cpp.cxxLanguageVersion set by user module conflicts with value in Qt.core

The Problem

Consider a product that

• depends on Qt.core (which sets cpp.cxxLanguageVersion)
• depends on another module "globals" that sets cpp.cxxLanguageVersion

Module "globals" might be user-provided to set project-global properties as we recommend to do.

Resolving such a product will print a warning:

WARNING: Conflicting scalar values at /home/jobor/.config/QtProject/qbs/1.10.0/profiles/qt591/modules/Qt/core/core.qbs:94:29 and /home/jobor/playground/collidingModuleProp/modules/globals/globals.qbs:3:29.

...because cpp.cxxLanguageVersion is a scalar property, and qbs doesn't know how to resolve the conflict between scalar properties.

Proposed Solution

Provide a way to resolve conflicts on scalar properties. This could be done like this:

// In the cpp module
PropertyOptions {
    name: "cxxLanguageVersion"
    resolveConflict: function (a, b) {
        return cxxLanguageVersionLessThan(a, b) ? b : a;
    }
}

The function resolveConflict either returns a valid value or throws an exception. The default implementation returns the first parameter and prints the warning above.