From c141d34592abb9f3c296d94fdc7107e11f7c0b1d Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Fri, 19 Dec 2025 09:52:24 -0300
Subject: [PATCH] cgen: fix fnptr indirections declaration (fix #25940)
 (#25987)

---
 vlib/v/checker/fn.v                                |  5 ++++-
 vlib/v/checker/tests/fn_ptr_undereferenced_err.out |  6 ++++++
 vlib/v/checker/tests/fn_ptr_undereferenced_err.vv  | 11 +++++++++++
 vlib/v/gen/c/assign.v                              |  2 +-
 vlib/v/gen/c/cgen.v                                |  3 +++
 vlib/v/gen/c/fn.v                                  | 11 ++++++++---
 vlib/v/gen/c/for.v                                 |  2 +-
 vlib/v/gen/c/if.v                                  |  9 +++++++--
 vlib/v/gen/c/testdata/assign_fn_addr.c.must_have   |  4 ++--
 vlib/v/tests/fns/anon_with_prefix_test.v           |  4 ++--
 vlib/v/tests/fns/fn_ptr_deref_test.v               | 14 ++++++++++++++
 vlib/v/tests/fns/iter_alias_fn_test.v              |  3 ++-
 vlib/v/tests/options/option_fn_guard_test.v        | 10 ++++++----
 13 files changed, 67 insertions(+), 17 deletions(-)
 create mode 100644 vlib/v/checker/tests/fn_ptr_undereferenced_err.out
 create mode 100644 vlib/v/checker/tests/fn_ptr_undereferenced_err.vv
 create mode 100644 vlib/v/tests/fns/fn_ptr_deref_test.v

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 63cddd97b..8025e8d1d 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1276,7 +1276,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 	}
 	// check for arg (var) of fn type
 	if !found {
-		mut typ := 0
+		mut typ := ast.no_type
 		if mut obj := node.scope.find(node.name) {
 			match mut obj {
 				ast.GlobalField {
@@ -1300,6 +1300,9 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 					}
 					node.is_fn_var = true
 					node.fn_var_type = typ
+					if typ.nr_muls() > 0 {
+						c.error('function pointer must be undereferenced first', node.pos)
+					}
 				}
 				else {}
 			}
diff --git a/vlib/v/checker/tests/fn_ptr_undereferenced_err.out b/vlib/v/checker/tests/fn_ptr_undereferenced_err.out
new file mode 100644
index 000000000..0e390b95f
--- /dev/null
+++ b/vlib/v/checker/tests/fn_ptr_undereferenced_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/fn_ptr_undereferenced_err.vv:10:2: error: function pointer must be undereferenced first
+    8 |     ref := &f
+    9 |     deref := ref 
+   10 |     deref()
+      |     ~~~~~~~
+   11 | }
diff --git a/vlib/v/checker/tests/fn_ptr_undereferenced_err.vv b/vlib/v/checker/tests/fn_ptr_undereferenced_err.vv
new file mode 100644
index 000000000..c24f12956
--- /dev/null
+++ b/vlib/v/checker/tests/fn_ptr_undereferenced_err.vv
@@ -0,0 +1,11 @@
+module main
+
+fn f_a() {
+}
+
+fn main() {
+	f := f_a
+	ref := &f
+	deref := ref
+	deref()
+}
diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 9e42d56b4..4ae99c533 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -799,7 +799,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					ret_styp := g.styp(g.unwrap_generic(val_type))
 					g.write('${ret_styp} ${fn_name}')
 				} else {
-					g.write_fntype_decl(fn_name, right_sym.info)
+					g.write_fntype_decl(fn_name, right_sym.info, var_type.nr_muls())
 				}
 			} else {
 				if is_decl {
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index b4c27027b..75d34d1b7 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -5747,6 +5747,8 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 				g.expr(node.expr)
 				g.write('), sizeof(${expr_styp})),._typ=${u32(expr_typ)}})')
 			} else {
+				old_inside_assign_fn_var := g.inside_assign_fn_var
+				g.inside_assign_fn_var = g.table.final_sym(expr_type).kind == .function
 				g.write('(')
 				if node.expr is ast.Ident {
 					if !node.typ.is_ptr() && node.expr_type.is_ptr() && node.expr.obj is ast.Var
@@ -5766,6 +5768,7 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 					g.write('(${expr_styp})')
 				}
 				g.expr(node.expr)
+				g.inside_assign_fn_var = old_inside_assign_fn_var
 				g.write('))')
 			}
 		}
diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index 2c6a13556..200369f70 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1080,8 +1080,13 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 					unwrapped_styp = unwrapped_styp[3..]
 				}
 				if node.is_return_used {
+					is_fn := g.table.final_sym(unwrapped_typ).kind == .function
 					// return value is used, so we need to write the unwrapped temporary var
-					g.write('\n ${cur_line}(*(${unwrapped_styp}*)${tmp_opt}.data)')
+					if is_fn && unwrapped_typ.nr_muls() > 0 {
+						g.write('\n ${cur_line}((${unwrapped_styp}*)${tmp_opt}.data)')
+					} else {
+						g.write('\n ${cur_line}(*(${unwrapped_styp}*)${tmp_opt}.data)')
+					}
 				} else {
 					g.write('\n ${cur_line}')
 				}
@@ -3051,7 +3056,7 @@ fn call_convention_attribute(cconvention string, is_cc_msvc bool) string {
 	return if is_cc_msvc { '__${cconvention} ' } else { '__attribute__((${cconvention})) ' }
 }
 
