“I often buy fruits when I go to the supermarket” – illogical?
I am an English assistant and I often hear my non-native students say:

I often buy fruits when I go to the supermarket.

I think it is wrong logically because "go" means "to move or travel from one place to another". I am not sure whether it implies "you are already there"

Should we change to:

I often buy fruits when I am at the supermarket
but it does not say that "I go to the supermarket".

How do native speakers express that idea?

Note: when could mean "after", so, I often buy fruits when I go to the supermarket=I often buy fruits after I go to the supermarket

Question

“I often buy fruits when I go to the supermarket” – illogical?
I am an English assistant and I often hear my non-native students say:

I often buy fruits when I go to the supermarket.

I think it is wrong logically because "go" means "to move or travel from one place to another". I am not sure whether it implies "you are already there"

Should we change to:

I often buy fruits when I am at the supermarket
but it does not say that "I go to the supermarket".

How do native speakers express that idea?

Note: when could mean "after", so, I often buy fruits when I go to the supermarket=I often buy fruits after I go to the supermarket

Answer

For me, as a native speaker (and frequent fruit-buyer), the only thing wrong with the sentence is the plural 'fruits'.  When I read your title, I assumed it was a question about  countable and uncountable nouns. If you change it to "I often buy fruit when I go to the supermarket", your sentence is 100% natural. There is no problem at all with 'go'.

Answer

Your suggestion would work too, but there is nothing wrong with saying "I often buy fruit when I go to the supermarket." Based on your definition of "go", you could reword the sentence like this: "I often buy fruit when I travel to the supermarket" or "When I travel to the supermarket, I often buy fruit."  "Go" does not imply that you already there.  It simply means that you usually purchase fruit when you are at the store.  Hope this helps.