fix(eslint-plugin): [no-unnecessary-type-assertion] allow non-null assertion for void type (typescript-eslint#8912)

yeonjuan · JoshuaKGoldberg · web-flow · commit b2552caecd93 · 2024-04-22T13:08:26.000-04:00
* fix(eslint-plugin): [no-unnecessary-type-assertion] allow non-null assertion for void type

* Update packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts

---------

Co-authored-by: Josh Goldberg ✨ &lt;git@joshuakgoldberg.com&gt;
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts
@@ -189,7 +189,7 @@ export default createRule<Options, MessageIds>({
 
         const type = getConstrainedTypeAtLocation(services, node.expression);
 
-        if (!isNullableType(type)) {
+        if (!isNullableType(type) && !isTypeFlagSet(type, ts.TypeFlags.Void)) {
           if (
             node.expression.type === AST_NODE_TYPES.Identifier &&
             isPossiblyUsedBeforeAssigned(node.expression)
@@ -217,6 +217,7 @@ export default createRule<Options, MessageIds>({
               ts.TypeFlags.Undefined,
             );
             const typeIncludesNull = isTypeFlagSet(type, ts.TypeFlags.Null);
+            const typeIncludesVoid = isTypeFlagSet(type, ts.TypeFlags.Void);
 
             const contextualTypeIncludesUndefined = isTypeFlagSet(
               contextualType,
@@ -226,6 +227,10 @@ export default createRule<Options, MessageIds>({
               contextualType,
               ts.TypeFlags.Null,
             );
+            const contextualTypeIncludesVoid = isTypeFlagSet(
+              contextualType,
+              ts.TypeFlags.Void,
+            );
 
             // make sure that the parent accepts the same types
             // i.e. assigning `string | null | undefined` to `string | undefined` is invalid
@@ -235,8 +240,11 @@ export default createRule<Options, MessageIds>({
             const isValidNull = typeIncludesNull
               ? contextualTypeIncludesNull
               : true;
+            const isValidVoid = typeIncludesVoid
+              ? contextualTypeIncludesVoid
+              : true;
 
-            if (isValidUndefined && isValidNull) {
+            if (isValidUndefined && isValidNull && isValidVoid) {
               context.report({
                 node,
                 messageId: 'contextuallyUnnecessary',
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts
@@ -88,6 +88,10 @@ let foo: number = bar!;
 declare const a: { data?: unknown };
 
 const x = a.data!;
+    `,
+    `
+declare function foo(arg?: number): number | void;
+const bar: number = foo()!;
     `,
     {
       code: `
@@ -692,6 +696,24 @@ y = 0;
     },
     {
       code: `
+declare function foo(arg?: number): number | void;
+const bar: number | void = foo()!;
+      `,
+      output: `
+declare function foo(arg?: number): number | void;
+const bar: number | void = foo();
+      `,
+      errors: [
+        {
+          messageId: 'contextuallyUnnecessary',
+          line: 3,
+          column: 28,
+          endColumn: 34,
+        },
+      ],
+    },
+    {
+      code: `
 declare function foo(): number;
 const a = foo()!;
       `,
