This code ``` template < auto Z = [](int l = int(0)) -> int { return l; } > int f() { return Z(); } int main() { return f(); } ``` causes this error ``` <source>: In function 'int f() [with auto Z = <lambda closure object>main()::<lambda(int)>{}]': <source>:5:13: internal compiler error: in gimplify_expr, at gimplify.c:12489 5 | return Z(); | ~^~ Please submit a full bug report, with preprocessed source if appropriate. See <https://gcc.gnu.org/bugs/> for instructions. ASM generation compiler returned: 1 ``` from `g++ (Compiler-Explorer-Build) 9.2.0` compiled with these options ``` -g -o /tmp/compiler-explorer-compiler120111-1689-nlrcj.y9iua/output.s -fdiagnostics-color=always -Wl,-rpath,/opt/compiler-explorer/gcc-9.2.0/lib -Wl,-rpath,/opt/compiler-explorer/gcc-9.2.0/lib32 -Wl,-rpath,/opt/compiler-explorer/gcc-9.2.0/lib64 -std=c++2a /tmp/compiler-explorer-compiler120111-1689-nlrcj.y9iua/example.cpp ``` See it on the compiler explorer: https://godbolt.org/z/TD8aHC ---- The reason I want to do this is to get source_location::current from an operator+'s call site. Since the operator cannot take an extra argument, I tried template arguments and then a template argument lambda that returns a source location.
Confirmed. $ ./cc1plus -quiet 93689.C -std=c++2a 93689.C: In instantiation of ‘int f() [with auto Z = <lambda closure object>main()::<lambda(int)>{}]’: 93689.C:9:14: required from here 93689.C:5:14: internal compiler error: unexpected expression ‘(int)(0)’ of kind cast_expr 5 | return Z(); | ^ 0x9a013c cxx_eval_constant_expression /home/mpolacek/src/gcc/gcc/cp/constexpr.c:6203 0x98cafa cxx_bind_parameters_in_call /home/mpolacek/src/gcc/gcc/cp/constexpr.c:1522 0x99018a cxx_eval_call_expression /home/mpolacek/src/gcc/gcc/cp/constexpr.c:2291 0x99ce51 cxx_eval_constant_expression /home/mpolacek/src/gcc/gcc/cp/constexpr.c:5395 0x9a14a5 cxx_eval_outermost_constant_expr /home/mpolacek/src/gcc/gcc/cp/constexpr.c:6404 0x9a2641 maybe_constant_value(tree_node*, tree_node*, bool, bool) /home/mpolacek/src/gcc/gcc/cp/constexpr.c:6692 0xa94da4 fold_for_warn(tree_node*) /home/mpolacek/src/gcc/gcc/cp/expr.c:401 0xcb3a85 maybe_warn_about_returning_address_of_local /home/mpolacek/src/gcc/gcc/cp/typeck.c:9472 0xcb6c2f check_return_expr(tree_node*, bool*) /home/mpolacek/src/gcc/gcc/cp/typeck.c:10081 0xc358a5 finish_return_stmt(tree_node*) /home/mpolacek/src/gcc/gcc/cp/semantics.c:955 0xbe0b11 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool) /home/mpolacek/src/gcc/gcc/cp/pt.c:17691 0xbe3622 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool) /home/mpolacek/src/gcc/gcc/cp/pt.c:17999 0xc07109 instantiate_decl(tree_node*, bool, bool) /home/mpolacek/src/gcc/gcc/cp/pt.c:25523 0xc07ac6 instantiate_pending_templates(int) /home/mpolacek/src/gcc/gcc/cp/pt.c:25639 0xa6c433 c_parse_final_cleanups() /home/mpolacek/src/gcc/gcc/cp/decl2.c:4875 0xd4adb6 c_common_parse_file() /home/mpolacek/src/gcc/gcc/c-family/c-opts.c:1208
The test was rejected with various errors, then with r9-4045-g0c1e0d63fe0ceabbd04384070f3b59f8bf50de09 we got this ICE: 93689.C: In function ‘int f() [with auto Z = <lambda closure object>main()::<lambda(int)>{}]’: 93689.C:5:13: internal compiler error: in gimplify_expr, at gimplify.c:12491 5 | return Z(); | ~^~ 0x6dbdf3 gimplify_expr(tree_node**, gimple**, gimple**, bool (*)(tree_node*), int) /home/mpolacek/src/gcc9/gcc/gimplify.c:12491 0xceaa9d gimplify_expr(tree_node**, gimple**, gimple**, bool (*)(tree_node*), int) /home/mpolacek/src/gcc9/gcc/gimplify.c:13226 0xcf3ce4 gimplify_expr /home/mpolacek/src/gcc9/gcc/gimplify.c:13512 0xcf5965 gimplify_call_expr /home/mpolacek/src/gcc9/gcc/gimplify.c:3405 ... and since r10-6527-gaaa26bf496a646778ac861aed124d960b5bf549f the ICE in Comment 1.
@Marek Polacek do you (or anyone) think this is a good first issue? I am curious to try contributing.
That's hard to say without really understanding what the issue is, but I'm afraid this might not be the best first issue -- it involves some pretty convoluted features, plus it seems there are two issues (that maybe_constant_value gets a template code and the gimplifier ICE). "Luckily" we have over 3000 C++ issues and some of them should be more straightforward: https://gcc.gnu.org/bugzilla/buglist.cgi?component=c%2B%2B&limit=0&list_id=240106&product=gcc&query_format=advanced&resolution=---&order=bug_id%20DESC&query_based_on= Note that we're in stage 4 now, meaning that we only fix regressions. Good luck.
Actually the issue might be just one, even the gimplifier ICE seems to be caused by a CAST_EXPR leaking where it should not. Maybe we fail to substitute default arguments in lambdas in a template parameter list.
It seems that it's the cast '(int)0' that causes the issue. If we remove the cast it works fine: ``` template < auto Z = [](int l = 0) -> int { return l; } > int f() { return Z(); } int main() { return f(); } ``` https://godbolt.org/z/zq3E7M
somewhat related 93595 ( https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93595 ) is still in UNCONFIRMED state.