Fix double error on invalid delete of readonly property

src/compiler/checker.ts

@@ -36680,7 +36680,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (isReadonlySymbol(symbol)) {
                 error(expr, Diagnostics.The_operand_of_a_delete_operator_cannot_be_a_read_only_property);
             }
-            checkDeleteExpressionMustBeOptional(expr, symbol);
+            else {
+                checkDeleteExpressionMustBeOptional(expr, symbol);
+            }
         }
         return booleanType;
     }

tests/baselines/reference/deleteReadonlyInStrictNullChecks.errors.txt

@@ -0,0 +1,10 @@
+deleteReadonlyInStrictNullChecks.ts(3,8): error TS2704: The operand of a 'delete' operator cannot be a read-only property.
+
+
+==== deleteReadonlyInStrictNullChecks.ts (1 errors) ====
+    interface Function { readonly name: string; }
+    class Foo {}
+    delete Foo.name;
+           ~~~~~~~~
+!!! error TS2704: The operand of a 'delete' operator cannot be a read-only property.
+

tests/baselines/reference/deleteReadonlyInStrictNullChecks.js

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/deleteReadonlyInStrictNullChecks.ts] ////
+
+//// [deleteReadonlyInStrictNullChecks.ts]
+interface Function { readonly name: string; }
+class Foo {}
+delete Foo.name;
+
+
+//// [deleteReadonlyInStrictNullChecks.js]
+var Foo = /** @class */ (function () {
+    function Foo() {
+    }
+    return Foo;
+}());
+delete Foo.name;

tests/baselines/reference/deleteReadonlyInStrictNullChecks.symbols

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/deleteReadonlyInStrictNullChecks.ts] ////
+
+=== deleteReadonlyInStrictNullChecks.ts ===
+interface Function { readonly name: string; }
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(deleteReadonlyInStrictNullChecks.ts, 0, 0))
+>name : Symbol(Function.name, Decl(deleteReadonlyInStrictNullChecks.ts, 0, 20))
+
+class Foo {}
+>Foo : Symbol(Foo, Decl(deleteReadonlyInStrictNullChecks.ts, 0, 45))
+
+delete Foo.name;
+>Foo.name : Symbol(Function.name, Decl(deleteReadonlyInStrictNullChecks.ts, 0, 20))
+>Foo : Symbol(Foo, Decl(deleteReadonlyInStrictNullChecks.ts, 0, 45))
+>name : Symbol(Function.name, Decl(deleteReadonlyInStrictNullChecks.ts, 0, 20))
+

tests/baselines/reference/deleteReadonlyInStrictNullChecks.types

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/deleteReadonlyInStrictNullChecks.ts] ////
+
+=== deleteReadonlyInStrictNullChecks.ts ===
+interface Function { readonly name: string; }
+>name : string
+
+class Foo {}
+>Foo : Foo
+
+delete Foo.name;
+>delete Foo.name : boolean
+>Foo.name : string
+>Foo : typeof Foo
+>name : string
+

tests/cases/compiler/deleteReadonlyInStrictNullChecks.ts

@@ -0,0 +1,5 @@
+// @strictNullChecks: true
+
+interface Function { readonly name: string; }
+class Foo {}
+delete Foo.name;