-fn (mut g Gen) write_fntype_decl(fn_name string, info ast.FnType) {
+fn (mut g Gen) write_fntype_decl(fn_name string, info ast.FnType, nr_muls int) {
 	ret_styp := g.styp(info.func.return_type)
 	mut call_conv := ''
 	mut msvc_call_conv := ''
@@ -3072,7 +3077,7 @@ fn (mut g Gen) write_fntype_decl(fn_name string, info ast.FnType) {
 	} else {
 		''
 	}
-	g.write('${ret_styp} (${msvc_call_conv}*${fn_name}) (')
+	g.write('${ret_styp} (${msvc_call_conv}${'*'.repeat(nr_muls + 1)}${fn_name}) (')
 	def_pos := g.definitions.len
 	g.fn_decl_params(info.func.params, unsafe { nil }, false, false)
 	g.definitions.go_back(g.definitions.len - def_pos)
diff --git a/vlib/v/gen/c/for.v b/vlib/v/gen/c/for.v
index 59a2a9a30..e216527c3 100644
--- a/vlib/v/gen/c/for.v
+++ b/vlib/v/gen/c/for.v
@@ -562,7 +562,7 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 				g.write('\tmemcpy(${val}, ${t_var}.data, sizeof(${val_styp}));')
 			} else {
 				if ret_sym.info is ast.FnType {
-					g.write_fntype_decl(val, ret_sym.info)
+					g.write_fntype_decl(val, ret_sym.info, 0)
 					g.writeln(' = **(${val_styp}**)&${t_var}.data;')
 				} else {
 					g.writeln('\t${val_styp} ${val} = *(${val_styp}*)${t_var}.data;')
diff --git a/vlib/v/gen/c/if.v b/vlib/v/gen/c/if.v
index 3eb7ed23b..90f94c9ea 100644
--- a/vlib/v/gen/c/if.v
+++ b/vlib/v/gen/c/if.v
@@ -402,11 +402,16 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 							}
 							expr_sym := g.table.sym(branch.cond.expr_type)
 							if expr_sym.info is ast.FnType {
-								g.write_fntype_decl(left_var_name, expr_sym.info)
+								g.write_fntype_decl(left_var_name, expr_sym.info, branch.cond.expr_type.nr_muls())
+								if branch.cond.expr_type.nr_muls() == 0 {
+									g.writeln(' = *(${base_type}*)${var_name}${dot_or_ptr}data;')
+								} else {
+									g.writeln(' = (${base_type}*)${var_name}${dot_or_ptr}data;')
+								}
 							} else {
 								g.write('\t${base_type} ${left_var_name}')
+								g.writeln(' = *(${base_type}*)${var_name}${dot_or_ptr}data;')
 							}
-							g.writeln(' = *(${base_type}*)${var_name}${dot_or_ptr}data;')
 						}
 					} else if branch.cond.vars.len > 1 {
 						sym := g.table.sym(branch.cond.expr_type)
diff --git a/vlib/v/gen/c/testdata/assign_fn_addr.c.must_have b/vlib/v/gen/c/testdata/assign_fn_addr.c.must_have
index 4433816d0..6fa13df4c 100644
--- a/vlib/v/gen/c/testdata/assign_fn_addr.c.must_have
+++ b/vlib/v/gen/c/testdata/assign_fn_addr.c.must_have
@@ -1,2 +1,2 @@
-string (*cb2) (void) = &cb1;
-string (*cb3) (void) = &cb2;
\ No newline at end of file
+string (**cb2) (void) = &cb1;
+string (***cb3) (void) = &cb2;
\ No newline at end of file
diff --git a/vlib/v/tests/fns/anon_with_prefix_test.v b/vlib/v/tests/fns/anon_with_prefix_test.v
index e7070ef7e..386588839 100644
--- a/vlib/v/tests/fns/anon_with_prefix_test.v
+++ b/vlib/v/tests/fns/anon_with_prefix_test.v
@@ -5,13 +5,13 @@ struct TestA {
 }
 
