Merge pull request #19770 from Microsoft/remove-readonly-from-spread-index-sigs

Remove readonly from index signatures of a spread

src/compiler/checker.ts

@@ -8000,11 +8000,16 @@ namespace ts {
                 }
             }
 
-            const spread = createAnonymousType(undefined, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
+            const spread = createAnonymousType(
+                symbol,
+                members,
+                emptyArray,
+                emptyArray,
+                getNonReadonlyIndexSignature(stringIndexInfo),
+                getNonReadonlyIndexSignature(numberIndexInfo));
             spread.flags |= propagatedFlags;
             spread.flags |= TypeFlags.FreshLiteral | TypeFlags.ContainsObjectLiteral;
             (spread as ObjectType).objectFlags |= (ObjectFlags.ObjectLiteral | ObjectFlags.ContainsSpread);
-            spread.symbol = symbol;
             return spread;
         }
 
@@ -8020,6 +8025,13 @@ namespace ts {
             return result;
         }
 
+        function getNonReadonlyIndexSignature(index: IndexInfo) {
+            if (index && index.isReadonly) {
+                return createIndexInfo(index.type, /*isReadonly*/ false, index.declaration);
+            }
+            return index;
+        }
+
         function isClassMethod(prop: Symbol) {
             return prop.flags & SymbolFlags.Method && find(prop.declarations, decl => isClassLike(decl.parent));
         }