Support backtick strings as constants

src/Extractor.ts

@@ -1442,6 +1442,8 @@ class Extractor {
   collectConstValue(node: ts.Expression): ConstValueNode | null {
     if (ts.isStringLiteral(node)) {
       return this.gql.string(node, node.text);
+    } else if (ts.isNoSubstitutionTemplateLiteral(node)) {
+      return this.gql.string(node, node.text);
     } else if (ts.isNumericLiteral(node)) {
       return node.text.includes(".")
         ? this.gql.float(node, node.text)

src/tests/fixtures/default_values/DefaultArgumentStringLiteralBackticks.ts

@@ -0,0 +1,7 @@
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello({ greeting = `hello` }: { greeting: string }): string {
+    return `${greeting} world!`;
+  }
+}

src/tests/fixtures/default_values/DefaultArgumentStringLiteralBackticks.ts.expected

@@ -0,0 +1,43 @@
+-----------------
+INPUT
+----------------- 
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello({ greeting = `hello` }: { greeting: string }): string {
+    return `${greeting} world!`;
+  }
+}
+
+-----------------
+OUTPUT
+-----------------
+-- SDL --
+type SomeType {
+  hello(greeting: String! = "hello"): String @metadata
+}
+-- TypeScript --
+import { GraphQLSchema, GraphQLObjectType, GraphQLString, GraphQLNonNull } from "graphql";
+export function getSchema(): GraphQLSchema {
+    const SomeTypeType: GraphQLObjectType = new GraphQLObjectType({
+        name: "SomeType",
+        fields() {
+            return {
+                hello: {
+                    name: "hello",
+                    type: GraphQLString,
+                    args: {
+                        greeting: {
+                            name: "greeting",
+                            type: new GraphQLNonNull(GraphQLString),
+                            defaultValue: "hello"
+                        }
+                    }
+                }
+            };
+        }
+    });
+    return new GraphQLSchema({
+        types: [SomeTypeType]
+    });
+}

src/tests/fixtures/default_values/DefaultArgumentStringLiteralBackticksInterpolated.invalid.ts

@@ -0,0 +1,9 @@
+const CONSTANT = "constant";
+
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello({ greeting = `hello ${CONSTANT}` }: { greeting: string }): string {
+    return `${greeting} world!`;
+  }
+}

src/tests/fixtures/default_values/DefaultArgumentStringLiteralBackticksInterpolated.invalid.ts.expected

@@ -0,0 +1,22 @@
+-----------------
+INPUT
+----------------- 
+const CONSTANT = "constant";
+
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello({ greeting = `hello ${CONSTANT}` }: { greeting: string }): string {
+    return `${greeting} world!`;
+  }
+}
+
+-----------------
+OUTPUT
+-----------------
+src/tests/fixtures/default_values/DefaultArgumentStringLiteralBackticksInterpolated.invalid.ts:6:22 - error: Expected GraphQL field argument default values to be a literal. Grats interprets argument defaults as GraphQL default values, which must be literals. For example: `10` or `"foo"`.
+
+If you think Grats should be able to infer this constant value, please report an issue at https://github.com/captbaritone/grats/issues.
+
+6   hello({ greeting = `hello ${CONSTANT}` }: { greeting: string }): string {
+                       ~~~~~~~~~~~~~~~~~~~