https://github.com/bfgroup/b2/commit/708353cdeb6006757e7c6971283efb53f718ae25
https://bugs.gentoo.org/970330
Fixed patch path to accommodate $S.

From: zyk2507 <93830642+zyk2507@users.noreply.github.com>
Date: Sun, 1 Feb 2026 12:36:17 +0800
Subject: [PATCH] Fix crash in var_defines when define string is empty (#535)
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* Fix crash in var_defines when define string is empty

b2 aborts in var_defines on empty define

* Redo no-value error fix to keep previous flow.

Still avoid errors for empty variables, but do so while not early continue, and to keep the use of string_view.

---------

Co-authored-by: René Ferdinand Rivera Morell <grafikrobot@gmail.com>
--- a/engine/variable.cpp
+++ b/engine/variable.cpp
@@ -78,8 +78,14 @@ void var_defines(struct module_t * module, const char * const * e, int preproces
 	for (; *e; ++e)
 	{
 		::b2::string_view def(*e);
-		::b2::string_view var(def.begin(), def.find('='));
-		::b2::string_view val(def.begin() + var.size() + 1);
+		::b2::string_view var = def;
+		::b2::string_view val;
+		auto eq = def.find('=');
+		if (eq != ::b2::string_view::npos)
+		{
+			var = ::b2::string_view(def.begin(), eq);
+			val = ::b2::string_view(def.begin() + eq + 1);
+		}
 		b2::jam::variable jam_var { module,
 			std::string { var.begin(), var.end() }.c_str() };
 		// std::printf(">> var_defines: *e = %s\n", *e);
@@ -89,7 +95,7 @@ void var_defines(struct module_t * module, const char * const * e, int preproces
 		// }
 
 		// No value to set var with.
-		if (var.size() == def.size()) continue;
+		if (val.empty()) continue;
 
 		// Skip pre-processing, to just set the raw value.
 		if (preprocess == 0)