all: mutable smartcasts (#6878)

vlib/v/ast/ast.v

@@ -533,7 +533,7 @@ pub:
 	body_pos     token.Position
 	comments     []Comment
 	left_as_name string // `name` in `if cond is SumType as name`
-	mut_name     bool // `if mut name is`
+	is_mut_name  bool // `if mut name is`
 pub mut:
 	stmts        []Stmt
 	smartcast    bool // true when cond is `x is SumType`, set in checker.if_expr // no longer needed with union sum types TODO: remove

vlib/v/checker/checker.v

@@ -3516,21 +3516,41 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, type_sym table.TypeSymbol
 				}
 				mut scope := c.file.scope.innermost(branch.pos.pos)
 				match node.cond as node_cond {
-					ast.SelectorExpr { scope.register_struct_field(ast.ScopeStructField{
+					ast.SelectorExpr {
+						expr_sym := c.table.get_type_symbol(node_cond.expr_type)
+						field := c.table.struct_find_field(expr_sym, node_cond.field_name) or {
+							table.Field{}
+						}
+						is_mut := field.is_mut
+						is_root_mut := scope.is_selector_root_mutable(c.table, node_cond)
+						// smartcast either if the value is immutable or if the mut argument is explicitly given
+						if (!is_root_mut && !is_mut) || node.is_mut {
+							scope.register_struct_field(ast.ScopeStructField{
 								struct_type: node_cond.expr_type
 								name: node_cond.field_name
 								typ: node.cond_type
 								sum_type_cast: expr_type
 								pos: node_cond.pos
-						}) }
-					ast.Ident { scope.register(node.var_name, ast.Var{
+							})
+						}
+					}
+					ast.Ident {
+						mut is_mut := false
+						if v := scope.find_var(node_cond.name) {
+							is_mut = v.is_mut
+						}
+						// smartcast either if the value is immutable or if the mut argument is explicitly given
+						if !is_mut || node.is_mut {
+							scope.register(node.var_name, ast.Var{
 								name: node.var_name
 								typ: node.cond_type
 								pos: node_cond.pos
 								is_used: true
 								is_mut: node.is_mut
 								sum_type_cast: expr_type
-						}) }
+							})
+						}
+					}
 					else {}
 				}
 			}
@@ -3762,7 +3782,9 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 								if v := scope.find_var(infix_left.name) {
 									is_mut = v.is_mut
 								}
-								if !is_mut && left_sym.kind == .union_sum_type {
+								// smartcast either if the value is immutable or if the mut argument is explicitly given
+								if (!is_mut || branch.is_mut_name) &&
+									left_sym.kind == .union_sum_type {
 									scope.register(branch.left_as_name, ast.Var{
 										name: branch.left_as_name
 										typ: infix.left_type
@@ -3780,7 +3802,10 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 								is_mut = field.is_mut
 								is_root_mut := scope.is_selector_root_mutable(c.table,
 									selector)
-								if !is_root_mut && !is_mut && left_sym.kind == .union_sum_type {
+								// smartcast either if the value is immutable or if the mut argument is explicitly given
+								if ((!is_root_mut && !is_mut) ||
+									branch.is_mut_name) &&
+									left_sym.kind == .union_sum_type {
 									scope.register_struct_field(ast.ScopeStructField{
 										struct_type: selector.expr_type
 										name: selector.field_name

vlib/v/fmt/fmt.v

@@ -1423,7 +1423,7 @@ pub fn (mut f Fmt) if_expr(it ast.IfExpr) {
 		}
 		if i < it.branches.len - 1 || !it.has_else {
 			f.write('${dollar}if ')
-			if branch.mut_name {
+			if branch.is_mut_name {
 				f.write('mut ')
 			}
 			f.expr(branch.cond)

vlib/v/parser/if_match.v

@@ -86,9 +86,9 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 		}
 		comments << p.eat_comments()
 		// `if mut name is T`
-		mut mut_name := false
+		mut is_mut_name := false
 		if p.tok.kind == .key_mut && p.peek_tok2.kind == .key_is {
-			mut_name = true
+			is_mut_name = true
 			p.next()
 			comments << p.eat_comments()
 		}
@@ -150,7 +150,7 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			body_pos: body_pos.extend(p.prev_tok.position())
 			comments: comments
 			left_as_name: left_as_name
-			mut_name: mut_name
+			is_mut_name: is_mut_name
 		}
 		comments = p.eat_comments()
 		if is_comptime {

vlib/v/tests/union_sum_type_test.v

@@ -346,10 +346,12 @@ fn test_zero_value_init() {
 }
 
 struct Milk {
+mut:
 	name string
 }
 
 struct Eggs {
+mut:
 	name string
 }
 
@@ -364,6 +366,50 @@ fn test_match_aggregate() {
 	}
 }
 
+fn test_match_mut() {
+	mut f := Food(Milk{'test'})
+	match union mut f {
+		Eggs {
+			f.name = 'eggs'
+			assert f.name == 'eggs'
+		}
+		Milk {
+			f.name = 'milk'
+			assert f.name == 'milk'
+		}
+	}
+}
+
+fn test_match_not_mut() {
+	mut f := Food(Milk{'test'})
+	match union f {
+		Eggs {
+			// only works without smartcast
+			assert f is Eggs
+		}
+		Milk {
+			// only works without smartcast
+			assert f is Milk
+		}
+	}
+}
+
+fn test_if_mut() {
+	mut f := Food(Milk{'test'})
+	if mut f is Milk {
+		f.name = 'milk'
+		assert f.name == 'milk'
+	}
+}
+
+fn test_if_not_mut() {
+	mut f := Food(Milk{'test'})
+	if f is Milk {
+		// only works without smartcast
+		assert f is Milk
+	}
+}
+
 fn test_sum_type_match() {
 	// TODO: Remove these casts
 	assert is_gt_simple('3', int(2))