From 9335a84eb35cbd58c71a559b397b38fd57697ea6 Mon Sep 17 00:00:00 2001
From: Swastik Baranwal <swstkbaranwal@gmail.com>
Date: Wed, 28 Jan 2026 01:30:10 +0530
Subject: [PATCH] parser, checker: check generic struct fields and
 initialisation (fix #26433) (fix #26436) (#26450)

---
 vlib/builtin/js/promise.js.v                     |  2 +-
 vlib/datatypes/doubly_linked_list.v              |  3 ++-
 vlib/v/checker/checker.v                         |  8 +++++---
 vlib/v/checker/fn.v                              | 10 +++++-----
 vlib/v/checker/struct.v                          |  4 ++--
 vlib/v/checker/tests/amod/amod.v                 |  4 ++++
 .../tests/modules/module_struct_noinit.out       | 16 +++++++++++-----
 .../modules/module_struct_noinit/src/main.v      |  3 +++
 .../tests/modules/module_struct_noinit/src/mod.v |  4 ++++
 .../v/checker/tests/struct_field_private_err.out | 13 ++++++++++---
 vlib/v/checker/tests/struct_field_private_err.vv |  4 ++++
 vlib/v/parser/fn.v                               |  2 +-
 vlib/v/parser/parse_type.v                       |  5 ++++-
 .../generics/modules/simplemodule/simplemodule.v |  1 +
 vlib/x/sessions/memory_store.v                   |  2 +-
 15 files changed, 58 insertions(+), 23 deletions(-)

diff --git a/vlib/builtin/js/promise.js.v b/vlib/builtin/js/promise.js.v
index 0d6cdc807..89a3f9656 100644
--- a/vlib/builtin/js/promise.js.v
+++ b/vlib/builtin/js/promise.js.v
@@ -14,7 +14,7 @@ pub fn JS.Promise.race(JS.Array) JS.Promise
 
 // Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.
 pub struct Promise[T] {
-mut:
+pub mut:
 	promise JS.Promise @[noinit]
 }
 
diff --git a/vlib/datatypes/doubly_linked_list.v b/vlib/datatypes/doubly_linked_list.v
index 70e5a413e..d7f068829 100644
--- a/vlib/datatypes/doubly_linked_list.v
+++ b/vlib/datatypes/doubly_linked_list.v
@@ -21,7 +21,8 @@ mut:
 	// of the list while iterating. TODO: use an option
 	// instead of a pointer to determine it is initialized.
 	iter &DoublyListIter[T] = unsafe { 0 }
-	len  int
+pub mut:
+	len int
 }
 
 // is_empty checks if the linked list is empty
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 02d7e556d..e3116b665 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -138,6 +138,7 @@ mut:
 	inside_decl_rhs                  bool
 	inside_if_guard                  bool // true inside the guard condition of `if x := opt() {}`
 	inside_assign                    bool
+	is_js_backend                    bool
 	// doing_line_info                  int    // a quick single file run when called with v -line-info (contains line nr to inspect)
 	// doing_line_path                  string // same, but stores the path being parsed
 	is_index_assign        bool
@@ -185,6 +186,7 @@ pub fn new_checker(table &ast.Table, pref_ &pref.Preferences) &Checker {
 	checker.checker_transformer.skip_array_transform = true
 	checker.type_resolver = type_resolver.TypeResolver.new(table, checker)
 	checker.comptime = &checker.type_resolver.info
+	checker.is_js_backend = checker.pref.backend.is_js()
 	return checker
 }
 
@@ -2815,7 +2817,7 @@ fn (mut c Checker) asm_stmt(mut stmt ast.AsmStmt) {
 		c.warn('inline assembly goto is not supported, it will most likely not work',
 			stmt.pos)
 	}
-	if c.pref.backend.is_js() {
+	if c.is_js_backend {
 		c.error('inline assembly is not supported in the js backend', stmt.pos)
 	}
 	mut aliases := c.asm_ios(mut stmt.output, mut stmt.scope, true)
@@ -2914,7 +2916,7 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 	if c.ct_cond_stack.len > 0 {
 		node.ct_conds = c.ct_cond_stack.clone()
 	}
-	if c.pref.backend.is_js() || c.pref.backend == .golang {
+	if c.pref.backend == .golang || c.is_js_backend {
 		// consider the best way to handle the .go.vv files
 		if !c.file.path.ends_with('.js.v') && !c.file.path.ends_with('.go.v')
 			&& !c.file.path.ends_with('.go.vv') {
@@ -3729,7 +3731,7 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	}
 	c.check_any_type(to_type, to_sym, node.pos)
 
