Use JSDoc @type tag type cast as a contextual type (#18690)

* Use JSDoc `@type` tag type cast as a contextual type

* Suggested changes

src/compiler/checker.ts

@@ -13305,8 +13305,11 @@ namespace ts {
                 case SyntaxKind.TemplateSpan:
                     Debug.assert(parent.parent.kind === SyntaxKind.TemplateExpression);
                     return getContextualTypeForSubstitutionExpression(<TemplateExpression>parent.parent, node);
-                case SyntaxKind.ParenthesizedExpression:
-                    return getContextualType(<ParenthesizedExpression>parent);
+                case SyntaxKind.ParenthesizedExpression: {
+                    // Like in `checkParenthesizedExpression`, an `/** @type {xyz} */` comment before a parenthesized expression acts as a type cast.
+                    const tag = isInJavaScriptFile(parent) ? getJSDocTypeTag(parent) : undefined;
+                    return tag ? getTypeFromTypeNode(tag.typeExpression.type) : getContextualType(<ParenthesizedExpression>parent);
+                }
                 case SyntaxKind.JsxExpression:
                     return getContextualTypeForJsxExpression(<JsxExpression>parent);
                 case SyntaxKind.JsxAttribute:
@@ -18127,13 +18130,9 @@ namespace ts {
         }
 
         function checkParenthesizedExpression(node: ParenthesizedExpression, checkMode?: CheckMode): Type {
-            if (isInJavaScriptFile(node) && node.jsDoc) {
-                const typecasts = flatMap(node.jsDoc, doc => filter(doc.tags, tag => tag.kind === SyntaxKind.JSDocTypeTag && !!(tag as JSDocTypeTag).typeExpression && !!(tag as JSDocTypeTag).typeExpression.type));
-                if (typecasts && typecasts.length) {
-                    // We should have already issued an error if there were multiple type jsdocs
-                    const cast = typecasts[0] as JSDocTypeTag;
-                    return checkAssertionWorker(cast, cast.typeExpression.type, node.expression, checkMode);
-                }
+            const tag = isInJavaScriptFile(node) ? getJSDocTypeTag(node) : undefined;
+            if (tag) {
+                return checkAssertionWorker(tag, tag.typeExpression.type, node.expression, checkMode);
             }
             return checkExpression(node.expression, checkMode);
         }

src/compiler/utilities.ts

@@ -4091,9 +4091,14 @@ namespace ts {
         return getFirstJSDocTag(node, SyntaxKind.JSDocTemplateTag) as JSDocTemplateTag;
     }
 
-    /** Gets the JSDoc type tag for the node if present */
+    /** Gets the JSDoc type tag for the node if present and valid */
     export function getJSDocTypeTag(node: Node): JSDocTypeTag | undefined {
-        return getFirstJSDocTag(node, SyntaxKind.JSDocTypeTag) as JSDocTypeTag;
+        // We should have already issued an error if there were multiple type jsdocs, so just use the first one.
+        const tag = getFirstJSDocTag(node, SyntaxKind.JSDocTypeTag) as JSDocTypeTag;
+        if (tag && tag.typeExpression && tag.typeExpression.type) {
+            return tag;
+        }
+        return undefined;
     }
 
     /**

tests/cases/fourslash/completionsJsdocTypeTagCast.ts

@@ -0,0 +1,7 @@
+/// <reference path="fourslash.ts" />
+
+// @allowJs: true
+// @Filename: /a.js
+////const x = /** @type {{ s: string }} */ ({ /**/ });
+
+verify.completionsAt("", ["s", "x"]);