Changes from stanc2

In the process of implementing stanc3, several bugs were found in stanc2 and were fixed in stanc3. We record these here.

Target is a reserved variable name

Printing without a semi-colon is not allowed

Function arguments should not be allowed to shadow

ODE solver type checking was wrong

Non-returning function calls should be followed by a semi-colon

Parse error in if-else

Stanc gives run-time errors when using function names (e.g. add) corresponding to operators (e.g. +), for certain signatures

(This could either be a Stan Math error or a Stanc error.)

Variable declarations and statements can be mixed

Array constructors work with sampling statements

Added pretty printing functionality, exposing a pre-processed version of the Stan program from the compiler

Fixed parser warnings for semicolons in function arguments

Parser points to the name of non-existent functions rather than the end of the line