spack
diff --git a/‎lib/spack/docs/packages_yaml.rst‎
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diff --git a/‎lib/spack/spack/schema/packages.py‎
Lines changed: 22 additions & 1 deletion b/‎lib/spack/spack/schema/packages.py‎
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@@ -487,6 +487,56 @@ present. For instance with a configuration like:
 
 you will use ``mvapich2~cuda %gcc`` as an ``mpi`` provider.
 
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Conflicts and strong preferences
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the semantic of requirements is too strong, you can also express "strong preferences" and "conflicts"
+from configuration files:
+
+.. code-block:: yaml
+
+   packages:
+     all:
+       prefer:
+       - '%clang'
+       conflict:
+       - '+shared'
+
+The ``prefer`` and ``conflict`` sections can be used whenever a ``require`` section is allowed.
+The argument is always a list of constraints, and each constraint can be either a simple string,
+or a more complex object:
+
+.. code-block:: yaml
+
+   packages:
+     all:
+       conflict:
+       - spec: '%clang'
+         when: 'target=x86_64_v3'
+         message: 'reason why clang cannot be used'
+
+The ``spec`` attribute is mandatory, while both ``when`` and ``message`` are optional.
+
+.. note::
+
+   Requirements allow for expressing both "strong preferences" and "conflicts".
+   The syntax for doing so, though, may not be immediately clear. For
+   instance, if we want to prevent any package from using ``%clang``, we can set:
+
+   .. code-block:: yaml
+
+      packages:
+        all:
+          require:
+          - one_of: ['%clang', '@:']
+
+   Since only one of the requirements must hold, and ``@:`` is always true, the rule above is
+   equivalent to a conflict. For "strong preferences" we need to substitute the ``one_of`` policy
+   with ``any_of``.
+
+
+
 .. _package-preferences:
 
 -------------------
 
@@ -54,6 +54,24 @@
     ]
 }
 
+prefer_and_conflict = {
+    "type": "array",
+    "items": {
+        "oneOf": [
+            {
+                "type": "object",
+                "additionalProperties": False,
+                "properties": {
+                    "spec": {"type": "string"},
+                    "message": {"type": "string"},
+                    "when": {"type": "string"},
+                },
+            },
+            {"type": "string"},
+        ]
+    },
+}
+
 permissions = {
     "type": "object",
     "additionalProperties": False,
@@ -85,6 +103,8 @@
                 "additionalProperties": False,
                 "properties": {
                     "require": requirements,
+                    "prefer": prefer_and_conflict,
+                    "conflict": prefer_and_conflict,
                     "version": {},  # Here only to warn users on ignored properties
                     "target": {
                         "type": "array",
@@ -133,6 +153,8 @@
                 "additionalProperties": False,
                 "properties": {
                     "require": requirements,
+                    "prefer": prefer_and_conflict,
+                    "conflict": prefer_and_conflict,
                     "version": {
                         "type": "array",
                         "default": [],
@@ -186,7 +208,6 @@
     }
 }
 
-
 #: Full schema with metadata
 schema = {
     "$schema": "http://json-schema.org/draft-07/schema#",