chore: ExecutorGroup returns errors

Author: vinniefalco

docs/modules/ROOT/pages/design-notes.adoc

@@ -39,3 +39,12 @@ We probably don't want to store any javadocs for namespaces either.
 The AST visitor and metadata all use forward slashes to represent file
 pathnames, even on Windows. This is so the generated reference documentation
 does not vary based on the platform.
+
+== Exceptions
+
+In functions which cannot return an error, such as work submitted to a thread
+pool or in a constructor, the implementation usually throws `Error`. These
+are caught and reported, and the process exits gracefully. If any exception
+is thrown which is not derived from `Error`, then it should not be caught.
+The uncaught exception handler should print a stack trace and exit the process
+immediately.

include/mrdox/Support/Error.hpp

@@ -121,6 +121,14 @@ class [[nodiscard]] MRDOX_DECL
         return text_;
     }
 
+    /** Return true if this equals other.
+    */
+    bool
+    operator==(Error const& other) const noexcept
+    {
+        return text_ == other.text_;
+    }
+
     /** Return a null-terminated error string.
     */
     char const*
@@ -147,6 +155,16 @@ success() noexcept
 } // mrdox
 } // clang
 
+template<>
+struct std::hash<::clang::mrdox::Error>
+{
+    std::size_t operator()(
+        ::clang::mrdox::Error const& err) const noexcept
+    {
+        return std::hash<std::string_view>()(err.message());
+    }
+};
+
 //------------------------------------------------
 
 template<>

include/mrdox/Support/ExecutorGroup.hpp

@@ -13,6 +13,7 @@
 
 #include <mrdox/Platform.hpp>
 #include <mrdox/Support/any_callable.hpp>
+#include <mrdox/Support/Error.hpp>
 #include <mrdox/Support/ThreadPool.hpp>
 #include <deque>
 #include <memory>
@@ -53,11 +54,17 @@ class MRDOX_DECL
     ExecutorGroupBase(ExecutorGroupBase&&) noexcept;
 
     /** Block until all work has completed.
+
+        @return Zero or more errors which were
+        thrown from submitted work.
     */
-    void
+    [[nodiscard]]
+    std::vector<Error>
     wait() noexcept;
 };
 
+//------------------------------------------------
+
 /** A set of execution agents for performing concurrent work.
 */
 template<class Agent>

include/mrdox/Support/Report.hpp

@@ -80,6 +80,14 @@ reportError(
         err.message()));
 }
 
+template<class Range>
+void
+reportErrors(Range const& errors)
+{
+    for(auto const& err : errors)
+        reportError(err.message());
+}
+
 /** Report a warning to the console.
 
     @param text The message contents. A newline