From 7cdb1ce9eb2ad2364dc50466f8f36cdf2c6a8534 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Sat, 5 Jul 2025 20:40:55 +0200
Subject: [PATCH] Catch accidental `foo == BAR;` where `foo = BAR;` was most
 likely intended. #2274

---
 releasenotes.md                               |  1 +
 src/compiler/sema_expr.c                      |  9 ++++++++-
 .../expressions/discarded_binary.c3           | 19 +++++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 test/test_suite/expressions/discarded_binary.c3

diff --git a/releasenotes.md b/releasenotes.md
index 2e27b438a..a10860fe4 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -9,6 +9,7 @@
 - `$typeof` may return a compile time type.
 - Improved error messages on missing qualifier on enum value. #2260
 - Add `--echo-prefix` to edit the prefix with `$echo` statements. Supports {FILE} and {LINE}
+- Catch accidental `foo == BAR;` where `foo = BAR;` was most likely intended. #2274
 
 ### Fixes
 - mkdir/rmdir would not work properly with substring paths on non-windows platforms.
diff --git a/src/compiler/sema_expr.c b/src/compiler/sema_expr.c
index 052dc00bf..255b98f18 100644
--- a/src/compiler/sema_expr.c
+++ b/src/compiler/sema_expr.c
@@ -11288,7 +11288,14 @@ bool sema_expr_check_discard(SemaContext *context, Expr *expr)
 		if (expr->call_expr.has_optional_arg) goto ERROR_ARGS;
 		return true;
 	}
-	if (!IS_OPTIONAL(expr)) return true;
+	if (!IS_OPTIONAL(expr))
+	{
+		if (expr->expr_kind == EXPR_BINARY && expr->binary_expr.operator == BINARYOP_EQ)
+		{
+			RETURN_SEMA_ERROR(expr, "This equals check was discarded, which isn't allowed. You can assign it to a variable or explicitly ignore it with a void cast '(void)' if this is what you want.");
+		}
+		return true;
+	}
 	RETURN_SEMA_ERROR(expr, "An optional value was discarded, you can assign it to a variable, ignore it with a void cast '(void)', rethrow on optional with '!' or panic '!!' to avoid this error.");
 ERROR_ARGS:
 	RETURN_SEMA_ERROR(expr, "The result of this call is optional due to its argument(s). This optional result may not be implicitly discarded. Please assign it to a variable, ignore it with '(void)', rethrow with '!' or panic with '!!'.");
diff --git a/test/test_suite/expressions/discarded_binary.c3 b/test/test_suite/expressions/discarded_binary.c3
new file mode 100644
index 000000000..f1f32751a
--- /dev/null
+++ b/test/test_suite/expressions/discarded_binary.c3
@@ -0,0 +1,19 @@
+enum Foo
+{
+	COMMENT
+}
+
+fn int main()
+{
+	Foo state = COMMENT;
+	char c;
+	switch (c)
+	{
+		case '#':
+			(void)(state == COMMENT);
+			state == COMMENT; // #error: This equals check was discarded
+			int a = 0;
+			a == 0; // #error: This equals check was discarded
+	}
+	return 0;
+}
\ No newline at end of file