fix(validator): allow nullable variables for nonnull args with default (#396)

lbarthon · web-flow · commit 5b4126931203 · 2025-10-29T21:00:16.000-04:00
The validation was failing for arguments that are defined as required, but have
a default value, preventing a non-required argument to override their value,
while it should work as expected.

Given the tests are imported from JS, I'm not sure how to add a new one there.
Here's an example schema and query to reproduce this:
```
type Query {
  user(username: String! = "foo"): User
}

type User {
  username: String!
}
```

```
query Foo($username: String) {
  user(username: $username) {
    username
  }
}
```
diff --git a/ast/value.go b/ast/value.go
@@ -29,9 +29,10 @@ type Value struct {
 	Comment  *CommentGroup
 
 	// Require validation
-	Definition         *Definition
-	VariableDefinition *VariableDefinition
-	ExpectedType       *Type
+	Definition             *Definition
+	VariableDefinition     *VariableDefinition
+	ExpectedType           *Type
+	ExpectedTypeHasDefault bool
 }
 
 type ChildValue struct {
diff --git a/validator/core/walk.go b/validator/core/walk.go
@@ -182,6 +182,7 @@ func (w *Walker) walkValue(value *ast.Value) {
 				fieldDef := value.Definition.Fields.ForName(child.Name)
 				if fieldDef != nil {
 					child.Value.ExpectedType = fieldDef.Type
+					child.Value.ExpectedTypeHasDefault = fieldDef.DefaultValue != nil && fieldDef.DefaultValue.Kind != ast.NullValue
 					child.Value.Definition = w.Schema.Types[fieldDef.Type.Name()]
 				}
 			}
@@ -208,6 +209,7 @@ func (w *Walker) walkValue(value *ast.Value) {
 func (w *Walker) walkArgument(argDef *ast.ArgumentDefinition, arg *ast.Argument) {
 	if argDef != nil {
 		arg.Value.ExpectedType = argDef.Type
+		arg.Value.ExpectedTypeHasDefault = argDef.DefaultValue != nil && argDef.DefaultValue.Kind != ast.NullValue
 		arg.Value.Definition = w.Schema.Types[argDef.Type.Name()]
 	}
 
diff --git a/validator/rules/variables_in_allowed_position.go b/validator/rules/variables_in_allowed_position.go
@@ -25,6 +25,11 @@ var VariablesInAllowedPositionRule = Rule{
 				}
 			}
 
+			// If the expected type has a default, the given variable can be null
+			if value.ExpectedTypeHasDefault {
+				tmp.NonNull = false
+			}
+
 			if !value.VariableDefinition.Type.IsCompatible(&tmp) {
 				addError(
 					Message(

Original file line number	Diff line number	Diff line change
`@@ -182,6 +182,7 @@ func (w Walker) walkValue(value ast.Value) {`
`182`	`182`	`fieldDef := value.Definition.Fields.ForName(child.Name)`
`183`	`183`	`if fieldDef != nil {`
`184`	`184`	`child.Value.ExpectedType = fieldDef.Type`
	`185`	`+ child.Value.ExpectedTypeHasDefault = fieldDef.DefaultValue != nil && fieldDef.DefaultValue.Kind != ast.NullValue`
`185`	`186`	`child.Value.Definition = w.Schema.Types[fieldDef.Type.Name()]`
`186`	`187`	`}`
`187`	`188`	`}`
`@@ -208,6 +209,7 @@ func (w Walker) walkValue(value ast.Value) {`
`208`	`209`	`func (w Walker) walkArgument(argDef ast.ArgumentDefinition, arg *ast.Argument) {`
`209`	`210`	`if argDef != nil {`
`210`	`211`	`arg.Value.ExpectedType = argDef.Type`
	`212`	`+ arg.Value.ExpectedTypeHasDefault = argDef.DefaultValue != nil && argDef.DefaultValue.Kind != ast.NullValue`
`211`	`213`	`arg.Value.Definition = w.Schema.Types[argDef.Type.Name()]`
`212`	`214`	`}`
`213`	`215`
Original file line number	Diff line number	Diff line change
`@@ -25,6 +25,11 @@ var VariablesInAllowedPositionRule = Rule{`
`25`	`25`	`}`
`26`	`26`	`}`
`27`	`27`
	`28`	`+ // If the expected type has a default, the given variable can be null`
	`29`	`+ if value.ExpectedTypeHasDefault {`
	`30`	`+ tmp.NonNull = false`
	`31`	`+ }`
	`32`	`+`
`28`	`33`	`if !value.VariableDefinition.Type.IsCompatible(&tmp) {`
`29`	`34`	`addError(`
`30`	`35`	`Message(`