cgen: allow == to be user defined if != is auto generated and vice versa (#8286)

vlib/v/gen/cgen.v

@@ -3171,13 +3171,42 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 		}
 		g.expr(node.right)
 		g.write(')')
-	} else if node.op in [.eq, .ne] && left_sym.kind == .struct_ && right_sym.kind == .struct_
-		&& has_eq_overloaded&& has_ne_overloaded {
-		ptr_typ := g.gen_struct_equality_fn(left_type)
-		if node.op == .eq {
-			g.write('${ptr_typ}_struct_eq(')
-		} else if node.op == .ne {
-			g.write('!${ptr_typ}_struct_eq(')
+	} else if node.op in [.eq, .ne] && left_sym.kind == .struct_ && right_sym.kind == .struct_ {
+		if has_eq_overloaded && !has_ne_overloaded {
+			// Define `!=` as negation of Autogenerated `==`
+			styp := g.typ(left_type)
+			if node.op == .eq {
+				g.write('!${styp}__ne(')
+			} else if node.op == .ne {
+				g.write('${styp}__ne(')
+			}
+		}
+		if !has_eq_overloaded && has_ne_overloaded {
+			// Define `==` as negation of Autogenerated `!=`
+			styp := g.typ(left_type)
+			if node.op == .ne {
+				g.write('!${styp}__eq(')
+			} else if node.op == .eq {
+				g.write('${styp}__eq(')
+			}
+		}
+		if !has_eq_overloaded && !has_ne_overloaded {
+			// Overload both User defined `==` and `!=`
+			styp := g.typ(left_type)
+			if node.op == .eq {
+				g.write('${styp}__eq(')
+			} else if node.op == .ne {
+				g.write('${styp}__ne(')
+			}
+		}
+		if has_eq_overloaded && has_ne_overloaded {
+			// Auto generate both `==` and `!=`
+			ptr_typ := g.gen_struct_equality_fn(left_type)
+			if node.op == .eq {
+				g.write('${ptr_typ}_struct_eq(')
+			} else if node.op == .ne {
+				g.write('!${ptr_typ}_struct_eq(')
+			}
 		}
 		if node.left_type.is_ptr() {
 			g.write('*')

vlib/v/tests/struct_eq_op_only_test.v

@@ -0,0 +1,15 @@
+struct User {
+	name string
+	num int
+}
+
+fn (u User) == (u1 User) bool {
+	return u.name == u1.name
+}
+
+fn test_eq_op() {
+	u1 := User{'Joe', 23}
+	u2 := User{'Joe', 24}
+	assert u1 == u2
+	assert (u1 != u2) == false
+}

vlib/v/tests/sturct_ne_op_only_test.v

@@ -0,0 +1,15 @@
+struct User {
+	name string
+	num int
+}
+
+fn (u User) != (u1 User) bool {
+	return u.num != u1.num
+}
+
+fn test_eq_op() {
+	u1 := User{'Joe', 23}
+	u2 := User{'Joe', 24}
+	assert u1 != u2
+	assert (u1 == u2) == false
+}