fix(eslint-plugin): restrict-plus-operands: generic constraint support (#440)

Author: bradzacher

Diff for: packages/eslint-plugin/src/rules/restrict-plus-operands.ts

@@ -9,7 +9,7 @@ export default util.createRule({
     docs: {
       description:
         'When adding two variables, operands must both be of type number or of type string.',
-      tslintRuleName: 'restrict-plus-operands',
+      tslintName: 'restrict-plus-operands',
       category: 'Best Practices',
       recommended: false,
     },
@@ -32,10 +32,18 @@ export default util.createRule({
 
     /**
      * Helper function to get base type of node
-     * @param type type to be evaluated
-     * @returns string, number or invalid
      */
     function getBaseTypeOfLiteralType(type: ts.Type): BaseLiteral {
+      const constraint = type.getConstraint();
+      if (
+        constraint &&
+        // for generic types with union constraints, it will return itself from getConstraint
+        // so we have to guard against infinite recursion...
+        constraint !== type
+      ) {
+        return getBaseTypeOfLiteralType(constraint);
+      }
+
       if (type.isNumberLiteral()) {
         return 'number';
       }

Diff for: packages/eslint-plugin/tests/rules/restrict-plus-operands.test.ts

@@ -60,6 +60,27 @@ var foo = ("5.5" as string) + pair.second;
     `const foo = 'hello' + (someBoolean ? 'a' : 'b') + (() => someBoolean ? 'c' : 'd')() + 'e';`,
     `const balls = true;`,
     `balls === true;`,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/230
+    `
+function foo<T extends string>(a: T) {
+    return a + "";
+}
+    `,
+    `
+function foo<T extends "a" | "b">(a: T) {
+    return a + "";
+}
+    `,
+    `
+function foo<T extends number>(a: T) {
+    return a + 1;
+}
+    `,
+    `
+function foo<T extends 1>(a: T) {
+    return a + 1;
+}
+    `,
   ],
   invalid: [
     {
@@ -305,5 +326,62 @@ var foo = pair + pair;
         },
       ],
     },
+    // https://github.com/typescript-eslint/typescript-eslint/issues/230
+    {
+      code: `
+function foo<T extends string>(a: T) {
+    return a + 1;
+}
+      `,
+      errors: [
+        {
+          messageId: 'notStrings',
+          line: 3,
+          column: 12,
+        },
+      ],
+    },
+    {
+      code: `
+function foo<T extends "a" | "b">(a: T) {
+    return a + 1;
+}
+      `,
+      errors: [
+        {
+          messageId: 'notStrings',
+          line: 3,
+          column: 12,
+        },
+      ],
+    },
+    {
+      code: `
+function foo<T extends number>(a: T) {
+    return a + "";
+}
+      `,
+      errors: [
+        {
+          messageId: 'notStrings',
+          line: 3,
+          column: 12,
+        },
+      ],
+    },
+    {
+      code: `
+function foo<T extends 1>(a: T) {
+    return a + "";
+}
+      `,
+      errors: [
+        {
+          messageId: 'notStrings',
+          line: 3,
+          column: 12,
+        },
+      ],
+    },
   ],
 });