checker: keep map-copy guard for option-map or-unwraps (#27867)

vlib/v/checker/assign.v

@@ -28,6 +28,111 @@ fn assign_expr_is_auto_deref(expr ast.Expr) bool {
 	return expr.is_auto_deref_var()
 }
 
+// assign_or_unwrap_source_is_immutable conservatively reports whether the source
+// of an `or {}` unwrap can never be mutated later (so a copy of its map cannot
+// observe a later mutation of the original). Only the plainly-immutable roots are
+// recognised; anything unknown returns false, keeping the map-copy guard.
+// A pointer/reference anywhere in the chain is treated as mutable/unknown, since
+// an immutable pointer can still alias storage that its owner mutates later
+// (e.g. `mut c := ...; p := &c; x := p.f or { ... }; c.f?[k] = v`). Types are
+// fully unaliased first, so a `type Ref = &T` receiver is treated as a pointer.
+fn (c &Checker) assign_or_unwrap_source_is_immutable(expr ast.Expr) bool {
+	return match expr {
+		ast.Ident {
+			if expr.obj is ast.Var {
+				!expr.is_mut() && !c.table.fully_unaliased_type(expr.obj.typ).is_ptr()
+			} else {
+				!expr.is_mut()
+			}
+		}
+		ast.SelectorExpr {
+			!c.table.fully_unaliased_type(expr.expr_type).is_ptr()
+				&& c.assign_or_unwrap_source_is_immutable(expr.expr)
+		}
+		ast.IndexExpr {
+			!c.table.fully_unaliased_type(expr.left_type).is_ptr()
+				&& c.assign_or_unwrap_source_is_immutable(expr.left)
+		}
+		ast.ParExpr {
+			c.assign_or_unwrap_source_is_immutable(expr.expr)
+		}
+		else {
+			false
+		}
+	}
+}
+
+// assign_expr_or_block returns the `or {}` block attached to `expr`, if any.
+fn assign_expr_or_block(expr ast.Expr) ast.OrExpr {
+	return match expr {
+		ast.Ident { expr.or_expr }
+		ast.IndexExpr { expr.or_expr }
+		ast.SelectorExpr { expr.or_block }
+		else { ast.OrExpr{} }
+	}
+}
+
+// assign_or_block_default_is_safe conservatively reports whether the value an
+// `or {}` block falls back to cannot alias mutable storage. `x := opt or { d }`
+// makes `x` the block's value `d` when the option is empty, so if `d` is any map
+// lvalue (e.g. `or { fallback }`) then `x` aliases it. Even an immutable lvalue
+// is unsafe: an immutable map parameter/field can alias caller-owned storage the
+// caller mutates later (the same copy `x := d` would reject). A call is unsafe
+// too unless it is known to produce fresh storage — an arbitrary map-returning
+// call may just return a caller-owned alias (`or { id(fallback) }`). Only a map
+// literal, a `clone`/`move` call, a `@[noreturn]` call (`panic`/`exit`), or a
+// block that yields no value by diverging (`return`/`break`/`continue`) is safe.
+// Anything else keeps the guard.
+fn (c &Checker) assign_or_block_default_is_safe(or_expr ast.OrExpr) bool {
+	if or_expr.kind != .block {
+		return true
+	}
+	// A trailing `;` leaves a `SemicolonStmt`; filter those out so the block's
+	// real value statement is classified, matching `check_or_expr`.
+	valid_stmts := or_expr.stmts.filter(it !is ast.SemicolonStmt)
+	if valid_stmts.len == 0 {
+		return true
+	}
+	last := valid_stmts.last()
+	return match last {
+		ast.ExprStmt {
+			// harmless parentheses around the value must not change classification
+			default_expr := last.expr.remove_par()
+			match default_expr {
+				ast.MapInit { true }
+				ast.CallExpr { c.assign_call_default_produces_fresh_map(default_expr) }
+				else { false }
+			}
+		}
+		ast.Return, ast.BranchStmt {
+			true
+		}
+		else {
+			false
+		}
+	}
+}
+
+// assign_call_default_produces_fresh_map reports whether a call used as an `or {}`
