gps: apply transitive constraints

gps/identifier.go

@@ -215,6 +215,8 @@ type completeDep struct {
 	workingConstraint
 	// The specific packages required from the ProjectDep
 	pl []string
+	// Flags the constraint as being defined on an indirect/transitive dependency
+	isTransitive bool
 }
 
 // dependency represents an incomplete edge in the depgraph. It has a

gps/solve_bimodal_test.go

@@ -132,6 +132,26 @@ var bimodalFixtures = map[string]bimodalFixture{
 			"b 1.0.0",
 		),
 	},
+	"transitive constraint": {
+		ds: []depspec{
+			dsp(mkDepspec("root 1.0.0", "foo 1.0.0"),
+				pkg("root", "foo"),
+			),
+			dsp(mkDepspec("foo 1.0.0", "bar 1.0.0", "baz =1.0.0"),
+				pkg("foo", "bar"),
+			),
+			dsp(mkDepspec("bar 1.0.0", "baz >=1.0.0"),
+				pkg("bar", "baz"),
+			),
+			dsp(mkDepspec("baz 1.0.1"), pkg("baz")),
+			dsp(mkDepspec("baz 1.0.0"), pkg("baz")),
+		},
+		r: mksolution(
+			"foo 1.0.0",
+			"bar 1.0.0",
+			"baz 1.0.0",
+		),
+	},
 	// Constraints apply only if the project that declares them has a
 	// reachable import
 	"constraints activated by import": {

gps/solver.go

@@ -645,6 +645,13 @@ func (s *solver) selectRoot() error {
 		}
 
 		s.sel.pushDep(dependency{depender: awp.a, dep: dep})
+
+		if dep.isTransitive {
+			// Do not add transitive dependencies to the queue immediately,
+			// instead wait until they are directly used
+			continue
+		}
+
 		// Add all to unselected queue
 		heap.Push(s.unsel, bimodalIdentifier{id: dep.Ident, pl: dep.pl, fromRoot: true})
 	}
@@ -792,6 +799,17 @@ func (s *solver) intersectConstraintsWithImports(deps []workingConstraint, reach
 		}
 	}
 
+	// Include transitive constraints, flagging them as transitive for special handling later on
+	for _, wc := range deps {
+		root := wc.Ident.ProjectRoot
+		if _, ok := dmap[root]; !ok {
+			dmap[root] = completeDep{
+				workingConstraint: wc, // TODO(carolynvs): deal with overrides and package prefix-foo (not sure if that's needed?)
+				isTransitive:      true,
+			}
+		}
+	}
+
 	// Dump all the deps from the map into the expected return slice
 	cdeps := make([]completeDep, 0, len(dmap))
 	for _, cdep := range dmap {