https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120197
Bug ID: 120197
Summary: ICE in GCC with decltype(cout << a) in default
function parameter leads to recursive diagnostics with
-std=c++20
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mario.rodriguezb1 at um dot es
Target Milestone: ---
Using decltype(cout << a) as a default argument causes GCC to enter
error-reporting routines recursively when a is deduced to a stream type,
leading to an internal compiler error.
```
#include <iostream>
using namespace std;
template <class T>
void myFunc(T a, decltype(cout << a) o=cout) {
cout << o << endl;
}
int main() {
myFunc(5);
}
```
Not a specific ICE is shown in trunk, just: internal compiler error: error
reporting routines re-entered.
```
internal compiler error: error reporting routines re-entered.
internal compiler error: error reporting routines re-entered.
0x293b1be diagnostic_context::report_diagnostic(diagnostic_info*)
???:0
0x293b2d5 diagnostic_context::diagnostic_impl(rich_location*,
diagnostic_metadata const*, diagnostic_option_id, char const*, __va_list_tag
(*) [1], diagnostic_t)
???:0
0x2954bdf error_at(unsigned long, char const*, ...)
???:0
0xb59baa satisfaction_cache::get()
???:0
0xb60248 constraints_satisfied_p(tree_node*, tree_node*)
???:0
0xd32096 tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0xd3176b tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0xd4e9bd instantiate_template(tree_node*, tree_node*, int)
???:0
0xd59dc6 fn_type_unification(tree_node*, tree_node*, tree_node*, tree_node*
const*, unsigned int, tree_node*, unification_kind_t, int, conversion**, bool,
bool)
???:0
0xb104a6 build_new_op(op_location_t const&, tree_code, int, tree_node*,
tree_node*, tree_node*, tree_node*, tree_node**, int)
???:0
0xdbada2 build_x_binary_op(op_location_t const&, tree_code, tree_node*,
tree_code, tree_node*, tree_code, tree_node*, tree_node**, int)
???:0
0xd3195d tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0x2978ca3 pretty_printer::format(text_info&)
???:0
0x297e463 pp_verbatim(pretty_printer*, char const*, ...)
???:0
0x29529a8 diagnostic_text_output_format::on_report_diagnostic(diagnostic_info
const&, diagnostic_t)
???:0
0x293ae26 diagnostic_context::report_diagnostic(diagnostic_info*)
???:0
0x293b2d5 diagnostic_context::diagnostic_impl(rich_location*,
diagnostic_metadata const*, diagnostic_option_id, char const*, __va_list_tag
(*) [1], diagnostic_t)
???:0
0x29552f6 internal_error(char const*, ...)
???:0
0xad6166 fancy_abort(char const*, int, char const*)
???:0
0x293a6eb diagnostic_context::finish()
???:0
```
It happens in trunk and from version 11.2
https://gcc.godbolt.org/z/zz6d3ezrf