-	if c.pref.backend.is_js() {
+	if c.is_js_backend {
 		if (to_sym.is_number() && from_sym.name == 'JS.Number')
 			|| (to_sym.is_number() && from_sym.name == 'JS.BigInt')
 			|| (to_sym.is_string() && from_sym.name == 'JS.String')
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 2da62c894..9334e5ba1 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1536,7 +1536,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 		}
 	}
 	c.set_node_expected_arg_types(mut node, func)
-	if !c.pref.backend.is_js() && args_len > 0 && func.params.len == 0 {
+	if !c.is_js_backend && args_len > 0 && func.params.len == 0 {
 		c.error('too many arguments in call to `${func.name}` (non-js backend: ${c.pref.backend})',
 			node.pos)
 	}
@@ -1925,7 +1925,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 				&& arg_typ !in [ast.voidptr_type, ast.nil_type] && arg_typ.nr_muls() == 0
 				&& func.name !in ['isnil', 'ptr_str'] && !func.name.starts_with('json.')
 				&& arg_typ_sym.kind !in [.float_literal, .int_literal, .charptr, .function]
-				&& !c.pref.backend.is_js() {
+				&& !c.is_js_backend {
 				c.warn('automatic ${arg_typ_sym.name} referencing/dereferencing into voidptr is deprecated and will be removed soon; use `foo(&x)` instead of `foo(x)`',
 					call_arg.pos)
 			}
@@ -2209,7 +2209,7 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 		&& !(left_sym.kind == .alias && left_sym.has_method(method_name)) {
 		unaliased_left_type := c.table.unaliased_type(left_type)
 		return c.map_builtin_method_call(mut node, unaliased_left_type)
-	} else if c.pref.backend.is_js() && left_sym.name.starts_with('Promise[') && node.kind == .wait {
+	} else if c.is_js_backend && left_sym.name.starts_with('Promise[') && node.kind == .wait {
 		info := left_sym.info as ast.Struct
 		if node.args.len > 0 {
 			c.error('wait() does not have any arguments', node.args[0].pos)
@@ -2903,7 +2903,7 @@ fn (mut c Checker) spawn_expr(mut node ast.SpawnExpr) ast.Type {
 			node.call_expr.left.pos())
 	}
 
-	if c.pref.backend.is_js() {
+	if c.is_js_backend {
 		return c.table.find_or_register_promise(c.unwrap_generic(ret_type))
 	} else {
 		return c.table.find_or_register_thread(c.unwrap_generic(ret_type))
@@ -2930,7 +2930,7 @@ fn (mut c Checker) go_expr(mut node ast.GoExpr) ast.Type {
 			node.call_expr.left.pos())
 	}
 
-	if c.pref.backend.is_js() {
+	if c.is_js_backend {
 		return c.table.find_or_register_promise(c.unwrap_generic(ret_type))
 	} else {
 		return c.table.find_or_register_thread(c.unwrap_generic(ret_type))
diff --git a/vlib/v/checker/struct.v b/vlib/v/checker/struct.v
index 0763042fc..745884a5b 100644
--- a/vlib/v/checker/struct.v
+++ b/vlib/v/checker/struct.v
@@ -1065,7 +1065,6 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 fn (mut c Checker) check_uninitialized_struct_fields_and_embeds(node ast.StructInit, type_sym ast.TypeSymbol, mut info ast.Struct, mut inited_fields []string) {
 	mut fields := c.table.struct_fields(type_sym)
 	mut checked_types := []ast.Type{}
-
 	for i, mut field in fields {
 		if field.name in inited_fields {
 			if c.mod != type_sym.mod {
@@ -1078,7 +1077,8 @@ fn (mut c Checker) check_uninitialized_struct_fields_and_embeds(node ast.StructI
 							} else {
 								parts.last()
 							}
-							if !c.inside_unsafe {
+							if !c.inside_unsafe && !(c.is_js_backend
+								&& mod_type.starts_with('Promise')) {
 								c.error('cannot access private field `${field.name}` on `${mod_type}`',
 									init_field.pos)
 
