Generalize validation of value literals

This PR is an editorial change to validation rules which does not affect which documents are valid or invalid, however seeks to generalize the language for validating input value literals and in doing so both de-duplicate the rules (previously duplicated between argument validation and default value validation) and ensure the validation of value literals applies to any new location values may be found in the future (such as the SDL).

This change to the spec mirrors a similar change to the reference implementation graphql/graphql-js#1144

spec/Section 5 -- Validation.md

@@ -649,47 +649,6 @@ and invalid.
   * {arguments} must be the set containing only {argument}.
 
 
-### Argument Values Type Correctness
-
-#### Compatible Values
-
-**Formal Specification**
-
-  * For each {argument} in the document
-  * Let {value} be the Value of {argument}
-  * If {value} is not a Variable
-    * Let {argumentName} be the Name of {argument}.
-    * Let {argumentDefinition} be the argument definition provided by the parent field or definition named {argumentName}.
-    * Let {type} be the type expected by {argumentDefinition}.
-    * The type of {literalArgument} must be coercible to {type}.
-
-**Explanatory Text**
-
-Literal values must be compatible with the type defined by the argument they are
-being provided to, as per the coercion rules defined in the Type System chapter.
-
-For example, an Int can be coerced into a Float.
-
-```graphql example
-fragment goodBooleanArg on Arguments {
-  booleanArgField(booleanArg: true)
-}
-
-fragment coercedIntIntoFloatArg on Arguments {
-  floatArgField(floatArg: 1)
-}
-```
-
-An incoercible conversion, is string to int. Therefore, the
-following example is invalid.
-
-```graphql counter-example
-fragment stringIntoInt on Arguments {
-  intArgField(intArg: "3")
-}
-```
-
-
 #### Required Non-Null Arguments
 
   * For each Field or Directive in the document.
@@ -1233,6 +1192,87 @@ and {Sentient}.
 ## Values
 
 
+### Values of Correct Type
+
+**Format Specification**
+
+  * For each input Value {value} in the document.
+    * Let {type} be the type expected in the position {value} is found.
+    * {value} must be coercible to {type}.
+
+**Explanatory Text**
+
+Literal values must be compatible with the type expected in the position they
+are found as per the coercion rules defined in the Type System chapter.
+
+The type expected in a position include the type defined by the argument a value
+is provided for, the type defined by an input object field a value is provided
+for, and the type of a variable definition a default value is provided for.
+
+The following examples are valid use of value literals:
+
+```graphql example
+fragment goodBooleanArg on Arguments {
+  booleanArgField(booleanArg: true)
+}
+
+fragment coercedIntIntoFloatArg on Arguments {
+  # Note: The input coercion rules for Float allow Int literals.
+  floatArgField(floatArg: 123)
+}
+
+query goodComplexDefaultValue($search: ComplexInput = { name: "Fido" }) {
+  findDog(complex: $search)
+}
+```
+
+Non-coercible values (such as a String into an Int) are invalid. The
+following examples are invalid:
+
+```graphql counter-example
+fragment stringIntoInt on Arguments {
+  intArgField(intArg: "123")
+}
+
+query badComplexValue {
+  findDog(complex: { name: 123 })
+}
+```
+
+
+### Input Object Field Names
+
+**Formal Specification**
+
+  * For each Input Object Field {inputField} in the document
+  * Let {inputFieldName} be the Name of {inputField}.
+  * Let {inputFieldDefinition} be the input field definition provided by the
+    parent input object type named {inputFieldName}.
+  * {inputFieldDefinition} must exist.
+
+**Explanatory Text**
+
+Every input field provided in an input object value must be defined in the set
+of possible fields of that input object's expected type.
+
+For example the following example input object is valid:
+
+```graphql example
+{
+  findDog(complex: { name: "Fido" })
+}
+```
+
+While the following example input-object uses a field "favoriteCookieFlavor"
+which is not defined on the expected type:
+
+```graphql counter-example
+{
+  findDog(complex: { favoriteCookieFlavor: "Bacon" })
+}
+```
+
+
 ### Input Object Field Uniqueness
 
 **Formal Specification**
@@ -1393,16 +1433,15 @@ fragment HouseTrainedFragment {
 ```
 
 
-### Variable Default Values Are Correctly Typed
+### Variable Default Value Is Allowed
 
 **Formal Specification**
 
-  * For every {operation} in a document
-  * For every {variable} on each {operation}
-    * Let {variableType} be the type of {variable}
-    * If {variableType} is non-null it cannot have a default value
-    * If {variable} has a default value it must be of the same type
-      or able to be coerced to {variableType}
+  * 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**
 
@@ -1431,31 +1470,6 @@ query houseTrainedQuery($atOtherHomes: Boolean! = true) {
 }
 ```
 
-Default values must be compatible with the types of variables.
-Types must match or they must be coercible to the type.
-
-Non-matching types fail, such as in the following example:
-
-```graphql counter-example
-query houseTrainedQuery($atOtherHomes: Boolean = "true") {
-  dog {
-    isHousetrained(atOtherHomes: $atOtherHomes)
-  }
-}
-```
-
-However if a type is coercible the query will pass validation.
-
-For example:
-
-```graphql example
-query intToFloatQuery($floatVar: Float = 1) {
-  arguments {
-    floatArgField(floatArg: $floatVar)
-  }
-}
-```
-
 
 ### Variables Are Input Types