checker: allow mut var.$(field.name) (#17911)

vlib/v/ast/ast.v

@@ -1973,6 +1973,7 @@ pub fn (expr Expr) is_lvalue() bool {
 		SelectorExpr { return expr.expr.is_lvalue() }
 		ParExpr { return expr.expr.is_lvalue() } // for var := &{...(*pointer_var)}
 		PrefixExpr { return expr.right.is_lvalue() }
+		ComptimeSelector { return expr.field_expr.is_lvalue() }
 		else {}
 	}
 	return false

vlib/v/checker/assign.v

@@ -466,7 +466,7 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			}
 		}
 		if left_sym.kind == .map && node.op in [.assign, .decl_assign] && right_sym.kind == .map
-			&& !left.is_blank_ident() && right.is_lvalue()
+			&& !left.is_blank_ident() && right.is_lvalue() && right !is ast.ComptimeSelector
 			&& (!right_type.is_ptr() || (right is ast.Ident && right.is_auto_deref_var())) {
 			// Do not allow `a = b`
 			c.error('cannot copy map: call `move` or `clone` method (or use a reference)',

vlib/v/tests/comptimeselector_mut_test.v

@@ -0,0 +1,33 @@
+struct Struct {
+mut:
+	a string
+	b int
+}
+
+fn decode_string[T](mut value T) {
+	value = 'gg'
+}
+
+fn decode_string_int[T](mut value T) {
+	value = 123
+}
+
+fn decode[T]() T {
+	key := 'a'
+	mut result := T{}
+	$for field in T.fields {
+		$if field.typ is string {
+			decode_string(mut result.$(field.name))
+		} $else $if field.typ is int {
+			decode_string_int(mut result.$(field.name))
+		}
+	}
+	return result
+}
+
+fn test_main() {
+	assert decode[Struct]() == Struct{
+		a: 'gg'
+		b: 123
+	}
+}