comptime: fix $match with fn type (#25271)

vlib/v/checker/match.v

@@ -145,10 +145,18 @@ fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 								node.cond)
 							c_str = '${expr} == ${c.table.type_to_str(branch_type)}'
 						} else {
+							is_function := c.table.final_sym(node.cond_type).kind == .function
+							if !is_function {
 								// $match a { $int {}
 								comptime_match_branch_result = c.check_compatible_types(node.cond_type,
 									'${node.cond}', expr)
 								c_str = '${c.table.type_to_str(node.cond_type)} == ${expr}'
+							} else {
+								// $match T { FnType {} }
+								branch_type := c.get_expr_type(expr)
+								comptime_match_branch_result = c.table.type_to_str(node.cond_type) == c.table.type_to_str(branch_type)
+								c_str = '${comptime_match_branch_result} == true'
+							}
 						}
 						if comptime_match_branch_result {
 							break

vlib/v/tests/comptime/comptime_match_fntype_test.v

@@ -0,0 +1,31 @@
+type FN1 = fn (a int) int
+
+type FN2 = fn (a int, b int) int
+
+type FN3 = FN1 | FN2
+
+fn invoke[T](cb T) int {
+	$match T {
+		FN1 {
+			return cb(1)
+		}
+		FN2 {
+			return cb(1, 2)
+		}
+		$else {
+			return 0
+		}
+	}
+}
+
+fn test_main() {
+	ret1 := invoke(fn (a int) int {
+		return a
+	})
+	assert ret1 == 1
+
+	ret2 := invoke(fn (a int, b int) int {
+		return a + b
+	})
+	assert ret2 == 3
+}