chore(client): fix multipart serialisation of Default() fields

Author: stainless-app[bot]

internal/apiform/encoder.go

@@ -265,6 +265,14 @@ func (e *encoder) newStructTypeEncoder(t reflect.Type) encoderFunc {
 					}
 					return typeEncoderFn(key, value, writer)
 				}
+			} else if ptag.defaultValue != nil {
+				typeEncoderFn := e.typeEncoder(field.Type)
+				encoderFn = func(key string, value reflect.Value, writer *multipart.Writer) error {
+					if value.IsZero() {
+						return typeEncoderFn(key, reflect.ValueOf(ptag.defaultValue), writer)
+					}
+					return typeEncoderFn(key, value, writer)
+				}
 			} else {
 				encoderFn = e.typeEncoder(field.Type)
 			}

internal/apiform/form_test.go

@@ -123,6 +123,11 @@ type StructUnion struct {
 	param.APIUnion
 }
 
+type ConstantStruct struct {
+	Anchor  string `form:"anchor" default:"created_at"`
+	Seconds int    `form:"seconds"`
+}
+
 type MultipartMarshalerParent struct {
 	Middle MultipartMarshalerMiddleNext `form:"middle"`
 }
@@ -554,6 +559,37 @@ Content-Disposition: form-data; name="union"
 			Union: UnionTime(time.Date(2010, 05, 23, 0, 0, 0, 0, time.UTC)),
 		},
 	},
+	"constant_zero_value": {
+		`--xxx
+Content-Disposition: form-data; name="anchor"
+
+created_at
+--xxx
+Content-Disposition: form-data; name="seconds"
+
+3600
+--xxx--
+`,
+		ConstantStruct{
+			Seconds: 3600,
+		},
+	},
+	"constant_explicit_value": {
+		`--xxx
+Content-Disposition: form-data; name="anchor"
+
+created_at_override
+--xxx
+Content-Disposition: form-data; name="seconds"
+
+3600
+--xxx--
+`,
+		ConstantStruct{
+			Anchor:  "created_at_override",
+			Seconds: 3600,
+		},
+	},
 	"deeply-nested-struct,brackets": {
 		`--xxx
 Content-Disposition: form-data; name="middle[middleNext][child]"