Implement String concatenation with + operator incorrect

[piotrszul]

String concatenation with + operator produces incorrect result (the value of the first argument).

(Math 6 exlusions)

Details

Expected: null but got: c ==> expected: <null> but was: <c>

• 'c' + Patient.name.given[0] [** Null in concatenation is treated as an empty collection (2)]

Expected: onetwo but got: one ==> expected: <onetwo> but was: <one>

• s5 + s6 [** Can concatenate two strings with +]

(FHIR R4 Exlclusions) Details

Expected: true but got: false ==> expected: <true> but was: <false>

• 'a'+'b' = 'ab' [** testPlus4]

[johngrimes]

We don't actually support this, so the task is to either:

1. Make it impossible
2. Implement it properly as a new feature

[piotrszul]

Changed to be a new feature as not implemented in previous versions.

[johngrimes]

We need to make a decision on what data types we are going to support for the math and comparison operators.

At the moment this doesn't seem to work at all for strings.

[johngrimes]

The current error message is au.csiro.pathling.errors.InvalidUserInputError: + operator does not support left operand: ??, which indicates a bug or incomplete implementation.