Select locked version compatible with the constraint

When selecting a preferred version for a lock (before solve)
if the constraint is a semver range, ensure the version
selected is compatible.

cmd/dep/root_analyzer.go

@@ -164,6 +164,15 @@ func lookupVersionForLockedProject(pi gps.ProjectIdentifier, c gps.Constraint, r
 	gps.SortPairedForUpgrade(versions) // Sort versions in asc order
 	for _, v := range versions {
 		if v.Revision() == rev {
+			// If the constraint is semver, make sure the version is acceptable.
+			// This prevents us from suggesting an incompatible version, which
+			// helps narrow the field when there are multiple matching versions.
+			if c != nil {
+				_, err := gps.NewSemverConstraint(c.String())
+				if err == nil && !c.Matches(v) {
+					continue
+				}
+			}
 			return v, nil
 		}
 	}

cmd/dep/root_analyzer_test.go

@@ -35,9 +35,8 @@ func TestLookupVersionForLockedProject_MatchRevisionToTag(t *testing.T) {
 	defer sm.Release()
 
 	pi := gps.ProjectIdentifier{ProjectRoot: gps.ProjectRoot("github.com/sdboyer/deptest")}
-	c, _ := gps.NewSemverConstraint("^0.8.1")
 	rev := gps.Revision("ff2948a2ac8f538c4ecd55962e919d1e13e74baf")
-	v, err := lookupVersionForLockedProject(pi, c, rev, sm)
+	v, err := lookupVersionForLockedProject(pi, nil, rev, sm)
 	h.Must(err)
 
 	wantV := "v1.0.0"
@@ -47,6 +46,29 @@ func TestLookupVersionForLockedProject_MatchRevisionToTag(t *testing.T) {
 	}
 }
 
+func TestLookupVersionForLockedProject_MatchRevisionToMultipleTags(t *testing.T) {
+	h := test.NewHelper(t)
+	defer h.Cleanup()
+
+	ctx := newTestContext(h)
+	sm, err := ctx.SourceManager()
+	h.Must(err)
+	defer sm.Release()
+
+	pi := gps.ProjectIdentifier{ProjectRoot: gps.ProjectRoot("github.com/sdboyer/deptest")}
+	// Both 0.8.0 and 1.0.0 use the same rev, force dep to pick the lower version
+	c, _ := gps.NewSemverConstraint("<1.0.0")
+	rev := gps.Revision("ff2948a2ac8f538c4ecd55962e919d1e13e74baf")
+	v, err := lookupVersionForLockedProject(pi, c, rev, sm)
+	h.Must(err)
+
+	wantV := "v0.8.0"
+	gotV := v.String()
+	if gotV != wantV {
+		t.Fatalf("Expected the locked version to satisfy the manifest's semver constraint: wanted '%s', got '%s'", wantV, gotV)
+	}
+}
+
 func TestLookupVersionForLockedProject_FallbackToConstraint(t *testing.T) {
 	h := test.NewHelper(t)
 	defer h.Cleanup()