Confirmed.

r12-6075 is exposing a latent bug with non-dependent decltype folding, namely we instantiate the expression only if it's potentially constant.  In this case, id(v) is not potentially constant because id is not a constexpr function, so we discard the expression without actually instantiating it and therefore never issue the narrowing warning.

(In reply to Patrick Palka from comment #2)
> r12-6075

Patrick, can you please start using latest git gcc-descr version:
	gcc-descr = "!f() { \"`git rev-parse --show-toplevel`/contrib/git-descr.sh\" $@; } ; f"
	gcc-undescr = "!f() { \"`git rev-parse --show-toplevel`/contrib/git-undescr.sh\" $@; } ; f"

So that hashes are appended to revisions:

$ git gcc-descr 
r12-7534-g7cce7b1c3d8291

Thanks

The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:ec0f53a3a542e762f8fc8c22b9d345f922be5867

commit r12-7564-gec0f53a3a542e762f8fc8c22b9d345f922be5867
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Mar 9 08:42:37 2022 -0500

    c++: non-constant non-dependent decltype folding [PR104823]
    
    When processing a non-dependent decltype operand we want to instantiate
    it even if it's non-constant, since non-dependent decltype is always
    resolved ahead of time.  But currently finish_decltype_type uses
    instantiate_non_dependent_expr, which instantiates only potentially
    constant expressions, and this causes us to miss diagnosing the narrowing
    conversion in S{id(v)} in the below testcase because we never instantiate
    this non-constant non-dependent decltype operand.
    
    In light of
    
      > On Mon, 7 Mar 2022, Jason Merrill wrote:
      >> On 3/7/22 14:41, Patrick Palka wrote:
      >>> instantiate_non_dependent_expr instantiates only potentially constant
      >>> expressions
      >>
      >> Hmm, that now strikes me as a problematic interface, as we don't know whether
      >> what we get back is template or non-template trees.
    
    this patch drops the potentially-constant check in i_n_d_e and turns
    its dependence check into a checking_assert, since most callers already
    check that the argument is non-dependent; thus i_n_d_e now instantiates
    even non-constant expressions and always returns non-templated trees.
    This patch also relaxes the dependence check in i_n_d_e to use the
    _uneval version (since that's what finish_decltype_type uses) and
    strengthens the dependence checks used by other callers accordingly.
    
    In cp_parser_parenthesized_expression_list_elt we were calling
    instantiate_non_dependent_expr (when parsing an attribute list) without
    first checking for non-dependence.  We could fix this by guarding the
    call appropriately, but I noticed we also fold non-dependent attributes
    later from cp_check_const_attribute, so this earlier folding is at best
    redundant.  And it currently causes us to reject constexpr-attribute4.C
    below due to the second folding seeing non-templated trees.  Thus the
    right solution here seems to be to remove this unguarded call to i_n_d_e
    so that we end up instantiating non-dependent attributes only once.
    
    Finally, after calling i_n_d_e in finish_decltype_type we need to keep
    processing_template_decl cleared for sake of the later call to
    lvalue_kind, which handles templated and non-templated COND_EXPR
    differently.  Otherwise we'd incorrectly reject the declaration of g in
    cpp0x/cond2.C with:
    
      error: 'g' declared as function returning a function
    
            PR c++/104823
    
    gcc/cp/ChangeLog:
    
            * except.cc (build_noexcept_spec): Strengthen dependence check
            to instantiation_dependent_expression_p.
            * parser.cc (cp_parser_parenthesized_expression_list_elt):
            Remove fold_expr_p parameter, and don't call
            instantiate_non_dependent_expr.
            (cp_parser_parenthesized_expression_list): Adjust accordingly.
            * pt.cc (expand_integer_pack): Strengthen dependence check
            to instantiation_dependent_expression_p.
            (instantiate_non_dependent_expr_internal): Adjust comment.
            (instantiate_non_dependent_expr_sfinae): Likewise.  Drop
            the potentially-constant check, and relax and turn the
            dependence check into a checking assert.
            (instantiate_non_dependent_or_null): Adjust comment.
            * semantics.cc (finish_decltype_type): Keep
            processing_template_decl cleared after calling
            instantiate_non_dependent_expr_sfinae.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/Wnarrowing19.C: New test.

Fixed.