I agree with @Cord's answer and comment about defaulting to the past tense. I simply wish to add some references so that the OP can make up his or her mind about the acceptability of sentences such as I told you it's impossible to fly.
Firstly, this is what the Collins Cobuild English Grammar (p327) has to say about the "default" use of the past tense:
When the reporting verb is in a past tense, a past tense is also
usually used for the verb in the reported clause even if the reported
situation still exists. For example, you could say "I told him I was
eighteen" even if you are still eighteen. You are concentrating on
the situation at the past time you are talking about.
The CCEG then goes on to state:
A present tense is sometimes used instead, to emphasize that the
situation still exists.
Swan in Practical English Usage (p251) writes:
If somebody talks about a situation that has still not changed - that
is to say, if the original speaker's present and future are still
present and future - a reporter can often choose whether to keep the
original speaker's tenses or to change them, after a past reporting
verb. Both structures are common.
- DIRECT: The earth goes round the sun.
- INDIRECT: He proved that the earth goes/went round the sun.
Huddleston and Pullum in A Student's Introduction to English Grammar (p47) state:
Even with preterite reporting verbs backshift is often optional: You
can keep the original tense instead of backshifting. Instead of [I
told Stacy that Kim had blue eyes], therefore, we can have: I told
Stacy that Kim has blue eyes.
Yule in Explaining English Grammar (p272) states:
The forms used in a reported version of previous talk can vary a great
deal and typically depend on the perspective of the reporter. If the
speaker thinks that the situation being reported is still true at the
time of the report, then there may be no backshifting.
Quirk et al. in A Comprehensive Grammar Of The English Language (p1027) state:
Backshift is optional when the time reference of the original
utterance is valid at the time of the reported utterance.
They list several examples, including:
- Their teacher had told them that the earth moves around the sun.
- I heard her say that she is studying Business Administration.
- I didn't know that our meeting is next Tuesday.
- They thought that prison conditions have improved.
Finally, Huddleston and Pullum, in the monumental Cambridge Grammar of the English Language, currently the most authoritative descriptive grammar (p155), state:
Very often, the use of a backshifted preterite is optional. Jill's "I
have too many commitments" may be given in indirect reported speech in
- Jill said she had too many commitments.
- Jill said she has too many commitments.
In a lengthy discussion of contexts in which the present tense is likely to be retained by the reporter, they state:
If I endorse or accept the original, this will somewhat favour the
deictic present version, and conversely if I reject it this will
She said she doesn't need it so I'll let Bill have it. [accepted:
She said that there was plenty left, but there's hardly any.
If we contextualise the flying example, we could say that the backshift is more likely if the telling happened a long time ago. Maybe 5 years ago your friend told you he was working on a pair of wings so he could fly to the shops. Now, after 5 years of fruitless attempts, he tells you of his failure. You then remind him of what you said:
I told you it was impossible to fly.
As Cord says: "We are more concerned about the report itself (the source and context of the information)".
Conversely, imagine that your friend comes in one day with a pair of wings and tells you she is going to fly around the garden. You tell her it's impossible. She then makes several unsuccessful attempts, at the end of which you remind her of what you said an hour before:
I told you it's impossible to fly.
The emphasis here is on the general (and present) impossibility of flying and not on the telling.