Updated based on feedback.

This updates this proposal to be a bit broader in scope however much narrower in breaking behavior changes.

Mirroring the changes in graphql/graphql-js#1274, this update better defines the difference between a "required" and "non-null" argument / input field as a non-null typed argument / input-field with a default value is no longer required. As such the validation rule which prohibited queries from using non-null variables and default values has been removed. This also adds clarity to the input field validation - this rule has existed in the GraphQL.js reference implementation however was found missing within the spec.

This also updates the CoerceVariableValues() and CoerceArgumentValues() algorithms to retain explicit null values overriding a default value (minimizing breaking changes), however critically adding additional protection to CoerceArgumentValues() to explicitly block null values from variables - thus allowing the older pattern of passing a nullable variable into a non-null argument while limiting the problematic case of an explicit null value at runtime.

spec/Section 5 -- Validation.md

@@ -710,25 +710,26 @@ and invalid.
   * {arguments} must be the set containing only {argument}.
 
 
-#### Required Non-Null Arguments
+#### Required Arguments
 
   * For each Field or Directive in the document.
   * Let {arguments} be the arguments provided by the Field or Directive.
   * Let {argumentDefinitions} be the set of argument definitions of that Field or Directive.
-  * For each {definition} in {argumentDefinitions}:
-    * Let {type} be the expected type of {definition}.
-    * If {type} is Non-Null:
-      * Let {argumentName} be the name of {definition}.
+  * For each {argumentDefinition} in {argumentDefinitions}:
+    * Let {type} be the expected type of {argumentDefinition}.
+    * Let {defaultValue} be the default value of {argumentDefinition}.
+    * If {type} is Non-Null and {defaultValue} does not exist:
+      * Let {argumentName} be the name of {argumentDefinition}.
       * Let {argument} be the argument in {arguments} named {argumentName}
       * {argument} must exist.
       * Let {value} be the value of {argument}.
       * {value} must not be the {null} literal.
 
 **Explanatory Text**
 
-Arguments can be required. If the argument type is non-null the argument is
-required and furthermore the explicit value {null} may not be provided.
-Otherwise, the argument is optional.
+Arguments can be required. If the argument type is non-null and does not have a
+default value, the argument is required and furthermore the explicit value
+{null} may not be provided. Otherwise, the argument is optional.
 
 For example the following are valid:
 
@@ -738,7 +739,7 @@ fragment goodBooleanArg on Arguments {
 }
 
 fragment goodNonNullArg on Arguments {
-  nonNullBooleanArgField(nonNullBooleanArg: true)
+  requiredBooleanArgField(requiredBooleanArg: true)
 }
 ```
 
@@ -752,19 +753,19 @@ fragment goodBooleanArgDefault on Arguments {
 }
 ```
 
-but this is not valid on a non-null argument.
+but this is not valid on a required argument.
 
 ```graphql counter-example
 fragment missingRequiredArg on Arguments {
-  nonNullBooleanArgField
+  requiredBooleanArgField
 }
 ```
 
 Providing the explicit value {null} is also not valid.
 
 ```graphql counter-example
 fragment missingRequiredArg on Arguments {
-  notNullBooleanArgField(nonNullBooleanArg: null)
+  requiredBooleanArgField(requiredBooleanArg: null)
 }
 ```
 
@@ -1358,6 +1359,31 @@ For example the following query will not pass validation.
 ```
 
 
+### Input Object Required Fields
+
+**Formal Specification**
+
+  * For each Input Object in the document.
+    * Let {fields} be the fields provided by that Input Object.
+    * Let {fieldDefinitions} be the set of input field definitions of that Input Object.
+  * For each {fieldDefinition} in {fieldDefinitions}:
+    * Let {type} be the expected type of {fieldDefinition}.
+    * Let {defaultValue} be the default value of {fieldDefinition}.
+    * If {type} is Non-Null and {defaultValue} does not exist:
+      * Let {fieldName} be the name of {fieldDefinition}.
+      * Let {field} be the input field in {fields} named {fieldName}
+      * {field} must exist.
+      * Let {value} be the value of {field}.
+      * {value} must not be the {null} literal.
+
+**Explanatory Text**
+
+Input object fields can be required. If the input object field type is non-null
+and does not have a default value, the input object field is required and
+furthermore the explicit value {null} may not be provided. Otherwise, the input
+object field is optional.
+
+
 ## Directives
 
 
@@ -1494,44 +1520,6 @@ fragment HouseTrainedFragment {
 ```
 
 
-### Variable Default Value Is Allowed
-
-**Formal Specification**
-
-  * For every Variable Definition {variableDefinition} in a document
-    * Let {variableType} be the type of {variableDefinition}
-    * Let {defaultValue} be the default value of {variableDefinition}
-    * If {variableType} is Non-null:
-      * {defaultValue} must be undefined.
-
-**Explanatory Text**
-
-Variables defined by operations are allowed to define default values
-if the type of that variable is not non-null.
-
-For example the following query will pass validation.
-
-```graphql example
-query houseTrainedQuery($atOtherHomes: Boolean = true) {
-  dog {
-    isHousetrained(atOtherHomes: $atOtherHomes)
-  }
-}
-```
-
-However if the variable is defined as non-null, default values
-are unreachable. Therefore queries such as the following fail
-validation
-
-```graphql counter-example
-query houseTrainedQuery($atOtherHomes: Boolean! = true) {
-  dog {
-    isHousetrained(atOtherHomes: $atOtherHomes)
-  }
-}
-```
-
-
 ### Variables Are Input Types
 
 **Formal Specification**
@@ -1906,8 +1894,12 @@ query booleanArgQuery($booleanArg: Boolean) {
 }
 ```
 
-A notable exception is when a default value are provided. They are, in effect,
-treated as non-nulls as long as the default value is not {null}.
+A notable exception is when a default value is provided. Variables which include
+a default value may be provided to a non-null argument as long as the default
+value is not {null}.
+
+Note: The value {null} could still be provided to a variable at runtime, and
+a non-null argument must produce a field error if provided such a variable.
 
