let expression binding of argument to a function fails.

[sonodtt]

NB it has been verified that let expressions do work in the simple scenario of binding a value to a variable.
https://github.com/sonodtt/gnat2goto/tree/irep_debug_framework_v0

The advantage of a let expression is that it allows to bind a parameter value to a
function in a single expression.

The alternative to this requires creating a unique function each call:
a) create a function with required value of parameter as variable in body
b) create a parameterless function call
c) pass expression to location where it is needed
(e.g the Do_Expression method of "tree_walk.adb")

As part of the cleanup work on arrays, there were a number of locations where 'let expressions' were used.
But in the scenario of using a let expression to bind a parameter to a function call - there are issues (the binding seems to fail, and it looks at a first glance like it is interpreted as a raw irep)
It is likely something minor is missing in the process.

• In the case of " Do_Aggregate_Literal_Array" in tree_walk.adb a let expression was used as so:

let_expr_irep := Make_Let_Expr (Symbol => Literal_Temp,
Value => Array_Expr,
Where => Result_Struct,
I_Type => Result_Type,
Source_Location => Sloc (N));

  return let_expr_irep;

This allows binding an array to a parameter of a function.
As this did not work, the entire callee was rewrote to avoid the let expression.

While this is a valid workaround - it seem worthwhile to investigate the original issue - let expressions, as clarified in the first paragraph, can be a useful tool to avoid trivial duplication of simple function calls.