eval: Fix crash on missing printValue tBlackhole case

Fixes #8119

src/libexpr/eval.cc

@@ -173,7 +173,17 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
     case tFloat:
         str << fpoint;
         break;
+    case tBlackhole:
+        // Although we know for sure that it's going to be an infinite recursion
+        // when this value is accessed _in the current context_, it's likely
+        // that the user will misinterpret a simpler «infinite recursion» output
+        // as a definitive statement about the value, while in fact it may be
+        // a valid value after `builtins.trace` and perhaps some other steps
+        // have completed.
+        str << "«potential infinite recursion»";
+        break;
     default:
+        printError("Nix evaluator internal error: Value::print(): invalid value type %1%", internalType);
         abort();
     }
 }

tests/lang.sh

@@ -5,12 +5,19 @@ export NIX_REMOTE=dummy://
 export NIX_STORE_DIR=/nix/store
 
 nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>&1 | grepQuiet Hello
+nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>/dev/null | grepQuiet 123
 nix-instantiate --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1
 nix-instantiate --trace-verbose --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grepQuiet Hello
 nix-instantiate --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grepQuietInverse Hello
 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1 | grepQuietInverse Hello
 expectStderr 1 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" (throw "Foo")' | grepQuiet Hello
 
+nix-instantiate --eval -E 'let x = builtins.trace { x = x; } true; in x' \
+  2>&1 | grepQuiet -E 'trace: { x = «potential infinite recursion»; }'
+
+nix-instantiate --eval -E 'let x = { repeating = x; tracing = builtins.trace x true; }; in x.tracing'\
+  2>&1 | grepQuiet -F 'trace: { repeating = «repeated»; tracing = «potential infinite recursion»; }'
+
 set +x
 
 fail=0