From ae04167c321fa30120f6cf6a8e78cb4540601cd5 Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Thu, 17 Oct 2024 15:48:44 -0300
Subject: [PATCH] v: allow checking of method comptime signature with `is`
 operator (fix #8843) (#22549)

---
 vlib/v/ast/types.v                            |  2 +-
 vlib/v/checker/if.v                           | 17 +++++++++-
 vlib/v/gen/c/comptime.v                       | 18 ++++++++++-
 .../comptime/comptime_call_is_check_test.v    | 31 +++++++++++++++++++
 4 files changed, 65 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/tests/comptime/comptime_call_is_check_test.v

diff --git a/vlib/v/ast/types.v b/vlib/v/ast/types.v
index fb7e8fa23..2c5148df3 100644
--- a/vlib/v/ast/types.v
+++ b/vlib/v/ast/types.v
@@ -1604,7 +1604,7 @@ pub fn (t &Table) fn_signature_using_aliases(func &Fn, import_aliases map[string
 		sb.write_string(func.name)
 	}
 	sb.write_string('(')
-	start := int(opts.skip_receiver)
+	start := int(func.is_method && opts.skip_receiver)
 	for i in start .. func.params.len {
 		if i != start {
 			sb.write_string(', ')
diff --git a/vlib/v/checker/if.v b/vlib/v/checker/if.v
index 13b3aa029..e267d6728 100644
--- a/vlib/v/checker/if.v
+++ b/vlib/v/checker/if.v
@@ -191,7 +191,22 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 									checked_type = c.unwrap_generic(var.smartcasts.last())
 								}
 							}
-							skip_state = c.check_compatible_types(checked_type, right as ast.TypeNode)
+							if sym.info is ast.FnType
+								&& c.comptime.comptime_for_method_var == left.name {
+								skip_state = if c.table.fn_signature(sym.info.func,
+									skip_receiver: true
+									type_only:     true
+								) == c.table.fn_signature(c.comptime.comptime_for_method,
+									skip_receiver: true
+									type_only:     true
+								) {
+									.eval
+								} else {
+									.skip
+								}
+							} else {
+								skip_state = c.check_compatible_types(checked_type, right as ast.TypeNode)
+							}
 						}
 					}
 					if branch.cond.op == .not_is && skip_state != .unknown {
diff --git a/vlib/v/gen/c/comptime.v b/vlib/v/gen/c/comptime.v
index 579c7cac6..9cf381769 100644
--- a/vlib/v/gen/c/comptime.v
+++ b/vlib/v/gen/c/comptime.v
@@ -574,7 +574,23 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) (bool, bool) {
 									return !is_true, true
 								}
 							}
-							if cond.op == .key_is {
+							if got_sym.info is ast.FnType && cond.left is ast.Ident
+								&& g.comptime.comptime_for_method_var == cond.left.name {
+								is_compatible := g.table.fn_signature(got_sym.info.func,
+									skip_receiver: true
+									type_only:     true
+								) == g.table.fn_signature(g.comptime.comptime_for_method,
+									skip_receiver: true
+									type_only:     true
+								)
+								if cond.op == .key_is {
+									g.write(int(is_compatible).str())
+									return is_compatible, true
+								} else {
+									g.write(int(!is_compatible).str())
+									return !is_compatible, true
+								}
+							} else if cond.op == .key_is {
 								g.write('${exp_type.idx()} == ${got_type.idx()} && ${exp_type.has_flag(.option)} == ${got_type.has_flag(.option)}')
 								return exp_type == got_type, true
 							} else {
diff --git a/vlib/v/tests/comptime/comptime_call_is_check_test.v b/vlib/v/tests/comptime/comptime_call_is_check_test.v
new file mode 100644
index 000000000..e4d62eaae
--- /dev/null
+++ b/vlib/v/tests/comptime/comptime_call_is_check_test.v
@@ -0,0 +1,31 @@
+module main
+
+struct Struct1 {
+	num f64
+}
+
+pub type DoThingFunc = fn (f32) bool
+
+fn (mut s Struct1) do_thing(a f32) bool {
+	return false
+}
+
+fn register[T]() {
+	obj := T{}
+	mut called := false
+	$for method in T.methods {
+		$if method !is DoThingFunc {
+			println('method ${method.name} is not do thing')
+			assert false
+		} $else {
+			println('method ${method.name} is do thing')
+			assert obj.$method(1.23) == false
+			called = true
+		}
+	}
+	assert called
+}
+
+fn test_main() {
+	register[Struct1]()
+}
-- 
2.39.5