[gcc r13-6716] c++: maybe_constant_init and unevaluated operands [PR109030]

[gcc r13-6716] c++: maybe_constant_init and unevaluated operands [PR109030]

[Patrick Palka]

https://gcc.gnu.org/g:31cdfdef04701e10cffcec4578b2337684f0e4bc

commit r13-6716-g31cdfdef04701e10cffcec4578b2337684f0e4bc
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Mar 16 14:47:43 2023 -0400

    c++: maybe_constant_init and unevaluated operands [PR109030]
    
    This testcase in this PR (already fixed by r13-6526-ge4692319fd5fc7)
    demonstrates that maybe_constant_init can be called on an unevaluated
    operand (e.g. from massage_init_elt) so this entry point should also
    limit constant evaluation in that case, like maybe_constant_value does.
    
            PR c++/109030
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (maybe_constant_init_1): For an unevaluated
            non-manifestly-constant operand, don't constant evaluate
            and instead call fold_to_constant as in maybe_constant_value.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/constexpr-inst2.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                          |  4 ++++
 gcc/testsuite/g++.dg/cpp2a/constexpr-inst2.C | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index abf6ee560c5..3de60cfd0f8 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8796,6 +8796,10 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
 			&& (TREE_STATIC (decl) || DECL_EXTERNAL (decl)));
       if (is_static)
 	manifestly_const_eval = true;
+
+      if (cp_unevaluated_operand && !manifestly_const_eval)
+	return fold_to_constant (t);
+
       t = cxx_eval_outermost_constant_expr (t, allow_non_constant, !is_static,
 					    mce_value (manifestly_const_eval),
 					    false, decl);
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-inst2.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-inst2.C
new file mode 100644
index 00000000000..7676b625321
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-inst2.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++11 } }
+
+struct __as_receiver {
+  int empty_env;
+};
+
+template<class T>
+constexpr int f(T t) {
+  return t.fail;
+};
+
+using type = decltype(__as_receiver{f(0)}); // OK, f<int> not instantiated