search: provide better error messages for why a package wasn't selected

This provides messages that better explain why a cps file could not be
found, whether there isn't one named in that path, or if there's a
version mismatch, or dependency that is missing.

src/cps/search.cpp

@@ -11,6 +11,7 @@
 #include "cps/version.hpp"
 
 #include <fmt/core.h>
+#include <fmt/ranges.h>
 #include <tl/expected.hpp>
 
 #include <algorithm>
@@ -27,6 +28,8 @@ namespace cps::search {
 
     namespace {
 
+        using version::to_string;
+
         /// @brief A CPS file, along with the components in that CPS file to
         /// load
         class Dependency {
@@ -222,10 +225,12 @@ namespace cps::search {
         tl::expected<std::shared_ptr<Node>, std::string>
         build_node(std::string_view name, const loader::Requirement & requirements, NodeFactory factory, Env env) {
             const std::vector<fs::path> paths = CPS_TRY(find_paths(name, env));
+            std::vector<std::string> errors{};
             for (auto && path : paths) {
 
                 auto maybe_node = factory.get(name, path);
                 if (!maybe_node) {
+                    errors.emplace_back(fmt::format("No CPS file for {} in path {}", name, path.string()));
                     continue;
                 }
                 auto node = maybe_node.value();
@@ -242,26 +247,38 @@ namespace cps::search {
                     // > If not provided, the CPS will not satisfy any request for
                     // > a specific version of the package.
                     if (!p.version) {
+                        errors.emplace_back(
+                            fmt::format("Tried {}, which does not specify a version, but the user requires version {}",
+                                        path.string(), requirements.version.value()));
                         continue;
                     }
                     if (version::compare(p.version.value(), version::Operator::lt, requirements.version.value(),
                                          p.version_schema)) {
+                        errors.emplace_back(fmt::format(
+                            "{} has a version of {}, which is less than the required {}, using the schema {}",
+                            path.string(), p.version.value(), requirements.version.value(),
+                            to_string(p.version_schema)));
                         continue;
                     }
                 }
 
                 if (!std::all_of(requirements.components.begin(), requirements.components.end(),
                                  [p](const std::string & c) { return p.components.find(c) != p.components.end(); })) {
+                    // TODO: more fine grained error message
+                    errors.emplace_back(fmt::format("{} does not implement all of the required components '{}'",
+                                                    path.string(), fmt::join(requirements.components, ", ")));
                     continue;
                 }
 
                 std::vector<std::shared_ptr<Node>> found;
                 found.reserve(p.require.size());
                 for (auto && [n, r] : p.require) {
                     auto && child = build_node(n, r, factory, env);
+
                     if (child) {
                         found.emplace_back(child.value());
                     } else {
+                        errors.emplace_back(child.error());
                         break;
                     }
                 }
@@ -273,7 +290,7 @@ namespace cps::search {
                 return node;
             }
 
-            return tl::unexpected(fmt::format("Could not find a dependency to satisfy {}", name));
+            return tl::unexpected(fmt::format("{}:\n  {}", name, fmt::join(errors, "\n  ")));
         }
 
         tl::expected<std::shared_ptr<Node>, std::string> build_node(std::string_view name,

tests/cases/cps-config.toml

@@ -82,7 +82,8 @@ expected = "-I/something -I/opt/include"
 name = "Requires version, but version not set"
 cps = "needs-version"
 args = ["flags", "--modversion", "--print-errors", "--errors-to-stdout"]
-expected = "Could not find a dependency to satisfy needs-version"
+expected = "Tried /.*/cps-files/lib/cps/multiple-components.cps, which does not specify a version, but the user requires version 1.0"
+re = true
 returncode = 1
 
 [[case]]

tests/cases/pkg-config-compat.toml

@@ -50,5 +50,6 @@ expected = "-I/usr/local/include -I/opt/include"
 name = "Requires version, but version not set"
 cps = "needs-version"
 args = ["pkg-config", "--modversion", "--print-errors", "--errors-to-stdout"]
-expected = "Could not find a dependency to satisfy needs-version"
+expected = "Tried /.*/cps-files/lib/cps/multiple-components.cps, which does not specify a version, but the user requires version 1.0"
 returncode = 1
+re = true

tests/cps-files/lib/cps/has-compat-version.cps

@@ -0,0 +1,15 @@
+{
+    "name": "has-compat-version",
+    "cps_version": "0.12.0",
+    "version": "1.0.700344",
+    "compat_version": "1.0.0",
+    "components": {
+        "default": {
+            "type": "archive",
+            "location": "/usr/lib/libfoo.a"
+        }
+    },
+    "default_components": [
+        "default"
+    ]
+}