Missing module version (ModuleVersionMissing)

Warnings that indicate the compiler cannot resolve an #if canImport(<ModuleName>, _version: <version>) directive because the module found was not built with a -user-module-version flag.

Overview

Developers may conditionalize which code is active in a source file based on the module version number of an imported dependency module, like this:

import Dependency

#if canImport(Dependency, _version: 1.2)
// Use declarations introduced in version 1.2 of Dependency
#else
// ...
#endif

A dependency's module version must be established by passing the -user-module-version flag when compiling the sources of the dependency the module. If no -user-module-version flag was specified when the dependency module was built, then the compiler will warn that it cannot resolve the #if canImport directive:

warning: cannot find user version number for module 'Dependency'; version number ignored [#ModuleVersionMissing]

If this diagnostic is emitted, then the #if canImport directive implicitly evaluates true.