Proposed resolution (approved by CWG 2026-04-17):

Change in 6.10.1 [intro.execution] paragraph 16 as follows:

During the evaluation of an expression as a core constant expression (7.7 [expr.const]), evaluations of operands of individual operators and of subexpressions of individual expressions that are otherwise either unsequenced or indeterminately sequenced are evaluated in lexical order. For a function call (7.6.1.3 [expr.call]), any default arguments used are evaluated in the order as-if an argument were present for the corresponding parameter.

(From submission #859.)

According to 6.10.1 [intro.execution] paragraph 16, lexical order is used for sequencing during constant evalution. It is unclear how that applies to default arguments used in a function call.