[WIP] Throw a better exception on invalid $ref targets

See: sourcemeta/jsonschema#624
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/compiler/compile.cc

@@ -342,9 +342,8 @@ auto compile(const sourcemeta::core::JSON &schema,
         entry.type != sourcemeta::core::SchemaFrame::LocationType::Resource &&
         entry.type != sourcemeta::core::SchemaFrame::LocationType::Anchor) {
       assert(reference_pointer != nullptr);
-      throw sourcemeta::core::SchemaReferenceError(
-          destination_uri, to_pointer(*reference_pointer),
-          "The target of the reference is not a valid schema");
+      throw CompilerReferenceTargetNotSchemaError(
+          destination_uri, to_pointer(*reference_pointer));
     }
 
     auto subschema{sourcemeta::core::get(context.root, entry.pointer)};
@@ -441,12 +440,7 @@ auto compile(const Context &context, const SchemaContext &schema_context,
   const auto &entry{context.frame.locations().at(
       {sourcemeta::core::SchemaReferenceType::Static, destination})};
   const auto &new_schema{get(context.root, entry.pointer)};
-
-  if (!is_schema(new_schema)) {
-    throw sourcemeta::core::SchemaReferenceError(
-        destination, to_pointer(schema_context.relative_pointer),
-        "The target of the reference is not a valid schema");
-  }
+  assert(is_schema(new_schema));
 
   const sourcemeta::core::WeakPointer destination_pointer{
       dynamic_context.keyword.empty()

src/compiler/include/sourcemeta/blaze/compiler_error.h

@@ -8,9 +8,10 @@
 #include <sourcemeta/core/jsonpointer.h>
 #include <sourcemeta/core/uri.h>
 
-#include <exception> // std::exception
-#include <string>    // std::string
-#include <utility>   // std::move
+#include <exception>   // std::exception
+#include <string>      // std::string
+#include <string_view> // std::string_view
+#include <utility>     // std::move
 
 namespace sourcemeta::blaze {
 
@@ -48,6 +49,34 @@ class SOURCEMETA_BLAZE_COMPILER_EXPORT CompilerError : public std::exception {
   std::string message_;
 };
 
+/// @ingroup jsonschema
+/// An error that represents a reference target that is not a valid schema
+class SOURCEMETA_BLAZE_COMPILER_EXPORT CompilerReferenceTargetNotSchemaError
+    : public std::exception {
+public:
+  CompilerReferenceTargetNotSchemaError(
+      const std::string_view identifier,
+      sourcemeta::core::Pointer schema_location)
+      : identifier_{identifier}, schema_location_{std::move(schema_location)} {}
+
+  [[nodiscard]] auto what() const noexcept -> const char * override {
+    return "The target of the reference is not a valid schema";
+  }
+
+  [[nodiscard]] auto identifier() const noexcept -> const std::string & {
+    return this->identifier_;
+  }
+
+  [[nodiscard]] auto location() const noexcept
+      -> const sourcemeta::core::Pointer & {
+    return this->schema_location_;
+  }
+
+private:
+  std::string identifier_;
+  sourcemeta::core::Pointer schema_location_;
+};
+
 #if defined(_MSC_VER)
 #pragma warning(default : 4251 4275)
 #endif

test/evaluator/evaluator_draft4_test.cc

@@ -9087,7 +9087,8 @@ TEST(Evaluator_draft4, reference_from_unknown_keyword) {
     sourcemeta::blaze::compile(schema, sourcemeta::core::schema_walker,
                                sourcemeta::core::schema_resolver,
                                sourcemeta::blaze::default_schema_compiler);
-  } catch (const sourcemeta::core::SchemaReferenceError &error) {
+  } catch (
+      const sourcemeta::blaze::CompilerReferenceTargetNotSchemaError &error) {
     EXPECT_EQ(error.identifier(), "#/$defs/bar");
     EXPECT_EQ(error.location(),
               sourcemeta::core::Pointer({"properties", "foo", "$ref"}));
@@ -9163,7 +9164,8 @@ TEST(Evaluator_draft4, ref_to_non_schema) {
                                sourcemeta::core::schema_resolver,
                                sourcemeta::blaze::default_schema_compiler);
     FAIL() << "The compile function was expected to throw";
-  } catch (const sourcemeta::core::SchemaReferenceError &error) {
+  } catch (
+      const sourcemeta::blaze::CompilerReferenceTargetNotSchemaError &error) {
     EXPECT_EQ(error.identifier(), "#/definitions/array/items");
     EXPECT_EQ(sourcemeta::core::to_string(error.location()),
               "/additionalProperties/$ref");

test/evaluator/evaluator_draft6_test.cc

@@ -1952,7 +1952,8 @@ TEST(Evaluator_draft6, reference_from_unknown_keyword) {
     sourcemeta::blaze::compile(schema, sourcemeta::core::schema_walker,
                                sourcemeta::core::schema_resolver,
                                sourcemeta::blaze::default_schema_compiler);
-  } catch (const sourcemeta::core::SchemaReferenceError &error) {
+  } catch (
+      const sourcemeta::blaze::CompilerReferenceTargetNotSchemaError &error) {
     EXPECT_EQ(error.identifier(), "#/$defs/bar");
     EXPECT_EQ(error.location(),
               sourcemeta::core::Pointer({"properties", "foo", "$ref"}));

test/evaluator/evaluator_draft7_test.cc

@@ -536,7 +536,8 @@ TEST(Evaluator_draft7, reference_from_unknown_keyword) {
     sourcemeta::blaze::compile(schema, sourcemeta::core::schema_walker,
                                sourcemeta::core::schema_resolver,
                                sourcemeta::blaze::default_schema_compiler);
-  } catch (const sourcemeta::core::SchemaReferenceError &error) {
+  } catch (
+      const sourcemeta::blaze::CompilerReferenceTargetNotSchemaError &error) {
     EXPECT_EQ(error.identifier(), "#/$defs/bar");
     EXPECT_EQ(error.location(),
               sourcemeta::core::Pointer({"properties", "foo", "$ref"}));