[gcc r16-9538] c++: is_const_eval init folding inside lambda scope [PR126483]

https://gcc.gnu.org/g:62b3305d69025d25a5b8cbc531b14a6adb3d1cbc

commit r16-9538-g62b3305d69025d25a5b8cbc531b14a6adb3d1cbc
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 664dd0990bf8..d29dbfcb65df 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1019,7 +1019,7 @@ 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" } }