Ensure we treat "type assertions" as JSX within a unary expression (#52667)

src/compiler/parser.ts

@@ -5648,6 +5648,11 @@ namespace Parser {
             case SyntaxKind.VoidKeyword:
                 return parseVoidExpression();
             case SyntaxKind.LessThanToken:
+                // Just like in parseUpdateExpression, we need to avoid parsing type assertions when
+                // in JSX and we see an expression like "+ <foo> bar".
+                if (languageVariant === LanguageVariant.JSX) {
+                    return parseJsxElementOrSelfClosingElementOrFragment(/*inExpressionContext*/ true);
+                }
                 // This is modified UnaryExpression grammar in TypeScript
                 //  UnaryExpression (modified):
                 //      < type > UnaryExpression
@@ -6163,6 +6168,7 @@ namespace Parser {
     }
 
     function parseTypeAssertion(): TypeAssertion {
+        Debug.assert(scriptKind === ScriptKind.TS, "Type assertions should never be parsed outside of TS; they should either be comparisons or JSX.");
         const pos = getNodePos();
         parseExpected(SyntaxKind.LessThanToken);
         const type = parseType();