go b/decode_test.go
index fb13b9a..07fc717 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -6,2 +6,3 @@ import (
        "fmt"
+       yaml2 "gopkg.in/yaml.v3"
        "io"
@@ -1752,3 +1753,3 @@ w: w_value
 `
-       if err := yaml.NewDecoder(strings.NewReader(src)).Decode(&v); err != nil {
+       if err := yaml2.NewDecoder(strings.NewReader(src)).Decode(&v); err != nil {
                t.Fatalf(`parsing should succeed: %s`, err)
diff --git a/go.mod b/go.mod
index 4d99408..2248eeb 100644
--- a/go.mod
+++ b/go.mod
@@ -9,2 +9,3 @@ require (
        golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
+       gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
 )
diff --git a/go.sum b/go.sum
index 8ce2a41..8d090ed 100644
--- a/go.sum
+++ b/go.sum
@@ -41 +41,3 @@ gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b h1:h8qDotaEPuJATrMmW04NCwg7v22aHH28wwpauUhK9Oo=
+gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

... they don't fail.

[Al2Klimov]

@goccy Now it should work!

[codecov-commenter]

Codecov Report

Merging #260 (c6bcb80) into master (b478465) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
+ Coverage   76.44%   76.48%   +0.03%     
==========================================
  Files          12       12              
  Lines        4216     4222       +6     
==========================================
+ Hits         3223     3229       +6     
  Misses        759      759              
  Partials      234      234              

[htriem]

LGTM! 👍

[goccy]

@Al2Klimov
Could you please describe this fixing in this PR description with example YAML.

[Al2Klimov]

@goccy Done.

[goccy]

@Al2Klimov Thank you ! LGTM