+// default cannot alias caller-owned map storage: a `@[noreturn]` call such as
+// `panic`/`exit` (yields no value at all), or the builtin map `clone`/`move`
+// (fresh storage). Only the builtin map operations qualify — a user-defined
+// function or method named `clone`/`move` (its `CallKind` also comes from the
+// name) may just return an aliased map. The receiver type must be a map
+// *directly*, not via an alias: a map cannot carry user methods, but an alias
+// (`type M = map[...]`) can define its own `clone`/`move`, which `fn.v` resolves
+// before the map builtin. Any other map-returning call keeps the map-copy guard.
+fn (c &Checker) assign_call_default_produces_fresh_map(expr ast.Expr) bool {
+	if expr is ast.CallExpr {
+		if expr.is_noreturn {
+			return true
+		}
+		return expr.is_method && expr.name in ['clone', 'move']
+			&& c.table.sym(expr.receiver_type).kind == .map
+	}
+	return false
+}
+
 fn (c &Checker) auto_deref_source_type_is_pointer(expr ast.Expr) bool {
 	if expr !is ast.Ident || c.table.cur_fn == unsafe { nil } || !expr.is_auto_deref_var() {
 		return false
@@ -886,8 +991,38 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 		} else {
 			right.is_lvalue()
 		}
+		// `x := opt_map or { ... }` unwraps an option/result into a new immutable
+		// variable, so `x` can never become a mutable alias of the underlying map
+		// and the shallow copy is safe. This mirrors how V already accepts the
+		// equivalent immutable `x := opt_array_field or { ... }`. Several things
+		// must hold for the copy to be safe, otherwise the guard is kept so
+		// aliasing still requires a `clone`/`move`:
+		//   - the destination is a new immutable variable (a mutable `mut x := ...`
+		//     or a reassignment `x = ...` could still mutate through `x`);
+		//   - the `or` actually clears the option/result — for `map[K]?map[...]`,
+		//     `v := m[k] or { none }` keeps the option-map type, so `v` is still an
+		//     option handle aliasing the map in `m`;
+		//   - the unwrapped source itself is immutable — otherwise a later mutation
+		//     of the source (e.g. `mut opt := ...; x := opt or { ... }; opt?[k] = v`)
+		//     would be observed through `x`.
+		// See vlang/v issue #27867.
+		// A `shared`/`atomic` destination is mutable under `lock`, so it must never
+		// be exempted even though it is a `:=` declaration. Its `is_mut` flag is
+		// already set by the parser, but check `share` explicitly so this safety
+		// bypass does not depend on that incidental detail.
+		left_is_lockable_dest := left is ast.Ident && left.info is ast.IdentVar
+			&& left.info.share in [.shared_t, .atomic_t]
+		mut right_is_immutable_or_unwrap := false
+		if node.op == .decl_assign && left is ast.Ident && !left.is_mut && !left_is_lockable_dest
+			&& !right_type.has_flag(.option) && !right_type.has_flag(.result) {
+			unwrapped_right := right.remove_par()
+			or_expr := assign_expr_or_block(unwrapped_right)
+			right_is_immutable_or_unwrap = or_expr.kind != .absent
+				&& c.assign_or_unwrap_source_is_immutable(unwrapped_right)
+				&& c.assign_or_block_default_is_safe(or_expr)
+		}
 		if left_sym.kind == .map && is_assign && right_sym.kind == .map && !c.inside_unsafe
-			&& !left.is_blank_ident() && right_is_lvalue
+			&& !left.is_blank_ident() && right_is_lvalue && !right_is_immutable_or_unwrap
 			&& (!right_type.is_ptr() || (right is ast.Ident && assign_expr_is_auto_deref(right))) {
 			// Do not allow `a = b`
 			c.error('cannot copy map: call `move` or `clone` method (or use a reference)',

