Set '.declarations' on a property of a homomorphic mapped type

src/compiler/checker.ts

@@ -5690,6 +5690,7 @@ namespace ts {
                     prop.type = propType;
                     if (propertySymbol) {
                         prop.syntheticOrigin = propertySymbol;
+                        prop.declarations = propertySymbol.declarations;
                     }
                     members.set(propName, prop);
                 }