From 2efab47ee393d97eb0faf8d75e7f11796310aad6 Mon Sep 17 00:00:00 2001
From: Kim Shrier <gitkim@westryn.net>
Date: Tue, 12 May 2026 05:47:10 -0600
Subject: [PATCH] ast: fix and test for list of generic bug (#27141)

---
 vlib/v/ast/table.v                            | 12 +++-
 ...eric_method_slice_two_param_sibling_test.v | 71 +++++++++++++++++++
 2 files changed, 81 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/generics/generic_method_slice_two_param_sibling_test.v

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index ccebd507b..ef6f94952 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -3775,7 +3775,14 @@ fn (mut t Table) unwrap_generic_type_ex_with_depth(typ Type, generic_names []str
 			// If concrete types still contain generic parameters (e.g. Vec3[U] where U
 			// is unresolved), don't create a partially-resolved struct entry. The struct
 			// will be properly instantiated when all type parameters are known.
-			if final_concrete_types.any(it.has_flag(.generic)) {
+			//
+			// Structural check via generic_type_names catches entries like `[]T`
+			// whose `.generic` Type-flag has been cleared but whose type structure
+			// still references an unresolved generic parameter. Without this check,
+			// such entries reach register_sym below and leak into codegen as
+			// `Array_T` / placeholder-typed struct fields.
+			if final_concrete_types.any(it.has_flag(.generic))
+				|| final_concrete_types.any(t.generic_type_names(it).len > 0) {
 				return typ
 			}
 		}
@@ -4532,7 +4539,8 @@ pub fn (mut t Table) generic_insts_to_concrete() {
 		}
 		if sym.info is Struct {
 			if sym.info.concrete_types.len > 0 && sym.info.parent_type.has_flag(.generic)
-				&& !sym.info.concrete_types.any(it.has_flag(.generic)) {
+				&& !sym.info.concrete_types.any(it.has_flag(.generic))
+				&& !sym.info.concrete_types.any(t.generic_type_names(it).len > 0) {
 				parent_sym := t.sym(sym.info.parent_type)
 				for method in parent_sym.methods {
 					if method.generic_names.len == sym.info.concrete_types.len
diff --git a/vlib/v/tests/generics/generic_method_slice_two_param_sibling_test.v b/vlib/v/tests/generics/generic_method_slice_two_param_sibling_test.v
new file mode 100644
index 000000000..c66ab025a
--- /dev/null
+++ b/vlib/v/tests/generics/generic_method_slice_two_param_sibling_test.v
@@ -0,0 +1,71 @@
+module main
+
+struct Pair[A, B] {
+	a A
+	b B
+}
+
+struct Container[T] {
+	value T
+}
+
+pub fn (c Container[T]) get() !T {
+	return c.value
+}
+
+fn make_pair[A, B](a A, b B) Container[Pair[A, B]] {
+	return Container[Pair[A, B]]{
+		value: Pair[A, B]{
+			a: a
+			b: b
+		}
+	}
+}
+
+fn make_list[T](items []T) Container[[]T] {
+	return Container[[]T]{
+		value: items
+	}
+}
+
+fn test_pair_container_get_returns_pair() {
+	c := make_pair[int, int](7, 11)
+	p := c.get() or {
+		assert false, 'get() on Container[Pair[int, int]] errored: ${err}'
+		return
+	}
+	assert p.a == 7, 'expected p.a == 7, got ${p.a}'
+	assert p.b == 11, 'expected p.b == 11, got ${p.b}'
+}
+
+fn test_list_container_get_returns_slice() {
+	c := make_list[int]([1, 2, 3])
+	lst := c.get() or {
+		assert false, 'get() on Container[[]int] errored: ${err}'
+		return
+	}
+	assert lst.len == 3, 'expected length 3, got ${lst.len}'
+	assert lst[0] == 1
+	assert lst[1] == 2
+	assert lst[2] == 3
+}
+
+fn test_sibling_factories_return_independent_shapes() {
+	pc := make_pair[string, int]('hello', 42)
+	lc := make_list[string](['a', 'b'])
+
+	p := pc.get() or {
+		assert false, 'pair get failed: ${err}'
+		return
+	}
+	assert p.a == 'hello'
+	assert p.b == 42
+
+	lst := lc.get() or {
+		assert false, 'list get failed: ${err}'
+		return
+	}
+	assert lst.len == 2
+	assert lst[0] == 'a'
+	assert lst[1] == 'b'
+}
-- 
2.39.5