bugfix: pure cxx and fortran deps should display with a compiler. (#49752)

I noticed that `abseil-cpp` was showing in `spack find` with "no compiler", and the only
difference between it and other nodes was that it *only* depends on `cxx` -- others
depend on `c` as well.

It turns out that the `select()` method on `EdgeMap` only takes `Sequence[str]` and doesn't
check whether they're actually just one `str`.  So asking for, e.g., `cxx` is like asking for
`c` or `x` or `x`, as the `str` is treated like a sequence. This causes Spack to miss `cxx`
and `fortran` language virtuals in `DeprecatedCompilerSpec`.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/spack/spec.py

@@ -1022,7 +1022,7 @@ def select(
         parent: Optional[str] = None,
         child: Optional[str] = None,
         depflag: dt.DepFlag = dt.ALL,
-        virtuals: Optional[Sequence[str]] = None,
+        virtuals: Optional[Union[str, Sequence[str]]] = None,
     ) -> List[DependencySpec]:
         """Selects a list of edges and returns them.
 
@@ -1062,7 +1062,10 @@ def select(
 
         # Filter by virtuals
         if virtuals is not None:
-            selected = (dep for dep in selected if any(v in dep.virtuals for v in virtuals))
+            if isinstance(virtuals, str):
+                selected = (dep for dep in selected if virtuals in dep.virtuals)
+            else:
+                selected = (dep for dep in selected if any(v in dep.virtuals for v in virtuals))
 
         return list(selected)
 
@@ -1602,7 +1605,11 @@ def edges_from_dependents(
         ]
 
     def edges_to_dependencies(
-        self, name=None, depflag: dt.DepFlag = dt.ALL, *, virtuals: Optional[Sequence[str]] = None
+        self,
+        name=None,
+        depflag: dt.DepFlag = dt.ALL,
+        *,
+        virtuals: Optional[Union[str, Sequence[str]]] = None,
     ) -> List[DependencySpec]:
         """Returns a list of edges connecting this node in the DAG to children.
 
@@ -1644,7 +1651,7 @@ def dependencies(
         name=None,
         deptype: Union[dt.DepTypes, dt.DepFlag] = dt.ALL,
         *,
-        virtuals: Optional[Sequence[str]] = None,
+        virtuals: Optional[Union[str, Sequence[str]]] = None,
     ) -> List["Spec"]:
         """Returns a list of direct dependencies (nodes in the DAG)
 

lib/spack/spack/test/spec_semantics.py

@@ -1960,6 +1960,23 @@ def test_edge_equality_does_not_depend_on_virtual_order():
     assert tuple(sorted(edge2.virtuals)) == edge1.virtuals
 
 
+def test_virtual_queries_work_for_strings_and_lists():
+    """Ensure that ``dependencies()`` works with both virtuals=str and virtuals=[str, ...]."""
+    parent, child = Spec("parent"), Spec("child")
+    parent._add_dependency(
+        child, depflag=dt.BUILD, virtuals=("cxx", "fortran")  # multi-char dep names
+    )
+
+    assert not parent.dependencies(virtuals="c")  # not in virtuals but shares a char with cxx
+
+    for lang in ["cxx", "fortran"]:
+        assert parent.dependencies(virtuals=lang)  # string arg
+        assert parent.edges_to_dependencies(virtuals=lang)  # string arg
+
+        assert parent.dependencies(virtuals=[lang])  # list arg
+        assert parent.edges_to_dependencies(virtuals=[lang])  # string arg
+
+
 def test_old_format_strings_trigger_error(default_mock_concretization):
     s = spack.concretize.concretize_one("pkg-a")
     with pytest.raises(SpecFormatStringError):