From fa83b865a2cfd21d7e63eedb206c1e07c8178965 Mon Sep 17 00:00:00 2001
From: Daniel Pauls <daniel1708.de+git@gmail.com>
Date: Wed, 30 Mar 2022 15:41:25 +0200
Subject: [PATCH 1/2] libexpr: Throw the correct error in toJSON

BaseError::addTrace(...) returns a BaseError, but we want to
throw a TypeError instead.

Fixes #6336.
---
 src/libexpr/value-to-json.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index 517da4c01..7b35abca2 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -84,7 +84,8 @@ void printValueAsJSON(EvalState & state, bool strict,
                 .msg = hintfmt("cannot convert %1% to JSON", showType(v)),
                 .errPos = v.determinePos(pos)
             });
-            throw e.addTrace(pos, hintfmt("message for the trace"));
+            e.addTrace(pos, hintfmt("message for the trace"));
+            throw e;
     }
 }
 

From 629edd43ba7550be835660fe5df3b65cc4a515c7 Mon Sep 17 00:00:00 2001
From: Daniel Pauls <daniel1708.de+git@gmail.com>
Date: Wed, 30 Mar 2022 17:30:47 +0200
Subject: [PATCH 2/2] libutil: Change return value of addTrace to void

The return value of BaseError::addTrace(...) is never used and
error-prone as subclasses calling it will return a BaseError instead of
the subclass.
This commit changes its return value to be void.
---
 src/libutil/error.cc | 3 +--
 src/libutil/error.hh | 6 +++---
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index b2dfb35b2..02bc5caa5 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -9,10 +9,9 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-BaseError & BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
+void BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
 {
     err.traces.push_front(Trace { .pos = e, .hint = hint });
-    return *this;
 }
 
 // c++ std::exception descendants must have a 'const char* what()' function.
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 93b789f0b..6a757f9ad 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -175,12 +175,12 @@ public:
     const ErrorInfo & info() const { calcWhat(); return err; }
 
     template<typename... Args>
-    BaseError & addTrace(std::optional<ErrPos> e, const std::string & fs, const Args & ... args)
+    void addTrace(std::optional<ErrPos> e, const std::string & fs, const Args & ... args)
     {
-        return addTrace(e, hintfmt(fs, args...));
+        addTrace(e, hintfmt(fs, args...));
     }
 
-    BaseError & addTrace(std::optional<ErrPos> e, hintformat hint);
+    void addTrace(std::optional<ErrPos> e, hintformat hint);
 
     bool hasTrace() const { return !err.traces.empty(); }
 };