Support using parametrize() with a single value. (pantsbuild#19920)

Closes pantsbuild#16978

src/python/pants/engine/internals/graph.py

@@ -366,8 +366,9 @@ def _target_parametrizations(
     target_type: type[Target],
     union_membership: UnionMembership,
 ) -> _TargetParametrization:
-    first, *rest = Parametrize.expand(address, target_adaptor.kwargs)
-    if rest:
+    expanded_parametrizations = tuple(Parametrize.expand(address, target_adaptor.kwargs))
+    first_address, first_kwargs = expanded_parametrizations[0]
+    if first_address is not address:
         # The target was parametrized, and so the original Target does not exist.
         generated = FrozenDict(
             (
@@ -380,7 +381,7 @@ def _target_parametrizations(
                     description_of_origin=target_adaptor.description_of_origin,
                 ),
             )
-            for parameterized_address, parameterized_fields in (first, *rest)
+            for parameterized_address, parameterized_fields in expanded_parametrizations
         )
         return _TargetParametrization(None, generated)
     else:

src/python/pants/engine/internals/graph_test.py

@@ -1464,6 +1464,25 @@ def test_parametrize_16910(generated_targets_rule_runner: RuleRunner, field_cont
         )
 
 
+def test_parametrize_single_value_16978(generated_targets_rule_runner: RuleRunner) -> None:
+    assert_generated(
+        generated_targets_rule_runner,
+        Address("demo"),
+        "generated(resolve=parametrize('demo'), source='f1.ext')",
+        ["f1.ext"],
+        {
+            MockGeneratedTarget(
+                {SingleSourceField.alias: "f1.ext", ResolveField.alias: "demo"},
+                Address("demo", parameters={"resolve": "demo"}),
+                residence_dir="demo",
+            ),
+        },
+        expected_dependencies={
+            "demo@resolve=demo": set(),
+        },
+    )
+
+
 # -----------------------------------------------------------------------------------------------
 # Test `SourcesField`. Also see `engine/target_test.py`.
 # -----------------------------------------------------------------------------------------------