About validating default: {} for type: object

[Huang-Wei]

I noticed with the introduction of #610, the following schema will fail (but it was not before #610):

openapi: 3.0.0
info:
  title: "Hello World REST APIs"
  version: "1.0"
paths: {}
components:
  schemas:
    SomeSchema:
      type: object
      default: {}
      required:
      - field
      properties:
        field:
          type: string
          default: `foo`

According to my limited knowledge on OpenAPI v3, this yaml is valid, so could you help confirm if it's a regression?

BTW: removing default: {} can make kin validator happy, but I don't think that's mandatory in OpenAPI spec's POV.

[Huang-Wei]

Unit test to reproduce this issue using the latest master code:

func TestIssue673(t *testing.T) {
	specf := func(dflt string) string {
		return `
openapi: 3.0.0
info:
  title: "Hello World REST APIs"
  version: "1.0"
paths: {}
components:
  schemas:
    SomeSchema:
      type: object
      default: {}
      required:
      - field
      properties:
        field:
          type: string
          default: ` + dflt + `
`
	}

	for _, testcase := range []struct {
		dflt, err string
	}{
		{
			dflt: `"foo"`,
			err:  "",
		},
		{
			dflt: `""`,
			err:  "",
		},
		{
			dflt: `1`,
			err:  "invalid components: invalid schema default: Field must be set to string or not be present",
		},
	} {
		t.Run(testcase.dflt, func(t *testing.T) {
			spec := specf(testcase.dflt)

			sl := NewLoader()

			doc, err := sl.LoadFromData([]byte(spec))
			require.NoError(t, err)

			err = doc.Validate(sl.Context)
			if testcase.err == "" {
				require.NoError(t, err)
			} else {
				require.Error(t, err, testcase.err)
			}
		})
	}
}

Output:

--- FAIL: TestIssue673 (0.00s)
    --- FAIL: TestIssue673/"foo" (0.00s)
        issue136_test.go:107: 
            	Error Trace:	/Users/weih/go/src/github.com/Huang-Wei/kin-openapi/openapi3/issue136_test.go:107
            	Error:      	Received unexpected error:
            	            	invalid components: schema "SomeSchema": invalid default: Error at "/field": property "field" is missing
            	            	Schema:
            	            	  {
            	            	    "default": {},
            	            	    "properties": {
            	            	      "field": {
            	            	        "default": "foo",
            	            	        "type": "string"
            	            	      }
            	            	    },
            	            	    "required": [
            	            	      "field"
            	            	    ],
            	            	    "type": "object"
            	            	  }
            	            	
            	            	Value:
            	            	  {}
            	Test:       	TestIssue673/"foo"
    --- FAIL: TestIssue673/"" (0.00s)
        issue136_test.go:107: 
            	Error Trace:	/Users/weih/go/src/github.com/Huang-Wei/kin-openapi/openapi3/issue136_test.go:107
            	Error:      	Received unexpected error:
            	            	invalid components: schema "SomeSchema": invalid default: Error at "/field": property "field" is missing
            	            	Schema:
            	            	  {
            	            	    "default": {},
            	            	    "properties": {
            	            	      "field": {
            	            	        "default": "",
            	            	        "type": "string"
            	            	      }
            	            	    },
            	            	    "required": [
            	            	      "field"
            	            	    ],
            	            	    "type": "object"
            	            	  }
            	            	
            	            	Value:
            	            	  {}
            	Test:       	TestIssue673/""
FAIL
FAIL	github.com/getkin/kin-openapi/openapi3	0.125s
FAIL