Learn English – Future tense in both main clause and subordinate clause

I think the difference between the two types of examples that you've exhibited is the relative placement in time of the action in the "if" clause, and the action in the other clause.

• If it rains in the evening, we won't go for a walk - here, the event of raining occurs BEFORE the decision about whether to go for a walk.
• My teeth will rot if I eat too much sugar - presumably, I'll be eating the sugar BEFORE my teeth rot.
• If it will significantly increase complexity, don't implement this feature - here, the increasing of complexity occurs AFTER the implementation of the feature.
• I will give you money if it will make you happier - here, you becoming happier occurs AFTER I give you the money.

In all the cases where the "if" part happens first chronologically, we use the present tense. In the cases where the "if" part happens second, we use the future tense. However, because sentences of the first type are far more common than sentences in the second type, a good rule for learners to adopt is "don't use the future tense with IF".

Past

If Canada's population had been 40 million, the Canadian economy would have been more dynamic.

Present

If Canada's population were 40 million, the Canadian economy would be more dynamic.

You could say is when the clause introduced by the if is not a condition but a consequence, as in:

If Canada's population is 40 million, it is because people are living longer.

You could also use is if you doubt a fact, e.g.

I do not believe the extrapolation of the last census. If Canada's population is 40 million, the Canadian economy must be more dynamic than what I thought.