 fn test_main() {
-	func := &fn () int {
+	func := fn () int {
 		arr := []TestA{}
 		return arr.len
 	}
 	assert func() == 0
 
-	func2 := &fn () int {
+	func2 := fn () int {
 		arr := []TestA{}
 		return arr.len
 	}
diff --git a/vlib/v/tests/fns/fn_ptr_deref_test.v b/vlib/v/tests/fns/fn_ptr_deref_test.v
new file mode 100644
index 000000000..a715bbef9
--- /dev/null
+++ b/vlib/v/tests/fns/fn_ptr_deref_test.v
@@ -0,0 +1,14 @@
+module main
+
+fn f_a() {
+}
+
+fn test_main() {
+	f := f_a
+	f()
+	assert voidptr(f) == voidptr(f_a)
+	ref := &f
+	deref := *ref
+	assert voidptr(deref) == voidptr(f_a)
+	deref()
+}
diff --git a/vlib/v/tests/fns/iter_alias_fn_test.v b/vlib/v/tests/fns/iter_alias_fn_test.v
index 4cab67ff2..84d6ef88e 100644
--- a/vlib/v/tests/fns/iter_alias_fn_test.v
+++ b/vlib/v/tests/fns/iter_alias_fn_test.v
@@ -18,7 +18,8 @@ fn test_main() {
 	mut c := 0
 	for p in Iter{} {
 		println(p)
-		p()
+		w := *p
+		w()
 		c += 1
 	}
 	assert c == 1
diff --git a/vlib/v/tests/options/option_fn_guard_test.v b/vlib/v/tests/options/option_fn_guard_test.v
index 89c52db1c..3b227744e 100644
--- a/vlib/v/tests/options/option_fn_guard_test.v
+++ b/vlib/v/tests/options/option_fn_guard_test.v
@@ -10,12 +10,14 @@ fn get_ptr() ?&MyFn {
 
 fn test_main() {
 	if p := get_ptr() {
-		assert voidptr(p) == voidptr(hello)
+		assert voidptr(*p) == voidptr(hello)
 		dump(p)
-		p()
+		w := *p
+		w()
 	}
 
 	p2 := get_ptr() or { return }
-	p2()
-	assert voidptr(p2) == voidptr(hello)
+	w2 := *p2
+	w2()
+	assert voidptr(*p2) == voidptr(hello)
 }
-- 
2.39.5