cgen: fix infix op when handling comptime selector (#19691)

vlib/v/gen/c/infix.v

@@ -96,8 +96,18 @@ fn (mut g Gen) infix_expr_arrow_op(node ast.InfixExpr) {
 
 // infix_expr_eq_op generates code for `==` and `!=`
 fn (mut g Gen) infix_expr_eq_op(node ast.InfixExpr) {
-	left := g.unwrap(node.left_type)
-	right := g.unwrap(node.right_type)
+	left_type := if node.left is ast.ComptimeSelector {
+		g.get_comptime_var_type(node.left)
+	} else {
+		node.left_type
+	}
+	right_type := if node.right is ast.ComptimeSelector {
+		g.get_comptime_var_type(node.right)
+	} else {
+		node.right_type
+	}
+	left := g.unwrap(left_type)
+	right := g.unwrap(right_type)
 	mut has_defined_eq_operator := false
 	mut eq_operator_expects_ptr := false
 	if m := g.table.find_method(left.sym, '==') {
@@ -110,7 +120,7 @@ fn (mut g Gen) infix_expr_eq_op(node ast.InfixExpr) {
 		g.gen_plain_infix_expr(node)
 		return
 	}
-	is_none_check := node.left_type.has_flag(.option) && node.right is ast.None
+	is_none_check := left_type.has_flag(.option) && node.right is ast.None
 	if is_none_check {
 		g.gen_is_none_check(node)
 	} else if (left.typ.is_ptr() && right.typ.is_int())

vlib/v/tests/comptime_eq_test.v

@@ -0,0 +1,34 @@
+struct Abc {
+	id     int
+	name   string
+	letter string
+}
+
+fn join[T](mut old T, new T) {
+	$if T is $struct {
+		default := T{}
+		$for field in T.fields {
+			if new.$(field.name) != default.$(field.name) {
+				old.$(field.name) = new.$(field.name)
+			}
+		}
+	}
+}
+
+fn test_main() {
+	mut a := Abc{
+		name: 'Peter'
+		letter: 'a'
+	}
+	b := Abc{
+		id: 1
+		letter: 'b'
+	}
+	join(mut a, b)
+
+	assert a == Abc{
+		name: 'Peter'
+		id: 1
+		letter: 'b'
+	}
+}