From 588c5c4120ca1972dce19655c0556cf6a539b125 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 15:14:28 +0300
Subject: [PATCH] checker: delete the default zero value in v (fixes #23940)

---
 doc/docs.md                                   | 19 ++++++++++++++++++
 vlib/v/checker/checker.v                      | 18 ++++++++++++++---
 vlib/v/checker/tests/strict_map_index_err.out |  6 ++++++
 vlib/v/checker/tests/strict_map_index_err.vv  |  9 +++++++++
 vlib/v/parser/module.v                        |  1 +
 .../builtin_maps/map_get_anon_fn_value_test.v | 20 +++++++++++++++++++
 6 files changed, 70 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/checker/tests/strict_map_index_err.out
 create mode 100644 vlib/v/checker/tests/strict_map_index_err.vv

diff --git a/doc/docs.md b/doc/docs.md
index bb0ad8972..29f32b54c 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -1535,6 +1535,25 @@ mm := map[string]int{}
 val := mm['bad_key'] or { panic('key not found') }
 ```
 
+Modules can opt into stricter map indexing:
+
+```v
+@[strict_map_index]
+module main
+
+fn main() {
+	m := {
+		'abc': 'xyz'
+	}
+	value := m['abc'] or { panic('missing map key') }
+	if other := m['abc'] {
+		println(other)
+	}
+	// m['bad_key'] // compile error in `@[strict_map_index]` modules
+	println(value)
+}
+```
+
 You can also check, if a key is present, and get its value, if it was present, in one go:
 
 ```v
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index f3dd26acd..0579622bf 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -72,6 +72,7 @@ pub mut:
 	expected_expr_type          ast.Type // if/match is_expr: expected_type
 	mod                         string   // current module name
 	has_globals_in_module       bool     // true if the current module has @[has_globals] attribute
+	strict_map_index_in_module  bool     // true if the current module has @[strict_map_index] attribute
 	const_var                   &ast.ConstField = unsafe { nil } // the current constant, when checking const declarations
 	const_deps                  []string
 	const_names                 []string
@@ -662,10 +663,16 @@ pub fn (mut c Checker) change_current_file(file &ast.File) {
 	}
 	// Check if the current module has has_globals attribute
 	c.has_globals_in_module = false
+	c.strict_map_index_in_module = false
 	for attr in file.mod.attrs {
-		if attr.name == 'has_globals' {
-			c.has_globals_in_module = true
-			break
+		match attr.name {
+			'has_globals' {
+				c.has_globals_in_module = true
+			}
+			'strict_map_index' {
+				c.strict_map_index_in_module = true
+			}
+			else {}
 		}
 	}
 }
@@ -7923,6 +7930,11 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 				err := c.map_key_expected_msg(index_type, key_type, node.index, '${node.left}')
 				c.error('invalid key: ${err}', node.pos)
 			}
+			if c.strict_map_index_in_module && node.or_expr.kind == .absent && !node.is_setter
+				&& !c.inside_if_guard {
+				c.error('`@[strict_map_index]` requires handling missing map keys with `or {}` or `if value := map[key] {}`',
+					node.pos)
+			}
 			value_sym := c.table.sym(info.value_type)
 			if !node.is_setter && value_sym.kind == .sum_type && node.or_expr.kind == .absent
 				&& !c.inside_unsafe && !c.inside_if_guard {
diff --git a/vlib/v/checker/tests/strict_map_index_err.out b/vlib/v/checker/tests/strict_map_index_err.out
new file mode 100644
index 000000000..acc734f0b
--- /dev/null
+++ b/vlib/v/checker/tests/strict_map_index_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/strict_map_index_err.vv:8:7: error: `@[strict_map_index]` requires handling missing map keys with `or {}` or `if value := map[key] {}`
+    6 |         'abc': 'xyz'
+    7 |     }
+    8 |     _ = m['missing']
+      |          ~~~~~~~~~~~
+    9 | }
diff --git a/vlib/v/checker/tests/strict_map_index_err.vv b/vlib/v/checker/tests/strict_map_index_err.vv
new file mode 100644
index 000000000..8a24251b1
--- /dev/null
+++ b/vlib/v/checker/tests/strict_map_index_err.vv
@@ -0,0 +1,9 @@
+@[strict_map_index]
+module main
+
+fn main() {
+	m := {
+		'abc': 'xyz'
+	}
+	_ = m['missing']
+}
diff --git a/vlib/v/parser/module.v b/vlib/v/parser/module.v
index 0f6aaad5f..1747d6ab1 100644
--- a/vlib/v/parser/module.v
+++ b/vlib/v/parser/module.v
@@ -182,6 +182,7 @@ fn (mut p Parser) module_decl() ast.Module {
 				'has_globals' {
 					p.has_globals = true
 				}
+				'strict_map_index' {}
 				'translated' {
 					p.is_translated = true
 				}
diff --git a/vlib/v/tests/builtin_maps/map_get_anon_fn_value_test.v b/vlib/v/tests/builtin_maps/map_get_anon_fn_value_test.v
index 6b0c10249..028ed55ed 100644
--- a/vlib/v/tests/builtin_maps/map_get_anon_fn_value_test.v
+++ b/vlib/v/tests/builtin_maps/map_get_anon_fn_value_test.v
@@ -1,3 +1,6 @@
+@[strict_map_index]
+module main
+
 const numbers = {
 	'one': fn () int {
 		return 1
@@ -23,3 +26,20 @@ fn test_map_get_anon_fn_value() {
 	println(ret2)
 	assert ret2 == 2
 }
+
+fn test_map_get_anon_fn_value_if_guard() {
+	values := {
+		'one': 1
+	}
+	mut found := false
+	if value := values['one'] {
+		found = true
+		assert value == 1
+	}
+	assert found
+	mut missing_found := false
+	if _ := values['two'] {
+		missing_found = true
+	}
+	assert !missing_found
+}
-- 
2.39.5