diff --git a/vlib/v/checker/tests/amod/amod.v b/vlib/v/checker/tests/amod/amod.v
index e67d2e948..eb22830ef 100644
--- a/vlib/v/checker/tests/amod/amod.v
+++ b/vlib/v/checker/tests/amod/amod.v
@@ -9,6 +9,10 @@ pub struct Bcg {
 	x int
 }
 
+pub struct Bcg2[T] {
+	x int
+}
+
 @[params]
 pub struct FooParams {
 	bar string
diff --git a/vlib/v/checker/tests/modules/module_struct_noinit.out b/vlib/v/checker/tests/modules/module_struct_noinit.out
index 7d9d36599..2c5a8f15b 100644
--- a/vlib/v/checker/tests/modules/module_struct_noinit.out
+++ b/vlib/v/checker/tests/modules/module_struct_noinit.out
@@ -1,6 +1,12 @@
-vlib/v/checker/tests/modules/module_struct_noinit/src/main.v:9:9: error: struct `mod.Foo` is declared with a `@[noinit]` attribute, so it cannot be initialized with `mod.Foo{}`
-    7 | 
-    8 | fn default_value[T]() T {
-    9 |     return T{}
+vlib/v/checker/tests/modules/module_struct_noinit/src/main.v:12:9: error: struct `mod.Foo` is declared with a `@[noinit]` attribute, so it cannot be initialized with `mod.Foo{}`
+   10 | 
+   11 | fn default_value[T]() T {
+   12 |     return T{}
       |            ~~~
-   10 | }
+   13 | }
+vlib/v/checker/tests/modules/module_struct_noinit/src/main.v:12:9: error: struct `mod.Foo2[int]` is declared with a `@[noinit]` attribute, so it cannot be initialized with `mod.Foo2[int]{}`
+   10 | 
+   11 | fn default_value[T]() T {
+   12 |     return T{}
+      |            ~~~
+   13 | }
diff --git a/vlib/v/checker/tests/modules/module_struct_noinit/src/main.v b/vlib/v/checker/tests/modules/module_struct_noinit/src/main.v
index 032fb44c3..7ee572f9d 100644
--- a/vlib/v/checker/tests/modules/module_struct_noinit/src/main.v
+++ b/vlib/v/checker/tests/modules/module_struct_noinit/src/main.v
@@ -3,6 +3,9 @@ import mod
 fn main() {
 	dump(default_value[mod.Foo]())
 	println(default_value[mod.Foo]())
+
+	dump(default_value[mod.Foo2[int]]())
+	println(default_value[mod.Foo2[int]]())
 }
 
