Resolved crash when mods make 2 techs require each other

core/src/com/unciv/models/ruleset/Ruleset.kt

@@ -751,9 +751,9 @@ class Ruleset {
                         ?: return emptySet()
                     val techHashSet = HashSet<String>()
                     techHashSet += technology.prerequisites
+                    prereqsHashMap[technologyName] = techHashSet
                     for (prerequisite in technology.prerequisites)
                         techHashSet += getPrereqTree(prerequisite)
-                    prereqsHashMap[technologyName] = techHashSet
                     return techHashSet
                 }
 
@@ -762,6 +762,9 @@ class Ruleset {
                     lines.add("No need to add $prereq as a prerequisite of ${tech.name} - it is already implicit from the other prerequisites!",
                         RulesetErrorSeverity.Warning)
                 }
+
+                if (getPrereqTree(prereq).contains(tech.name))
+                    lines += "Techs ${tech.name} and $prereq require each other!"
             }
             if (tech.era() !in eras)
                 lines += "Unknown era ${tech.era()} referenced in column of tech ${tech.name}"

docs/Modders/uniques.md

@@ -1724,6 +1724,11 @@ Simple unique parameters are explained by mouseover. Complex parameters are expl
 
 	Applicable to: Conditional
 
+??? example  "<upon discovering [tech]>"
+	Example: "<upon discovering [Agriculture]>"
+
+	Applicable to: Conditional
+
 ??? example  "<after adopting [policy]>"
 	Example: "<after adopting [Oligarchy]>"
 

tests/src/com/unciv/uniques/UniqueErrorTests.kt

@@ -0,0 +1,26 @@
+package com.unciv.uniques
+
+import com.unciv.models.ruleset.RulesetCache
+import com.unciv.testing.GdxTestRunner
+import org.junit.Test
+import org.junit.runner.RunWith
+
+
+@RunWith(GdxTestRunner::class)
+class UniqueErrorTests {
+    @Test
+    fun testCodependantTechs() {
+        RulesetCache.loadRulesets()
+        val ruleset = RulesetCache.getVanillaRuleset()
+
+        // Create a prerequisite loop
+        val techWithPrerequisites = ruleset.technologies.values.first { it.prerequisites.isNotEmpty() }
+        val prereq = ruleset.technologies[techWithPrerequisites.prerequisites.first()]!!
+        prereq.prerequisites.add(techWithPrerequisites.name)
+        ruleset.modOptions.isBaseRuleset = true
+
+        // Check mod links and ensure we don't get a crash, instead we get errors
+        val errors = ruleset.checkModLinks(false)
+        assert(errors.isNotOK())
+    }
+}