Tests are failing, because of duplicated warning message, I'll try to come up with minimal example.

UPDT: New issue: #2603

Currently singleton variables warning is done in loader.pl during goal expansion, if this PR will be accepted, I can move that code to library(warnings).

Conceptually very interesting! This is an aspect that I think would also generally benefit from more thought during development: Which parts of the system must be provided as foundation, and which can be built on top?

Ideas for warnings:

• Warn if user tries to re-define built-in static predicate
• Warn if a non-list is found eg.: [a,b,c|d].
• Detect used, but not visible predicates + suggest which module to load (needs some additional support from loader)
• Warn about duplicated cuts (!/0) in a single predicate body (can be quite tricky in the presence of lambdas)
• Warn if predicate calls one of its arguments, but isn't declared as meta-predicate. Suggest correct meta-predicate declaration.
• Warn when more than 2 negations are nested: \+ \+ \+ foo(X).. Double negation has legit use-case, but I don't think that more nested negations are useful.
• Detect unsound type test predicates like integer/1 and suggest using library(si). (Thanks @triska)
• Branch singleton variables warnings would be useful #2189
• Warn about invalid arithmetic expressions

This list will be expanded in the future. Regarding implementation... if community thinks that this is a good idea, I will implement them. Currently I only gather ideas.

Important: those warnings will be optional, user is not required to import the library and see them.

One question that may be worth taking into account: What makes most programs wrong? I think the type tests integer/1, atom/1 are candidates for this: Their use makes many programs yield wrong answers. The safe type tests integer_si/1 etc. from library(si) could be automatically suggested in such cases.

There seems to be a pattern in the expansion code, so maybe this code can be generated from shorter declarative descriptions of warnings?

#2189 may be a candidate to address with this technique.

I've added quite interesting and less trivial warning. It now detects missing meta_predicate declaration. I don't like how I implemented it (and I expect a lot of critique :) ), mainly because that hook predicate doesn't seem elegant to me. There is a lot of useful code in loader, but it is hard to use it in different contexts.

You may be able to find a better design for the loading process, to make adding such features more elegant!

I've copied code from #2458, that particular task should be fixed in the compiler itself, but Prolog code is useful enough to give it second life here.