 fn default_value[T]() T {
diff --git a/vlib/v/checker/tests/modules/module_struct_noinit/src/mod.v b/vlib/v/checker/tests/modules/module_struct_noinit/src/mod.v
index 0db32d06f..d6afd0f1f 100644
--- a/vlib/v/checker/tests/modules/module_struct_noinit/src/mod.v
+++ b/vlib/v/checker/tests/modules/module_struct_noinit/src/mod.v
@@ -3,3 +3,7 @@ module mod
 @[noinit]
 pub struct Foo {
 }
+
+@[noinit]
+pub struct Foo2[T] {
+}
diff --git a/vlib/v/checker/tests/struct_field_private_err.out b/vlib/v/checker/tests/struct_field_private_err.out
index 286a91df2..c2a1420dd 100644
--- a/vlib/v/checker/tests/struct_field_private_err.out
+++ b/vlib/v/checker/tests/struct_field_private_err.out
@@ -5,8 +5,15 @@ vlib/v/checker/tests/struct_field_private_err.vv:8:2: error: cannot access priva
       |     ~~~~
     9 | }
    10 |
-vlib/v/checker/tests/struct_field_private_err.vv:11:10: error: cannot access private field `bar` on `amod.FooParams`
-    9 | }
+vlib/v/checker/tests/struct_field_private_err.vv:12:2: error: cannot access private field `x` on `amod.Bcg2[int]`
    10 | 
-   11 | amod.foo(bar: 'bar')
+   11 | _ := amod.Bcg2[int]{
+   12 |     x: 0
+      |     ~~~~
+   13 | }
+   14 |
+vlib/v/checker/tests/struct_field_private_err.vv:15:10: error: cannot access private field `bar` on `amod.FooParams`
+   13 | }
+   14 | 
+   15 | amod.foo(bar: 'bar')
       |          ~~~~~~~~~~
diff --git a/vlib/v/checker/tests/struct_field_private_err.vv b/vlib/v/checker/tests/struct_field_private_err.vv
index d1a8f91c1..f03d714f7 100644
--- a/vlib/v/checker/tests/struct_field_private_err.vv
+++ b/vlib/v/checker/tests/struct_field_private_err.vv
@@ -8,4 +8,8 @@ _ := amod.Bcg{
 	x: 0
 }
 
+_ := amod.Bcg2[int]{
+	x: 0
+}
+
 amod.foo(bar: 'bar')
diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index 9216f6287..966f77329 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -842,7 +842,7 @@ run them via `v file.v` instead',
 			is_non_local = elem_type_sym.mod.len > 0 && elem_type_sym.mod != p.mod
 				&& elem_type_sym.language == .v
 		}
-		if is_non_local {
+		if is_non_local && !(p.file_backend_mode == .js && type_sym.mod.starts_with('Promise')) {
 			p.error_with_pos('cannot define new methods on non-local type ${type_sym.name}. Define an alias and use that instead like `type AliasName = ${type_sym.name}` ',
 				rec.type_pos)
 			return ast.FnDecl{
diff --git a/vlib/v/parser/parse_type.v b/vlib/v/parser/parse_type.v
index e24893501..21313b42d 100644
--- a/vlib/v/parser/parse_type.v
+++ b/vlib/v/parser/parse_type.v
@@ -1026,12 +1026,15 @@ fn (mut p Parser) parse_generic_inst_type(name string, name_pos token.Pos) ast.T
 				}
 			}
 		}
+		// mod.Foo[int] -> mod
+		// mod.submod.Foo[int] -> mod.submod
+		mod := name.all_before_last('.')
 
 		idx := p.table.register_sym(ast.TypeSymbol{
 			kind:  .generic_inst
 			name:  bs_name
 			cname: util.no_dots(bs_cname)
-			mod:   p.mod
+			mod:   mod
 			info:  ast.GenericInst{
 				parent_idx:     parent_idx
 				concrete_types: concrete_types
diff --git a/vlib/v/tests/generics/modules/simplemodule/simplemodule.v b/vlib/v/tests/generics/modules/simplemodule/simplemodule.v
index 3b3ced78d..debaf239c 100644
--- a/vlib/v/tests/generics/modules/simplemodule/simplemodule.v
+++ b/vlib/v/tests/generics/modules/simplemodule/simplemodule.v
@@ -9,6 +9,7 @@ pub fn imul(x int, y int) int {
 }
 
 pub struct ThisIsGeneric[T] {
+pub:
 	msg T
 }
 
diff --git a/vlib/x/sessions/memory_store.v b/vlib/x/sessions/memory_store.v
index e868b3de4..25b80fe94 100644
--- a/vlib/x/sessions/memory_store.v
+++ b/vlib/x/sessions/memory_store.v
@@ -10,7 +10,7 @@ pub mut:
 
 // MemoryStore stores sessions in a `map` in memory only.
 pub struct MemoryStore[T] {
-mut:
+pub mut:
 	data map[string]MemoryStoreSessions[T]
 }
 
-- 
2.39.5