From 5d41166750e0f05da436bf30c7a642749f7a71c5 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Mon, 10 Aug 2026 10:12:48 +0100
Subject: Prevent singleton struct from being generic

---
 .../compiler/ast/untyped/UntypedStructDefinitionNode.java      |  7 +++++++
 .../zwobble/hobgoblin/compiler/typechecker/TypeChecker.java    |  4 ++++
 .../errors/SingletonStructCannotHaveTypeParamsError.java       | 10 ++++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SingletonStructCannotHaveTypeParamsError.java

(limited to 'src/main/java')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedStructDefinitionNode.java b/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedStructDefinitionNode.java
index 9742068..29cb885 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedStructDefinitionNode.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/ast/untyped/UntypedStructDefinitionNode.java
@@ -60,5 +60,12 @@ public record UntypedStructDefinitionNode(
     }
 
     // Custom area start: org.zwobble.hobgoblin.compiler.ast.untyped.UntypedStructDefinitionNode body
+    public boolean isGeneric() {
+        return this.typeParams.isPresent();
+    }
+
+    public boolean isSingleton() {
+        return this.fields.isEmpty();
+    }
     // Custom area end: org.zwobble.hobgoblin.compiler.ast.untyped.UntypedStructDefinitionNode body
 }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
index 63fc357..95ce840 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -227,6 +227,10 @@ public class TypeChecker {
         UntypedStructDefinitionNode untyped,
         TypeCheckerNamespaceContext context
     ) {
+        if (untyped.isSingleton() && untyped.isGeneric()) {
+            throw new SingletonStructCannotHaveTypeParamsError(untyped.source());
+        }
+
         var metaType = lookupTypeLevelValue(untyped.name(), untyped.source(), context);
         var typeOrConstructor = switch (metaType) {
             case SimpleStructType type ->
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SingletonStructCannotHaveTypeParamsError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SingletonStructCannotHaveTypeParamsError.java
new file mode 100644
index 0000000..a630646
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SingletonStructCannotHaveTypeParamsError.java
@@ -0,0 +1,10 @@
+package org.zwobble.hobgoblin.compiler.typechecker.errors;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+
+public class SingletonStructCannotHaveTypeParamsError extends SourceError {
+    public SingletonStructCannotHaveTypeParamsError(Source source) {
+        super("singleton struct cannot have type params", source);
+    }
+}
-- 
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