ycb_sympy_pr21379

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Unexpected `PolynomialError` when using simple `subs()` for particular expressions
I am seeing weird behavior with `subs` for particular expressions with hyperbolic sinusoids with piecewise arguments. When applying `subs`, I obtain an unexpected `PolynomialError`. For context, I was umbrella-applying a casting from int to float of all int atoms for a bunch of random expressions before using a tensorflow lambdify to avoid potential tensorflow type errors. You can pretend the expression below has a `+ 1` at the end, but below is the MWE that I could produce.

See the expression below, and the conditions in which the exception arises.

Sympy version: 1.8.dev

```python
from sympy import *
from sympy.core.cache import clear_cache

x, y, z = symbols('x y z')

clear_cache()
expr = exp(sinh(Piecewise((x, y > x), (y, True)) / z))
# This works fine
expr.subs({1: 1.0})

clear_cache()
x, y, z = symbols('x y z', real=True)
expr = exp(sinh(Piecewise((x, y > x), (y, True)) / z))
# This fails with "PolynomialError: Piecewise generators do not make sense"
expr.subs({1: 1.0})  # error
# Now run it again (isympy...) w/o clearing cache and everything works as expected without error
expr.subs({1: 1.0})
```

I am not really sure where the issue is, but I think it has something to do with the order of assumptions in this specific type of expression. Here is what I found-

- The error only (AFAIK) happens with `cosh` or `tanh` in place of `sinh`, otherwise it succeeds
- The error goes away if removing the division by `z`
- The error goes away if removing `exp` (but stays for most unary functions, `sin`, `log`, etc.)
- The error only happens with real symbols for `x` and `y` (`z` does not have to be real)

Not too sure how to debug this one.

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