From 605a7c4091f0c960c977293a270cf8eceea7ac81 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Mon, 23 Jun 2025 23:47:19 +0200
Subject: [PATCH] Improve error message on pointer diff #2239.

---
 releasenotes.md                 |  3 ++-
 src/compiler/sema_casts.c       | 13 +++++++++++++
 test/test_suite/cast/ptrdiff.c3 |  8 ++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 test/test_suite/cast/ptrdiff.c3

diff --git a/releasenotes.md b/releasenotes.md
index f011644cc..a2914a20c 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -25,7 +25,8 @@
 - Support distrinct types as the base type of bitstructs. #2218
 - Add hash::sha512 module to stdlib. #2227
 - Compile time type assignment (eg `$Foo = int`) is no longer an expression.
-- Add `@allow_deprecated` attribute to functions to selectively allow deprecated declarations #2223
+- Add `@allow_deprecated` attribute to functions to selectively allow deprecated declarations #2223.
+- Improve error message on pointer diff #2239.
 
 ### Fixes
 - `-2147483648`, MIN literals work correctly.
diff --git a/src/compiler/sema_casts.c b/src/compiler/sema_casts.c
index 4ed297b84..d948cbd5e 100644
--- a/src/compiler/sema_casts.c
+++ b/src/compiler/sema_casts.c
@@ -1275,6 +1275,14 @@ static bool rule_int_to_float(CastContext *cc, bool is_explicit, bool is_silent)
 	return true;
 }
 
+INLINE bool expr_is_pointer_diff(Expr *expr)
+{
+	if (expr->type != type_isz) return false;
+	if (expr->expr_kind != EXPR_BINARY) return false;
+	if (expr->binary_expr.operator != BINARYOP_SUB) return false;
+	return type_is_pointer(exprptr(expr->binary_expr.left)->type) && type_is_pointer(exprptr(expr->binary_expr.right)->type);
+}
+
 static bool rule_widen_narrow(CastContext *cc, bool is_explicit, bool is_silent)
 {
 	if (is_explicit) return true;
@@ -1327,6 +1335,11 @@ static bool rule_widen_narrow(CastContext *cc, bool is_explicit, bool is_silent)
 				expr_const_to_error_string(&expr->const_expr),
 				type_quoted_error_string(cc->to_type));
 		}
+		if (expr_is_pointer_diff(expr))
+		{
+			RETURN_CAST_ERROR(expr, "A pointer diff has the type %s which cannot implicitly be converted to %s. You can use an explicit cast if you know the conversion is safe.",
+					   type_quoted_error_string(expr->type), type_quoted_error_string(cc->to_type));
+		}
 		RETURN_CAST_ERROR(expr, "%s cannot implicitly be converted to %s, but you may use a cast.",
 		           type_quoted_error_string(expr->type), type_quoted_error_string(cc->to_type));
 	}
diff --git a/test/test_suite/cast/ptrdiff.c3 b/test/test_suite/cast/ptrdiff.c3
new file mode 100644
index 000000000..3818107ea
--- /dev/null
+++ b/test/test_suite/cast/ptrdiff.c3
@@ -0,0 +1,8 @@
+import std;
+fn int main()
+{
+	void* a;
+	void* b;
+	int z = a - b; // #error: A pointer diff has the type
+	return 0;
+}
\ No newline at end of file