"invoke" for functions

The spec relies on the notion of "invoking" a function in several places, but the process is never defined.

For example §6.12.1:9 (fls_yrr1s0tucgvh) says:

If the callee type is a function item type or a function pointer type, then the value is the result of invoking the corresponding function with the argument operands.

Compare §6.19:7 (fls_bqmwlona6l5w) for return expression, which is written in terms of activation frames and transfer of control.

Something needs to say that a pattern match happens between the argument operands and the patterns represented by the function's FunctionParameterPatterns, effectively resulting in additional local variables.

Compare this text from the Rust Reference:

The block of a function is conceptually wrapped in a block that binds the argument patterns and then returns the value of the function's block.

This needs to tie up with §14.4.1:3 "The binding of a function parameter is in scope within the related function body", and with §15.5 "Passing Conventions".

A different definition would be needed for external functions (and that one has to deal with variadics).

Read §6.12.1:9, §6.19:7, §14.4.1:3, and §15.5 to compare invocation with return expressions, parameter binding, and passing conventions. Define invocation for function items and function pointers, then address external functions and variadics; the specification is complete when these cases and their parameter-pattern bindings are explicitly connected.