Stack traces are unusable for array-to-string conversions when using the yield-based system

[stof]

I got a case where a template was doing {{ value }} and received an empty array as value (due to a bug in my code). The stack trace for that was totally useless:

In the old buffer-based architecture, converting the array to a string would happen in the compiled template at the location of this {{ value }}, allowing to debug this.
The new architecture documents that Template::doDisplay should return an iterable<scalar|\Stringable|null>, but the compiled code does not actually ensure this is respected. PrintNode does not ensure that the expression result satisfies that type before yielding it.

This has 2 impact:

• when using TemplateWrapper::render or TemplateWrapper::display (or the equivalent block-related methods), you will get warning with a useless stack trace (see my example)
• when using TemplateWrapper::stream (or TemplateWrapper::streamBlock), the returned iterable will not satisfy the contract (and whether you get an error or warning will depend on how the iterable is consumed, but it won't have a stack trace reaching to the root cause in the PrintNode).

[fabpot]

That's a tricky one. The only "fix" I can imagine is wrapping every single yield with a function call, but it would add a lot of overhead. Any other idea?

[stof]

we could add a (string) cast in the PrintNode (when not printing from a generator)