From 14db530ac0532a0df29fd6c30e23f736ee7aef92 Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Sat, 5 Apr 2025 21:57:20 +0200
Subject: [PATCH] libexpr: restrict EvalErrorBuilder to building EvalErrors

Change-Id: Iadd0fee4c12c5d0b6ee74dccbf8785c829d20d95
---
 lix/libexpr/eval-error.cc | 20 ++++++++++----------
 lix/libexpr/eval-error.hh |  8 +++++---
 lix/libexpr/eval.hh       |  5 +++--
 3 files changed, 18 insertions(+), 15 deletions(-)

diff --git a/lix/libexpr/eval-error.cc b/lix/libexpr/eval-error.cc
index c8254a7be..516ea3b90 100644
--- a/lix/libexpr/eval-error.cc
+++ b/lix/libexpr/eval-error.cc
@@ -5,27 +5,27 @@
 
 namespace nix {
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::withExitStatus(unsigned int exitStatus) &&
 {
     error->withExitStatus(exitStatus);
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::atPos(PosIdx pos) &&
 {
     error->err.pos = positions[pos];
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::atPos(Value & value, PosIdx fallback) &&
 {
     return std::move(*this).atPos(value.determinePos(fallback));
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::withTrace(PosIdx pos, const std::string_view text) &&
 {
     error->err.traces.push_front(
@@ -33,14 +33,14 @@ EvalErrorBuilder<T> EvalErrorBuilder<T>::withTrace(PosIdx pos, const std::string
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::withSuggestions(Suggestions & s) &&
 {
     error->err.suggestions = s;
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::withFrame(const Env & env, const Expr & expr) &&
 {
     if (debug) {
@@ -55,14 +55,14 @@ EvalErrorBuilder<T> EvalErrorBuilder<T>::withFrame(const Env & env, const Expr &
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 EvalErrorBuilder<T> EvalErrorBuilder<T>::addTrace(PosIdx pos, HintFmt hint) &&
 {
     error->addTrace(positions[pos], hint);
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 template<typename... Args>
 EvalErrorBuilder<T>
 EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs) &&
@@ -72,7 +72,7 @@ EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const A
     return std::move(*this);
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 void EvalErrorBuilder<T>::debugThrow(NeverAsync) &&
 {
     if (debug) {
@@ -86,7 +86,7 @@ void EvalErrorBuilder<T>::debugThrow(NeverAsync) &&
     throw *error;
 }
 
-template<class T>
+template<std::derived_from<EvalError> T>
 void EvalErrorBuilder<T>::throw_() &&
 {
     throw *error;
diff --git a/lix/libexpr/eval-error.hh b/lix/libexpr/eval-error.hh
index 2d010293c..e8f2a6a5c 100644
--- a/lix/libexpr/eval-error.hh
+++ b/lix/libexpr/eval-error.hh
@@ -6,6 +6,7 @@
 #include "lix/libutil/types.hh"
 #include "lix/libexpr/pos-idx.hh"
 #include "lix/libexpr/pos-table.hh"
+#include <concepts>
 
 namespace nix {
 
@@ -15,13 +16,14 @@ struct Env;
 struct Expr;
 struct Value;
 
+class EvalError;
 class EvalState;
-template<class T>
+template<std::derived_from<EvalError> T>
 class EvalErrorBuilder;
 
 class EvalError : public Error
 {
-    template<class T>
+    template<std::derived_from<EvalError> T>
     friend class EvalErrorBuilder;
 
     std::shared_ptr<const DebugTrace> frame;
@@ -54,7 +56,7 @@ public:
     }
 };
 
-template<class T>
+template<std::derived_from<EvalError> T>
 class [[nodiscard]] EvalErrorBuilder final
 {
     const PosTable & positions;
diff --git a/lix/libexpr/eval.hh b/lix/libexpr/eval.hh
index 5b70fcbcb..1634fcd2d 100644
--- a/lix/libexpr/eval.hh
+++ b/lix/libexpr/eval.hh
@@ -18,6 +18,7 @@
 #include "lix/libexpr/repl-exit-status.hh"
 #include "lix/libutil/backed-string-view.hh"
 
+#include <concepts>
 #include <map>
 #include <optional>
 #include <unordered_map>
@@ -181,7 +182,7 @@ public:
     class TraceFrame
     {
         friend struct DebugState;
-        template<class T>
+        template<std::derived_from<EvalError> T>
         friend class EvalErrorBuilder;
 
         // holds both the data for this frame *and* a deleter that pulls this frame
@@ -356,7 +357,7 @@ struct EvalErrorContext
     const PosTable & positions;
     DebugState * debug;
 
-    template<class T, typename... Args>
+    template<std::derived_from<EvalError> T, typename... Args>
     [[gnu::noinline]]
     EvalErrorBuilder<T> make(const Args & ... args) {
         return EvalErrorBuilder<T>(positions, debug, args...);