Fix sympify() for expressions with vector element .subs() (backport #14641) #14642
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Summary
This commit fixes an issue in the .sympify() method when building the Sympy expression for a
ParameterExpression
that was constructed by callingParameterExpression.subs()
with a ParameterVectorElement type. The previous implementation of this method was passing the symbols for substitution by string which required sympy to "parse" the input. The square brackets for indexing the vector element in it's string representation was not valid for sympy to parse and this caused the error. To avoid this problem this commit adjusts the usage to pass a sypy expression object instead of a string. This is the intent of the function as string parsing has other issues in sympy and this was an oversight in the implementation that was doing this. By passing a sympy object to sympy it is able to handle the vector's name with square brackets.Details and comments
Fixes #14640
This is an automatic backport of pull request #14641 done by [Mergify](https://mergify.com).