Fix it correctly this time

Author: jakebailey

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 && token() === SyntaxKind.LessThanToken && lookAhead(nextTokenIsIdentifierOrKeywordOrGreaterThan)) {
+                    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 || scriptKind === ScriptKind.TSX, "Type assertions should never be parsed outside of TS/TSX; they should either be comparisons or JSX.");
         const pos = getNodePos();
         parseExpected(SyntaxKind.LessThanToken);
         const type = parseType();

src/compiler/program.ts

@@ -2936,8 +2936,7 @@ export function createProgram(rootNamesOrOptions: readonly string[] | CreateProg
                         diagnostics.push(createDiagnosticForNode((node as SatisfiesExpression).type, Diagnostics.Type_satisfaction_expressions_can_only_be_used_in_TypeScript_files));
                         return "skip";
                     case SyntaxKind.TypeAssertionExpression:
-                        diagnostics.push(createDiagnosticForNode(node, Diagnostics.Type_assertion_expressions_can_only_be_used_in_TypeScript_files));
-                        return "skip";
+                        Debug.fail(); // Won't parse these in a JS file anyway, as they are interpreted as JSX.
                 }
             }
 

tests/baselines/reference/parseUnaryExpressionNoTypeAssertionInJsx.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/index.js(2,13): error TS17004: Cannot use JSX unless the '--jsx' flag is provided.
+tests/cases/compiler/index.js(2,14): error TS17008: JSX element 'number' has no corresponding closing tag.
+tests/cases/compiler/index.js(3,1): error TS1005: '</' expected.
+
+
+==== tests/cases/compiler/index.js (3 errors) ====
+    const x = "oops";
+    const y = + <number> x;
+                ~~~~~~~~
+!!! error TS17004: Cannot use JSX unless the '--jsx' flag is provided.
+                 ~~~~~~
+!!! error TS17008: JSX element 'number' has no corresponding closing tag.
+    
+    
+!!! error TS1005: '</' expected.

tests/baselines/reference/typeAssertionExpressionInJsNoCrash.symbols renamed to tests/baselines/reference/parseUnaryExpressionNoTypeAssertionInJsx.symbols

@@ -2,7 +2,6 @@
 const x = "oops";
 >x : Symbol(x, Decl(index.js, 0, 5))
 
-const y = delete <number> x;
+const y = + <number> x;
 >y : Symbol(y, Decl(index.js, 1, 5))
->x : Symbol(x, Decl(index.js, 0, 5))
 

tests/baselines/reference/parseUnaryExpressionNoTypeAssertionInJsx.types

@@ -0,0 +1,13 @@
+=== tests/cases/compiler/index.js ===
+const x = "oops";
+>x : "oops"
+>"oops" : "oops"
+
+const y = + <number> x;
+>y : number
+>+ <number> x; : number
+><number> x; : any
+>number : any
+
+> : any
+

tests/baselines/reference/typeAssertionExpressionInJsNoCrash.errors.txt

@@ -4,4 +4,4 @@
 // @filename: index.js
 
 const x = "oops";
-const y = delete <number> x;
+const y = + <number> x;