Fix variable validation for type as table

Before this patch, describing argument type in schema with a table
resulted in "unsupported Lua type 'function'" error on variable
type validation.

Based on PR #22 by @no1seman

Author: DifferentialOrange

graphql/rules.lua

@@ -517,6 +517,13 @@ local function isVariableTypesValid(argument, argumentType, context,
       variableType = types.nonNull(variableType)
     end
 
+    -- This line of code provides support to using
+    -- `arg = { kind = type, description = desc }`
+    -- type declaration in query input arguments
+    -- instead of `arg = type` one when passing
+    -- argument with a variable.
+    if argumentType.kind ~= nil then argumentType = argumentType.kind end
+
     if not isTypeSubTypeOf(variableType, argumentType, context) then
       return false, ('Variable "%s" type mismatch: the variable type "%s" ' ..
         'is not compatible with the argument type "%s"'):format(variableName,

test/integration/graphql_test.lua

@@ -1613,3 +1613,29 @@ function g.test_schema_input_arg_described_with_kind()
     local _, errors = check_request(query, query_schema, {})
     t.assert_equals(errors, nil)
 end
+
+function g.test_schema_input_arg_described_with_kind_variable_pass()
+    local function callback(_, args)
+        return args[1].value
+    end
+
+    local query_schema = {
+        ['test'] = {
+            kind = types.string.nonNull,
+            arguments = {
+                arg = {
+                    kind = types.string.nonNull,
+                },
+            },
+            resolve = callback,
+        }
+    }
+
+    local query = [[
+        query ($arg: String!) { test(arg: $arg) }
+    ]]
+    local variables = { arg = 'B' }
+
+    local _, errors = check_request(query, query_schema, nil, nil, { variables = variables })
+    t.assert_equals(errors, nil)
+end