vlib/v/checker/tests/option_map_or_unwrap_alias_ptr_source_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_map_or_unwrap_alias_ptr_source_err.vv:20:9: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   18 |     }
+   19 |     p := CategoryRef(&c)
+   20 |     x := p.translations or { panic('missing') }
+      |            ~~~~~~~~~~~~
+   21 |     c.translations?['x'] = 2
+   22 |     println(x)

vlib/v/checker/tests/option_map_or_unwrap_alias_ptr_source_err.vv

@@ -0,0 +1,23 @@
+// A `type Ref = &Category` receiver is a pointer once unaliased, so or-unwrapping
+// an option map through it can still alias storage mutated through the original
+// owner. The immutable-source exemption from issue #27867 must unalias types
+// before its pointer check, so an alias-to-pointer receiver keeps the map-copy
+// guard just like a plain `&Category` receiver.
+struct Category {
+mut:
+	translations ?map[string]int
+}
+
+type CategoryRef = &Category
+
+fn main() {
+	mut c := Category{
+		translations: {
+			'x': 1
+		}
+	}
+	p := CategoryRef(&c)
+	x := p.translations or { panic('missing') }
+	c.translations?['x'] = 2
+	println(x)
+}

vlib/v/checker/tests/option_map_or_unwrap_alias_user_clone_default_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_map_or_unwrap_alias_user_clone_default_err.vv:28:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   26 |     }
+   27 |     opt := maybe_none()
+   28 |     x := opt or { holder.m.clone() }
+      |          ~~~
+   29 |     holder.m['x'] = 2
+   30 |     println(x)

vlib/v/checker/tests/option_map_or_unwrap_alias_user_clone_default_err.vv

@@ -0,0 +1,31 @@
+// Only the *builtin* map clone/move produce fresh storage. A map type alias can
+// define its own `clone`/`move`, which `fn.v` resolves before the map builtin, so
+// `holder.m.clone()` here runs the user method that returns an alias of `holder.m`.
+// The exemption must require a direct map receiver (not one that merely unaliases
+// to a map), so this default keeps the map-copy guard. See vlang/v issue #27867.
+type MapAlias = map[string]int
+
+fn (m MapAlias) clone() map[string]int {
+	return map[string]int(m)
+}
+
+fn maybe_none() ?map[string]int {
+	return none
+}
+
+struct Holder {
+mut:
+	m MapAlias
+}
+
+fn main() {
+	mut holder := Holder{
+		m: MapAlias({
+			'x': 1
+		})
+	}
+	opt := maybe_none()
+	x := opt or { holder.m.clone() }
+	holder.m['x'] = 2
+	println(x)
+}

vlib/v/checker/tests/option_map_or_unwrap_call_default_alias_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_map_or_unwrap_call_default_alias_err.vv:19:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   17 |     }
+   18 |     opt := maybe_none()
+   19 |     x := opt or { id(fallback) }
+      |          ~~~
+   20 |     fallback['x'] = 2
+   21 |     println(x)

vlib/v/checker/tests/option_map_or_unwrap_call_default_alias_err.vv

@@ -0,0 +1,22 @@
+// An `or {}` default that is an arbitrary map-returning call can return a
+// caller-owned alias (`id` just returns its argument), so `x := opt or { id(fb) }`
+// would alias `fb` and observe later mutations — the same as `or { fb }`. Only
+// calls known to produce fresh storage (`clone`/`move`) or `@[noreturn]` calls
+// are exempted. See vlang/v issue #27867.
+fn maybe_none() ?map[string]int {
+	return none
+}
+
+fn id(m map[string]int) map[string]int {
+	return m
+}
+
+fn main() {
+	mut fallback := {
+		'x': 1
+	}
+	opt := maybe_none()
+	x := opt or { id(fallback) }
+	fallback['x'] = 2
+	println(x)
+}

vlib/v/checker/tests/option_map_or_unwrap_immut_default_alias_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/option_map_or_unwrap_immut_default_alias_err.vv:13:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   11 |     opt := maybe_none()
+   12 |     // immutable parameter fallback: `x` can alias the caller's map.
+   13 |     x := opt or { fallback }
+      |          ~~~
+   14 |     return x
+   15 | }
+vlib/v/checker/tests/option_map_or_unwrap_immut_default_alias_err.vv:23:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   21 |     opt := maybe_none()
+   22 |     // immutable local lvalue fallback is rejected too (cannot prove freshness).
+   23 |     y := opt or { local }
+      |          ~~~
+   24 | 
+   25 |     println(pick({

