Funny, than name of the map and topic implies that French colonization of Indochina did not quite end in 1949
If our Thai bros prefer to see it like that, no problem

In a strict sense, Indochina comprises the territory of the former French Indochina:
Cambodia
Laos
Vietnam

However, in a wider sense, the cultural region is better described as Mainland Southeast Asia in which sense it also includes:
Myanmar (formerly Burma—part of British India until 1937)
Singapore (also considered part of Maritime Southeast Asia if the Johor-Singapore Causeway is not taken into account)
Thailand (formerly Siam)
And, sometimes, Peninsular Malaysia