Allow nullable variables for non-null arguments that have defaults; allow scalar variable to list types

[Shane32]

See:

• https://spec.graphql.org/June2018/#sec-Type-System.List

Sample query which would fail validation:

query ($arg: String) {
  dummy(dummyArg: $arg)
}

where dummyArg was defined as [String]. However, these scenarios worked correctly:

query {
  dummy(dummyArg: "test")
}

and

query ($arg: [String]) {
  dummy(dummyArg: $arg)
}

where variables were:

{
  "arg": "test"
}

Now the original query at the top passes validation and works as described in the GraphQL spec.

Note that the spec is slightly in conflict with the description of rule All Variable Usages Are Allowed.

See:

• [Feature request]: Support non-list variables for list arguments graphql/graphql-spec#1033
• Support non-list variables for list arguments graphql/graphql-spec#1043

This PR also fixes some out-of-spec behavior during coercion of arguments that have default values:

# this should pass validation, providing that null is not provided to the arg variable
query ($arg: String = "abc") {
  dummy (nonNullArg: $arg)
}

# also this should pass validation, providing that null is not provided to the arg variable
query ($arg: String) {
  dummy (nonNullArgWithDefaultValue: $arg)
}

# also this should pass validation, providing that null is not provided to the arg variable
query ($arg: String = "abc") {
  dummy (inputArg: { nonNullValue: $arg })
}

# also this should pass validation, providing that null is not provided to the arg variable
query ($arg: String) {
  dummy (inputArg: { nonNullValueWithDefaultValue: $arg })
}

And I added a lot more tests to cover various of these scenarios.

[Shane32]

Todo: double check we have tests that ensure that type validation disallows [Int] matching [[Int]], either as literal, or variable, or with this new feature.

[Shane32]

Todo: fix final failing test to prevent allowing null for a non-null input object field

[Shane32]

These error messages could be cleaned up.

[codecov-commenter]

Codecov Report

Merging #3662 (3960020) into master (86a3d71) will increase coverage by 0.09%.
Report is 6 commits behind head on master.
The diff coverage is 92.80%.

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@@            Coverage Diff             @@
##           master    #3662      +/-   ##
==========================================
+ Coverage   84.14%   84.24%   +0.09%     
==========================================
  Files         409      410       +1     
  Lines       18234    18523     +289     
  Branches     2856     2909      +53     
==========================================
+ Hits        15343    15604     +261     
- Misses       2215     2236      +21     
- Partials      676      683       +7     

Files	Coverage Δ
src/GraphQL/ExperimentalFeatures.cs	100.00% <100.00%> (ø)
src/GraphQL/Validation/TypeInfo.cs	97.64% <100.00%> (+0.10%)	⬆️
...aphQL/Validation/ValidationContext.GetVariables.cs	98.48% <100.00%> (+0.48%)	⬆️
src/GraphQL/Validation/ValidationContext.cs	89.73% <100.00%> (+0.53%)	⬆️
src/GraphQL/Validation/VariableUsage.cs	64.28% <100.00%> (-35.72%)	⬇️
src/GraphQL/Execution/ExecutionHelper.cs	94.11% <96.00%> (+2.65%)	⬆️
...s/5.8 - Variables/5. VariablesInAllowedPosition.cs	92.45% <92.85%> (+1.88%)	⬆️
src/GraphQL/Validation/Variable.cs	73.68% <71.42%> (-26.32%)	⬇️
src/GraphQL/Validation/Variables.cs	64.86% <77.77%> (-5.14%)	⬇️
src/GraphQL/Extensions/GraphQLExtensions.cs	76.30% <82.35%> (-0.26%)	⬇️

... and 6 files with indirect coverage changes

[Shane32]

@sungam3r This PR contains bug fixes for type validation of variables:

• When a non-null argument (or input object field) has a default value, the variable type should allow a nullable type -- except with the null value is passed as the variable.
• And, if the value is not specified (with no variable default), it should use the argument default.

It also allows passing String variables to [String] arguments, but since this technically is not part of the spec, it is locked behind an experimental feature flag. See:

• [Feature request]: Support non-list variables for list arguments graphql/graphql-spec#1033
• Support non-list variables for list arguments graphql/graphql-spec#1043

I will merge this shortly unless you have comments.