vlib/v/checker/tests/option_map_or_unwrap_immut_default_alias_err.vv

@@ -0,0 +1,29 @@
+// An `or {}` block whose fallback is an immutable map lvalue is still unsafe:
+// an immutable map parameter (or any immutable alias) can point at caller-owned
+// storage the caller mutates later, so `x := opt or { fallback }` would alias it
+// — the same copy `x := fallback` rejects. Only fresh/owned fallbacks (map
+// literal / by-value call) are exempted. See vlang/v issue #27867.
+fn maybe_none() ?map[string]int {
+	return none
+}
+
+fn pick(fallback map[string]int) map[string]int {
+	opt := maybe_none()
+	// immutable parameter fallback: `x` can alias the caller's map.
+	x := opt or { fallback }
+	return x
+}
+
+fn main() {
+	local := {
+		'y': 9
+	}
+	opt := maybe_none()
+	// immutable local lvalue fallback is rejected too (cannot prove freshness).
+	y := opt or { local }
+
+	println(pick({
+		'x': 1
+	}))
+	println(y)
+}

vlib/v/checker/tests/option_map_or_unwrap_mut_alias_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/option_map_or_unwrap_mut_alias_err.vv:14:17: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   12 |         'x': 1
+   13 |     }]
+   14 |     mut a := xs[0] or { panic('missing') }
+      |                    ~~~~~~~~~~~~~~~~~~~~~~~
+   15 |     a['x'] = 2
+   16 |
+vlib/v/checker/tests/option_map_or_unwrap_mut_alias_err.vv:22:13: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   20 |         }
+   21 |     }
+   22 |     mut b := c.translations or { panic('missing') }
+      |                ~~~~~~~~~~~~
+   23 |     b['x'] = 2
+   24 |

vlib/v/checker/tests/option_map_or_unwrap_mut_alias_err.vv

@@ -0,0 +1,29 @@
+// Or-unwrapping an option map into a *mutable* destination would let the new
+// variable alias the container/field storage, so it must still require an
+// explicit `clone`/`move`. See vlang/v issue #27867 (the immutable
+// `x := opt_map or { ... }` form is allowed, the mutable form is not).
+struct Category {
+mut:
+	translations ?map[string]int
+}
+
+fn main() {
+	mut xs := [{
+		'x': 1
+	}]
+	mut a := xs[0] or { panic('missing') }
+	a['x'] = 2
+
+	mut c := Category{
+		translations: {
+			'x': 1
+		}
+	}
+	mut b := c.translations or { panic('missing') }
+	b['x'] = 2
+
+	println(xs)
+	println(c)
+	println(a)
+	println(b)
+}

vlib/v/checker/tests/option_map_or_unwrap_mut_default_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_map_or_unwrap_mut_default_err.vv:15:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   13 |     }
+   14 |     opt := maybe_none()
+   15 |     x := opt or { fallback }
+      |          ~~~
+   16 |     fallback['x'] = 2
+   17 |     println(x)

vlib/v/checker/tests/option_map_or_unwrap_mut_default_err.vv

@@ -0,0 +1,18 @@
+// `x := opt or { fallback }` makes `x` the `or` block's value when the option is
+// empty, so if that default is a mutable map lvalue, `x` aliases it and observes
+// later mutations (`fallback['x'] = 2`). The immutable-destination exemption from
+// issue #27867 must not apply when the default path can return a mutable map
+// lvalue, even if the option source itself is immutable.
+fn maybe_none() ?map[string]int {
+	return none
+}
+
+fn main() {
+	mut fallback := {
+		'x': 1
+	}
+	opt := maybe_none()
+	x := opt or { fallback }
+	fallback['x'] = 2
+	println(x)
+}

vlib/v/checker/tests/option_map_or_unwrap_mut_source_err.out

@@ -0,0 +1,21 @@
+vlib/v/checker/tests/option_map_or_unwrap_mut_source_err.vv:18:7: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   16 |         'x': 1
+   17 |     })
+   18 |     a := opt or { panic('missing') }
+      |          ~~~
+   19 | 
+   20 |     mut c := Category{
+vlib/v/checker/tests/option_map_or_unwrap_mut_source_err.vv:25:9: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   23 |         }
+   24 |     }
+   25 |     b := c.translations or { panic('missing') }
+      |            ~~~~~~~~~~~~
+   26 | 
+   27 |     inner := {
+vlib/v/checker/tests/option_map_or_unwrap_mut_source_err.vv:33:14: error: cannot copy map: call `move` or `clone` method (or use a reference)
+   31 |         'a': ?map[string]int(inner)
+   32 |     }
+   33 |     d := m['a'] or { panic('missing') }
+      |                 ~~~~~~~~~~~~~~~~~~~~~~~
+   34 | 
+   35 |     println(a)