Legacy resources in modular build should fail

[elharo]

Affected version

HEAD 4.1.x

Bug description

See #11612 and #11505 and #11632 on master branch.
Maven 4.x introduces a unified <sources> element that supports modular project layouts (src/<module>/<scope>/<lang>). However, resource handling did not follow the modular layout - resources were always loaded from the legacy <resources> element which defaults to src/main/resources. This change implements automatic module-aware resource injection.

• For modular projects without resource configuration in <sources>, automatically inject resource roots following the modular layout: src/<module>/main/resources and src/<module>/test/resources.
• Resources configured via <sources> take priority over legacy <resources>.
• Issue warnings (as ModelProblem) when explicit legacy resources are ignored.

The first two are fine. The last isn't. Resources the user clearly meant to include are dropped from the build. This should be a full build failure that requires the user to fix their project one way or another, not a simple warning.

[ascheman]

Thanks, this is a serious problem we obviously overlooked when designing and implementing #11612. As @desruisseaux suggested somewhere else, we should perhaps address this first on the master branch, then rebase the backport (for #11700) accordingly.

[ascheman]

We should add a new acceptance criterion to the concept of #11612:

AC8 (Legacy configuration error): When a modular project has explicit legacy configuration (, , , or ) that differs from Super POM defaults and would be ignored, the build must fail with an ERROR (not just a WARNING). This prevents silent
loss of user-configured sources/resources.

Cf. #11612 (comment)

[ascheman]

Based on further questions and discussions the AC8 will now supersede AC7:

AC8 (Legacy configuration error): When a modular project has legacy sources or resources that exist but would be ignored, the build must fail with an ERROR (not just a WARNING).
This includes:

• Explicit <sourceDirectory>/<testSourceDirectory> differing from defaults
• Default src/main/java or src/test/java directories existing on filesystem
• Explicit <resources>/<testResources> differing from Super POM defaults

This prevents silent loss of user-configured sources/resources.

Note: AC8 supersedes AC7. AC7 originally specified WARNING for ignored legacy directories. AC8 upgrades this to ERROR. Tests and comments referencing AC7 should be updated to mention that AC8 supersedes AC7 (not just replace AC7 with AC8 - preserve the history that AC7 existed but is no longer valid).

[ascheman]

AC9 (extends AC8 to classic projects):

For classic (non-modular) projects, explicit legacy <sourceDirectory> and <testSourceDirectory> configurations also trigger an ERROR when <sources> elements are configured for the same scope/language.

This ensures consistency with resource handling and prevents silent configuration loss.

Rationale:

• Resources already fail for both modular AND classic projects when <sources> conflicts with legacy <resources>/<testResources>
• Source directories should behave consistently
• Silent configuration loss is confusing for users

See discussion: #11702 (comment)

[ascheman]

AC8 and AC9: Legacy Directory Handling with <sources>

This comment clarifies the acceptance criteria for handling legacy directories (<sourceDirectory>, <testSourceDirectory>) when <sources> is configured (based on respective discussions in the course of #11702).

AC8: Modular Projects

For modular projects (projects with <module> attribute in <source> elements), all legacy directories are rejected:

1. Explicit configuration: If legacy directory differs from Super POM default → ERROR
2. Physical presence: If default directory (src/main/java) exists on filesystem → ERROR

Rationale: Modular projects use paths like src/<module>/main/java. Content in legacy directories cannot be dispatched between modules and would be silently ignored.

AC9: Non-Modular Projects

For non-modular projects using <sources>:

1. When <sources> has Java for a scope: Legacy directories are rejected if they differ from the default
2. When <sources> has no Java for a scope: Legacy directories are used as implicit fallback only if they match the default

<sources> has Java?	Legacy differs from default?	Result
Yes	Yes	ERROR
Yes	No	OK (ignored, use <sources>)
No	Yes	ERROR
No	No	OK (implicit fallback)

Rationale: This allows incremental adoption of <sources>:

• Customize only specific source types (e.g., resources) while keeping default Java directories
• Explicit legacy overrides always trigger errors (detectable configuration mixing)

Limitation

Maven works with the effective model after inheritance resolution. It cannot distinguish whether a value matching the default was inherited (from Super POM or any parent) or explicitly configured. For modular projects, the physical presence check provides an additional safeguard.

Example: AC9 Implicit Fallback

<build>
  <sources>
    <!-- Only customize resources -->
    <source>
      <lang>resources</lang>
      <scope>main</scope>
      <directory>src/main/custom-resources</directory>
    </source>
  </sources>
  <!-- src/main/java is used implicitly (AC9 fallback) -->
</build>