Don't drop build deps on overwrite install (#40252)

If you `spack install x ^y` where `y` is a pure build dep of `x`, and
then uninstall `y`, and then `spack install --overwrite x ^y`, the build
fails because `y` is not re-installed.

Same can happen when you install a develop spec, run `spack gc`,
modify sources, and install again; develop specs rely on overwrite
install to work correctly.

lib/spack/spack/installer.py

@@ -847,10 +847,11 @@ def get_depflags(self, pkg: "spack.package_base.PackageBase") -> int:
         else:
             cache_only = self.install_args.get("dependencies_cache_only")
 
-        # Include build dependencies if pkg is not installed and cache_only
+        # Include build dependencies if pkg is going to be built from sources, or
-        # is False, or if build depdencies are explicitly called for
+        # if build deps are explicitly requested.
-        # by include_build_deps.
+        if include_build_deps or not (
-        if include_build_deps or not (cache_only or pkg.spec.installed):
+            cache_only or pkg.spec.installed and not pkg.spec.dag_hash() in self.overwrite
+        ):
             depflag |= dt.BUILD
         if self.run_tests(pkg):
             depflag |= dt.TEST

lib/spack/spack/test/installer.py

@@ -20,6 +20,7 @@
 import spack.concretize
 import spack.config
 import spack.database
+import spack.deptypes as dt
 import spack.installer as inst
 import spack.package_base
 import spack.package_prefs as prefs
@@ -1388,6 +1389,26 @@ def test_single_external_implicit_install(install_mockery, explicit_args, is_exp
     assert spack.store.STORE.db.get_record(pkg).explicit == is_explicit
 
 
+def test_overwrite_install_does_install_build_deps(install_mockery, mock_fetch):
+    """When overwrite installing something from sources, build deps should be installed."""
+    s = spack.spec.Spec("dtrun3").concretized()
+    create_installer([(s, {})]).install()
+
+    # Verify there is a pure build dep
+    edge = s.edges_to_dependencies(name="dtbuild3").pop()
+    assert edge.depflag == dt.BUILD
+    build_dep = edge.spec
+
+    # Uninstall the build dep
+    build_dep.package.do_uninstall()
+
+    # Overwrite install the root dtrun3
+    create_installer([(s, {"overwrite": [s.dag_hash()]})]).install()
+
+    # Verify that the build dep was also installed.
+    assert build_dep.installed
+
+
 @pytest.mark.parametrize("run_tests", [True, False])
 def test_print_install_test_log_skipped(install_mockery, mock_packages, capfd, run_tests):
     """Confirm printing of install log skipped if not run/no failures."""