https://gcc.gnu.org/g:83c83219a531024921e56e6cbc1c21094b11561e
commit r15-11543-g83c83219a531024921e56e6cbc1c21094b11561e Author: Patrick Palka <[email protected]> Date: Fri Aug 14 15:53:12 2026 -0400 c++: is_const_eval init folding inside lambda scope [PR126483] Here when processing a's copy-initialization we overeagerly fold its initializer as if the lambda scope is not constexpr, and thus assume is_constant_evaluated is false. Subsequent constant evaluation of the lambda gives the wrong answer due to this premature folding. The culprit cp_fully_fold_init call is guarded by DECL_DECLARED_CONSTEXPR_P but that can't be relied on for maybe-constexpr lambdas until the lambda is fully parsed and its body is checked as a whole for constexpr suitability. This patch makes us check maybe_constexpr_fn instead, which considers such lambdas. This however means we no longer fold a's copy-init even for the runtime version of the lambda body, hence the is_constant_evaluated3b.C xfail. Making maybe_constexpr_fn distinguish between fully parsed constexpr and non-constexpr lambdas doesn't help because we don't hit this code path again. And cp_fold_function doesn't fold the copy-init because it's represented as an INIT_EXPR of an AGGR_INIT_EXPR which it doesn't yet handle. The subsequent patch will enable folding of INIT_EXPR initializers to address this xfail. PR c++/126483 gcc/cp/ChangeLog: * typeck2.cc (store_init_value): Check maybe_constexpr_fn instead of DECL_DECLARED_CONSTEXPR_P of the function scope. gcc/testsuite/ChangeLog: * g++.dg/opt/is_constant_evaluated3b.C: xfail a1 initializer folding. * g++.dg/cpp2a/is-constant-evaluated16.C: New test. Reviewed-by: Jason Merrill <[email protected]> (cherry picked from commit b3bca9f73eb6b29bddf5fec92e929f32675b8d20) Diff: --- gcc/cp/typeck2.cc | 2 +- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated16.C | 20 ++++++++++++++++++++ gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C | 2 +- 3 files changed, 22 insertions(+), 2 deletions(-) diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc index 1ed20ce921c0..d8544cd7262e 100644 --- a/gcc/cp/typeck2.cc +++ b/gcc/cp/typeck2.cc @@ -931,7 +931,7 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags) that might fold away something that needs to be diagnosed at constexpr evaluation time. */ if (!current_function_decl - || !DECL_DECLARED_CONSTEXPR_P (current_function_decl) + || !maybe_constexpr_fn (current_function_decl) || TREE_STATIC (decl)) value = cp_fully_fold_init (value); diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated16.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated16.C new file mode 100644 index 000000000000..c727967673e3 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated16.C @@ -0,0 +1,20 @@ +// PR c++/126483 +// { dg-do compile { target c++17 } } + +struct A { + int m; + constexpr A(int n) : m(__builtin_is_constant_evaluated() ? 42 : n) { } +}; + +constexpr auto v = [] { + A a = 0; + return a.m; +}; +static_assert(v() == 42); + +template<class> +constexpr auto vt = [] { + A a = 0; + return a.m; +}; +static_assert(vt<void>() == 42); diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C index ff708e23832b..ac6d21d632e1 100644 --- a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C +++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C @@ -17,7 +17,7 @@ auto f = [] { A a5{}; }; -// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } } +// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } } // { dg-final { scan-tree-dump "a2 = {\\.n=42, \\.m=0}" "original" } } // { dg-final { scan-tree-dump "a3 = {\\.n=42, \\.m=0}" "original" } } // { dg-final { scan-tree-dump "a4 = {\\.n=42, \\.m=0}" "original" } }
