fix: type-checking for foreach loops in trait methods

[anton-trunov]

Issue

Closes #971.

Checklist

• I have updated CHANGELOG.md
• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[jeshecdom]

Ok, I get it now. resolveDescriptors.ts copies inherited trait functions into the contract. This means that before the fix for the foreach statement in the cloneNode function, resolveDescriptors.ts would duplicate the expression id of the map expression in foreach statements inside functions in the contract. This would later produce the error in resolveExpression.ts as follows (taking the code from the test):

trait TraitWithForeach {
    m: map<Int, Int>;

    fun test() { foreach(_, _ in self.m) { } }
}


contract Test with TraitWithForeach {
    m: map<Int, Int>;
}

The test function in the trait gets copied into contract Test, but with the problem that the expression self.m has the same expression id for the subexpression self in both functions (the one in the trait and the copy in the contract). Hence, when resolveExpression registers the type of self when processing function test in the trait, it does so with type TraitWithForEach; but when it processes the copy of test in the contract, it attempts to register self again (because self has the same expression id), but this time with type Test, producing the error.

This is why changing the code like the following does not produce the error, because let statements correctly clone expressions:

trait TraitWithForeach {
    m: map<Int, Int>;

    fun test() { let a = self.m; foreach(_, _ in a) { } }
}


contract Test with TraitWithForeach {
    m: map<Int, Int>;
}