From cb2d0e798ec633389b1620519a0b5104771d8d01 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Mon, 29 Sep 2025 01:59:38 +0200
Subject: [PATCH] Prevent `foo.bar = {}` when `bar` is a flexible array member.

---
 releasenotes.md                            |  1 +
 src/compiler/sema_expr.c                   | 10 +++++++++
 src/compiler/sema_internal.h               |  2 ++
 test/test_suite/struct/flex_more_errors.c3 | 25 ++++++++++++++++++++++
 4 files changed, 38 insertions(+)
 create mode 100644 test/test_suite/struct/flex_more_errors.c3

diff --git a/releasenotes.md b/releasenotes.md
index 39a4f1759..a0d47581c 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -41,6 +41,7 @@
 - Issue not correctly aborting compilation on recursive generics.
 - Crash during codegen when taking the typeid of an empty enum with associated values.
 - Assert when the binary doesn't get created and --run-once is used. #2502
+- Prevent `foo.bar = {}` when `bar` is a flexible array member.
 
 ### Stdlib changes
 - Added generic `InterfaceList` to store a list of values that implement a specific interface
diff --git a/src/compiler/sema_expr.c b/src/compiler/sema_expr.c
index 7b5bae818..07bc95915 100644
--- a/src/compiler/sema_expr.c
+++ b/src/compiler/sema_expr.c
@@ -6862,6 +6862,10 @@ static bool sema_expr_analyse_assign(SemaContext *context, Expr *expr, Expr *lef
 		default:
 			break;
 	}
+	if (left->type && left->type->type_kind == TYPE_FLEXIBLE_ARRAY)
+	{
+		RETURN_SEMA_ERROR(left, "You can't assign to a flexible array member, but you may index into it and mutate it that way.");
+	}
 
 	// 2. Check assignability
 	if (!sema_expr_check_assign(context, left, failed_ref)) return false;
@@ -7174,7 +7178,13 @@ static bool sema_expr_analyse_op_assign(SemaContext *context, Expr *expr, Expr *
 			break;
 	}
 
+	if (left->type->type_kind == TYPE_FLEXIBLE_ARRAY)
+	{
+		RETURN_SEMA_ERROR(left, "You can't assign to a flexible array member, but you may index into it and mutate it that way.");
+	}
+
 	// 2. Verify that the left side is assignable.
+
 	if (!sema_expr_check_assign(context, left, NULL)) return false;
 
 	Type *left_type_canonical = left->type->canonical;
diff --git a/src/compiler/sema_internal.h b/src/compiler/sema_internal.h
index 3c90a6bf9..80f1cf9f6 100644
--- a/src/compiler/sema_internal.h
+++ b/src/compiler/sema_internal.h
@@ -275,6 +275,8 @@ static inline StorageType sema_resolve_storage_type(SemaContext *context, Type *
 			if (!sema_analyse_decl(context, type->decl)) return false;
 			type = type->decl->distinct->type;
 			goto RETRY;
+		case TYPE_FLEXIBLE_ARRAY:
+			return STORAGE_UNKNOWN;
 		default:
 			return STORAGE_NORMAL;
 	}
diff --git a/test/test_suite/struct/flex_more_errors.c3 b/test/test_suite/struct/flex_more_errors.c3
new file mode 100644
index 000000000..485ce9c01
--- /dev/null
+++ b/test/test_suite/struct/flex_more_errors.c3
@@ -0,0 +1,25 @@
+module main;
+
+struct OwnedString
+{
+	usz len;
+	char[*] data;
+}
+
+fn void test1()
+{
+	OwnedString owned;
+	owned.data = {}; // #error: You can't assign to a flexible array member
+}
+
+fn void test2()
+{
+	$typefrom(OwnedString.data.typeid) x = 1; // #error:  'char[*]' has unknown size, and
+}
+
+fn int main()
+{
+	OwnedString owned;
+	owned.data[0] = 1;
+    return 0;
+}
\ No newline at end of file