 ```graphql example
 query booleanArgQueryWithDefault($booleanArg: Boolean = true) {

spec/Section 6 -- Execution.md

@@ -88,21 +88,22 @@ CoerceVariableValues(schema, operation, variableValues):
       name {variableName}.
     * Let {value} be the value provided in {variableValues} for the
       name {variableName}.
-    * If {hasValue} is not {true} or if {value} is {null}:
-      * If {variableType} is a Non-Nullable type, throw a query error.
-      * Otherwise if {defaultValue} exists (including {null}):
-        * Add an entry to {coercedValues} named {variableName} with the
-          value {defaultValue}.
-      * Otherwise if {hasValue} is {true} and {value} is {null}:
+    * If {hasValue} is not {true} and {defaultValue} exists (including {null}):
+      * Add an entry to {coercedValues} named {variableName} with the
+        value {defaultValue}.
+    * Otherwise if {variableType} is a Non-Nullable type, and either {hasValue}
+      is not {true} or {value} is {null}, throw a query error.
+    * Otherwise if {hasValue} is true:
+      * If {value} is {null}:
         * Add an entry to {coercedValues} named {variableName} with the
           value {null}.
-    * Otherwise, {value} must not be {null}:
-      * If {value} cannot be coerced according to the input coercion
-        rules of {variableType}, throw a query error.
-      * Let {coercedValue} be the result of coercing {value} according to the
-        input coercion rules of {variableType}.
-      * Add an entry to {coercedValues} named {variableName} with the
-        value {coercedValue}.
+      * Otherwise:
+        * If {value} cannot be coerced according to the input coercion
+          rules of {variableType}, throw a query error.
+        * Let {coercedValue} be the result of coercing {value} according to the
+          input coercion rules of {variableType}.
+        * Add an entry to {coercedValues} named {variableName} with the
+          value {coercedValue}.
   * Return {coercedValues}.
 
 Note: This algorithm is very similar to {CoerceArgumentValues()}.
@@ -552,34 +553,35 @@ CoerceArgumentValues(objectType, field, variableValues):
     * Let {defaultValue} be the default value for {argumentDefinition}.
     * Let {hasValue} be {true} if {argumentValues} provides a value for the
       name {argumentName}.
-    * Let {value} be the value provided in {argumentValues} for the
+    * Let {argumentValue} be the value provided in {argumentValues} for the
       name {argumentName}.
-    * If {hasValue} is not {true} or if {value} is {null}:
-      * If {argumentType} is a Non-Nullable type, throw a field error.
-      * Otherwise, if {defaultValue} exists (including {null}):
-        * Add an entry to {coercedValues} named {argumentName} with the
-          value {defaultValue}.
-      * Otherwise, if {hasValue} is {true} and {value} is {null}:
+    * If {argumentValue} is a {Variable}:
+      * Let {variableName} be the name of {argumentValue}.
+      * Let {hasValue} be {true} if {variableValues} provides a value for the
+        name {variableName}.
+      * Let {value} be the value provided in {variableValues} for the
+        name {variableName}.
+    * Otherwise:
+      * Let {value} be {argumentValue}.
+    * If {hasValue} is not {true} and {defaultValue} exists (including {null}):
+      * Add an entry to {coercedValues} named {argumentName} with the
+        value {defaultValue}.
+    * Otherwise if {argumentType} is a Non-Nullable type, and either {hasValue}
+      is not {true} or {value} is {null}, throw a field error.
+    * Otherwise if {hasValue} is true:
+      * If {value} is {null}:
         * Add an entry to {coercedValues} named {argumentName} with the
           value {null}.
-    * Otherwise, if {value} is a {Variable}:
-      * Let {variableName} be the name of {value}.
-      * If {variableValues} provides a value for the name {variableName}:
-        * Let {variableValue} be the value provided in {variableValues} for the
-          name {variableName}.
+      * Otherwise, if {argumentValue} is a {Variable}:
         * Add an entry to {coercedValues} named {argumentName} with the
-          value {variableValue}.
-      * Otherwise, if {argumentType} is a Non-Nullable type, throw a field error.
-      * Otherwise, if {defaultValue} exists (including {null}):
+          value {value}.
+      * Otherwise:
+        * If {value} cannot be coerced according to the input coercion
+            rules of {variableType}, throw a field error.
+        * Let {coercedValue} be the result of coercing {value} according to the
+          input coercion rules of {variableType}.
         * Add an entry to {coercedValues} named {argumentName} with the
-          value {defaultValue}.
-    * Otherwise, {value} must be a {Value} and not {null}:
-      * If {value} cannot be coerced according to the input coercion
-        rules of {argType}, throw a field error.
-      * Let {coercedValue} be the result of coercing {value} according to the
-        input coercion rules of {argType}.
-      * Add an entry to {coercedValues} named {argumentName} with the
-        value {coercedValue}.
+          value {coercedValue}.
   * Return {coercedValues}.
 
 Note: Variable values are not coerced because they are expected to be coerced