Libraries dependencies: output error if no valid solution found

commands/lib/resolve_deps.go

@@ -42,6 +42,20 @@ func LibraryResolveDependencies(ctx context.Context, req *rpc.LibraryResolveDepe
 
 	// Resolve all dependencies...
 	deps := lm.Index.ResolveDependencies(reqLibRelease)
+
+	// If no solution has been found
+	if len(deps) == 0 {
+		// Check if there is a problem with the first level deps
+		for _, directDep := range reqLibRelease.GetDependencies() {
+			if _, ok := lm.Index.Libraries[directDep.GetName()]; !ok {
+				return nil, fmt.Errorf("dependency '%s' is not available", directDep.GetName())
+			}
+		}
+
+		// Otherwise there is no possible solution, the depends field has an invalid formula
+		return nil, fmt.Errorf("no valid solution found")
+	}
+
 	res := []*rpc.LibraryDependencyStatus{}
 	for _, dep := range deps {
 		// ...and add information on currently installed versions of the libraries

test/test_lib.py

@@ -60,6 +60,13 @@ def test_install(run_command):
     assert run_command("lib install {}".format(" ".join(libs)))
     assert run_command("lib install {}".format(" ".join(libs)))
 
+    # Test failing-install of library with wrong dependency
+    # (https://github.com/arduino/arduino-cli/issues/534)
+    result = run_command("lib install [email protected]")
+    assert (
+        "Error resolving dependencies for [email protected]: dependency 'MD_MAX72xx' is not available"
+        in result.stderr
+    )
 
 def test_update_index(run_command):
     result = run_command("lib update-index")