From 97edd610b08ab5f51106c22ae01b0fa05acccbea Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Tue, 16 Sep 2025 14:37:15 -0300
Subject: [PATCH] v: implement comptime regular func params traversing (support
 `$if T is $function { $for param in T.params {`) (#25322)

---
 vlib/builtin/builtin.v                       |  6 ++--
 vlib/v/checker/comptime.v                    |  6 ++--
 vlib/v/gen/c/comptime.v                      | 14 ++++-----
 vlib/v/markused/walker.v                     |  2 +-
 vlib/v/parser/comptime.v                     |  2 +-
 vlib/v/tests/comptime/comptime_params_test.v | 30 ++++++++++++++++++++
 6 files changed, 46 insertions(+), 14 deletions(-)
 create mode 100644 vlib/v/tests/comptime/comptime_params_test.v

diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index 3a8347f81..f8916a027 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -88,8 +88,8 @@ fn __print_assert_failure(i &VAssertMetaInfo) {
 	}
 }
 
-// MethodParam holds type information for function and/or method arguments.
-pub struct MethodParam {
+// FunctionParam holds type information for function and/or method arguments.
+pub struct FunctionParam {
 pub:
 	typ  int
 	name string
@@ -100,7 +100,7 @@ pub struct FunctionData {
 pub:
 	name        string
 	attrs       []string
-	args        []MethodParam
+	args        []FunctionParam
 	return_type int
 	typ         int
 }
diff --git a/vlib/v/checker/comptime.v b/vlib/v/checker/comptime.v
index e8d99be0d..e9c0bac40 100644
--- a/vlib/v/checker/comptime.v
+++ b/vlib/v/checker/comptime.v
@@ -364,11 +364,13 @@ fn (mut c Checker) comptime_for(mut node ast.ComptimeFor) {
 				node.typ_pos)
 			return
 		}
-		method := c.comptime.comptime_for_method
+
+		func := if sym.info is ast.FnType { &sym.info.func } else { c.comptime.comptime_for_method }
+		params := if func.is_method { func.params[1..] } else { func.params }
 		// example: fn (mut d MyStruct) add(x int, y int) string
 		// `d` is params[0], `x` is params[1], `y` is params[2]
 		// so we at least has one param (`d`) for method
-		for param in method.params[1..] {
+		for param in params {
 			c.push_new_comptime_info()
 			c.comptime.inside_comptime_for = true
 			c.comptime.comptime_for_method_param_var = node.val_var
diff --git a/vlib/v/gen/c/comptime.v b/vlib/v/gen/c/comptime.v
index 3c811f5a1..f5a6dc810 100644
--- a/vlib/v/gen/c/comptime.v
+++ b/vlib/v/gen/c/comptime.v
@@ -649,10 +649,10 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 			}
 			if method.params.len < 2 {
 				// 0 or 1 (the receiver) args
-				g.writeln('\t${node.val_var}.args = builtin____new_array_with_default(0, 0, sizeof(MethodParam), 0);')
+				g.writeln('\t${node.val_var}.args = builtin____new_array_with_default(0, 0, sizeof(FunctionParam), 0);')
 			} else {
 				len := method.params.len - 1
-				g.write('\t${node.val_var}.args = builtin__new_array_from_c_array(${len}, ${len}, sizeof(MethodParam), _MOV((MethodParam[${len}]){')
+				g.write('\t${node.val_var}.args = builtin__new_array_from_c_array(${len}, ${len}, sizeof(FunctionParam), _MOV((FunctionParam[${len}]){')
 				// Skip receiver arg
 				for j, arg in method.params[1..] {
 					typ := arg.typ.idx()
@@ -841,12 +841,12 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 			}
 		}
 	} else if node.kind == .params {
-		method := g.comptime.comptime_for_method
-
-		if method.params.len > 0 {
-			g.writeln('\tMethodParam ${node.val_var} = {0};')
+		func := if sym.info is ast.FnType { &sym.info.func } else { g.comptime.comptime_for_method }
+		if func.params.len > 0 {
+			g.writeln('\tFunctionParam ${node.val_var} = {0};')
 		}
-		for param in method.params[1..] {
+		params := if func.is_method { func.params[1..] } else { func.params }
+		for param in params {
 			g.push_new_comptime_info()
 			g.comptime.inside_comptime_for = true
 			g.comptime.comptime_for_method_param_var = node.val_var
diff --git a/vlib/v/markused/walker.v b/vlib/v/markused/walker.v
index 263d3e65e..ce63ea569 100644
--- a/vlib/v/markused/walker.v
+++ b/vlib/v/markused/walker.v
@@ -1354,7 +1354,7 @@ fn (mut w Walker) mark_resource_dependencies() {
 		w.mark_by_sym_name('FunctionData')
 	}
 	if w.uses_ct_params {
-		w.mark_by_sym_name('MethodParam')
+		w.mark_by_sym_name('FunctionParam')
 	}
 	if w.uses_ct_values {
 		w.mark_by_sym_name('EnumData')
diff --git a/vlib/v/parser/comptime.v b/vlib/v/parser/comptime.v
index 67ca8dfd4..51f74bbc3 100644
--- a/vlib/v/parser/comptime.v
+++ b/vlib/v/parser/comptime.v
@@ -423,7 +423,7 @@ fn (mut p Parser) comptime_for() ast.ComptimeFor {
 		'params' {
 			p.scope.register(ast.Var{
 				name: val_var
-				typ:  p.table.find_type('MethodParam')
+				typ:  p.table.find_type('FunctionParam')
 				pos:  var_pos
 			})
 			kind = .params
diff --git a/vlib/v/tests/comptime/comptime_params_test.v b/vlib/v/tests/comptime/comptime_params_test.v
new file mode 100644
index 000000000..7fff5bdbd
--- /dev/null
+++ b/vlib/v/tests/comptime/comptime_params_test.v
@@ -0,0 +1,30 @@
+fn demo(a int, b string) {
+}
+
+fn g[T](cb T) {
+	mut params := []FunctionParam{}
+	$if T is $function {
+		$for param in T.params {
+			params << param
+		}
+	}
+	assert params.len == 2
+	assert params[0].name == 'a'
+	assert params[0].typ == 8
+	assert params[1].name == 'b'
+	assert params[1].typ == 21
+}
+
+fn test_main() {
+	g(demo)
+
+	mut params := []FunctionParam{}
+	$for param in demo.params {
+		params << param
+	}
+	assert params.len == 2
+	assert params[0].name == 'a'
+	assert params[0].typ == 8
+	assert params[1].name == 'b'
+	assert params[1].typ == 21
+}
-- 
2.39.5