[gcc r16-9537] c++: testsuite: Add some opt/is_constant_evaluated3.C variants

https://gcc.gnu.org/g:7ed28af4fcad61780d5f4b38b5a99fdfcc2e6b03

commit r16-9537-g7ed28af4fcad61780d5f4b38b5a99fdfcc2e6b03
Author: Patrick Palka <[email protected]>
Date:   Fri Aug 14 15:53:10 2026 -0400

    c++: testsuite: Add some opt/is_constant_evaluated3.C variants
    
    r13-6422 added opt/is_constant_evaluated3.C to verify that we constant
    fold copy-init such as a1.  (Later, r14-6506 enabled constant folding of
    the a2-a5 inits.)  r13-6422 turns out to trigger a latent bug and causes
    PR c++/126483, premature folding of is_constant_evaluated in copy-init
    inside a lambda scope.
    
    This patch adds a couple extra variants of this test, one where the
    containing function is constexpr (3a) and one where it's a lambda (3b).
    
    The new 3a.C demonstrates however that we still don't constant fold a1's
    copy-init when inside a constexpr function scope.  3b.C demonstrates that
    we do fold it when inside a (maybe-constexpr) lambda scope.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/opt/is_constant_evaluated3a.C: New test.
            * g++.dg/opt/is_constant_evaluated3b.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit 9b0341a7a6a38912e667982003bcbfebdfb86335)

Diff:
---
 gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C | 24 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C | 24 ++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C
new file mode 100644
index 000000000000..4ec3ecdf7b5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3a.C
@@ -0,0 +1,24 @@
+// A version of is_constant_evaluated3.C where the containing function is
+// constexpr.
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-O -fdump-tree-original" }
+
+struct A {
+  constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { }
+  constexpr A() : A(42) { }
+  int n, m;
+};
+
+constexpr void f() {
+  A a1 = {42};
+  A a2{42};
+  A a3(42);
+  A a4;
+  A a5{};
+}
+
+// { 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" } }
+// { dg-final { scan-tree-dump "a5 = {\\.n=42, \\.m=0}" "original" } }
diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C
new file mode 100644
index 000000000000..ff708e23832b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3b.C
@@ -0,0 +1,24 @@
+// A version of is_constant_evaluated3.C where the containing function is
+// a (maybe-constexpr) lambda.
+// { dg-do compile { target c++17 } }
+// { dg-additional-options "-O -fdump-tree-original" }
+
+struct A {
+  constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { }
+  constexpr A() : A(42) { }
+  int n, m;
+};
+
+auto f = [] {
+  A a1 = {42};
+  A a2{42};
+  A a3(42);
+  A a4;
+  A a5{};
+};
+
+// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } }
+// { 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" } }
+// { dg-final { scan-tree-dump "a5 = {\\.n=42, \\.m=0}" "original" } }