Charizard evolves to Mega Charizard Y and stands against pokemon with Chlorophyll ability. Looking at https://bulbapedia.bulbagarden.net/wiki/User:FIQ/Turn_sequence

I think that Chlorophyll should activate and the Chlorophyll pokemon speed should be doubled, so in most of cases it should move before Charizard Y. But this not happens and the battle system do not respect Chlorophyll ability on Charizard mega evolution. Is this a bug or I misunderstand the sequence?

 

 

Hello @Demenelizator!

 

Apologies for the issue. 

 

It is not a bug. 

 

To explain briefly, Speed is calculated before the weather change due to a Mega Evolution. However even if this changed in new generation, since mega official last shown was in generation 7 we keep this mechanic.

 

Hope it answer your question.

 

Take care,

 

Zarachi 

Kind of paradox here: if speed is calculated before Mega evolution then Mega evolution should not change the evolved poke base speed stat. But I get your point.

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if speed is calculated before Mega evolution then Mega evolution should not change the evolved poke base speed stat

Speed is not calculated before the Mega evolution.

As Zarachi said 

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Speed is calculated before the weather change due to a Mega Evolution

Speed is calculated BEFORE the weather change, that happens DUE to a Mega Evolution - not before the Pokemon Mega evolved.

So 
1. Mega evolution 
2. Speed calculation
3. Weather change.

This behavior may change due to adjustments introduced in Generation 8 regarding how speed modifications are handled. However, since there are no Mega Evolutions beyond Generation 7, it is currently impossible to test this interaction in a valid environment - and neither Bulbapedia nor Showdown nor any other page is infallible.