From 2b0c8377729a3c62a05897136666574ab939aaab Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Tue, 26 Apr 2022 11:15:04 -0400
Subject: [PATCH] c++: constexpr ref to array of array [PR102307]

The problem here is that first check_initializer calls
build_aggr_init_full_exprs, which does overload resolution, but then in the
case of failed constexpr throws away the result and does it again in
build_functional_cast.  But in the first overload resolution,
reshape_init_array_1 decided to reuse the inner CONSTRUCTORs because
tf_error is set, so we know we're committed.  But the second pass gets
confused by the CONSTRUCTORs with non-init-list types.

Fixed by avoiding a second pass: instead, pass the call from build_aggr_init
to build_cplus_new, which will turn it into a TARGET_EXPR.  I don't bother
to change the object argument because it will be replaced later in
simplify_aggr_init_expr.

	PR c++/102307

gcc/cp/ChangeLog:

	* decl.cc (check_initializer): Use build_cplus_new in case of
	constexpr failure.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-array2.C: New test.
---
 gcc/cp/decl.cc                                | 17 ++++++++++++-----
 gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C | 12 ++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 5654bc754e6..381259cb9cf 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7413,12 +7413,19 @@ check_initializer (tree decl, tree init, int flags, vec<tree, va_gc> **cleanups)
 	      /* Declared constexpr or constinit, but no suitable initializer;
 		 massage init appropriately so we can pass it into
 		 store_init_value for the error.  */
-	      if (CLASS_TYPE_P (type)
-		  && (!init || TREE_CODE (init) == TREE_LIST))
+	      tree new_init = NULL_TREE;
+	      if (!processing_template_decl
+		  && TREE_CODE (init_code) == CALL_EXPR)
+		new_init = build_cplus_new (type, init_code, tf_none);
+	      else if (CLASS_TYPE_P (type)
+		       && (!init || TREE_CODE (init) == TREE_LIST))
+		new_init = build_functional_cast (input_location, type,
+						  init, tf_none);
+	      if (new_init)
 		{
-		  init = build_functional_cast (input_location, type,
-						init, tf_none);
-		  if (TREE_CODE (init) == TARGET_EXPR)
+		  init = new_init;
+		  if (TREE_CODE (init) == TARGET_EXPR
+		      && !(flags & LOOKUP_ONLYCONVERTING))
 		    TARGET_EXPR_DIRECT_INIT_P (init) = true;
 		}
 	      init_code = NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C
new file mode 100644
index 00000000000..c30e3f2361d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C
@@ -0,0 +1,12 @@
+// PR c++/102307
+// { dg-do compile { target c++11 } }
+
+#include <array>
+template <unsigned N, unsigned M> struct Matrix {
+  constexpr Matrix(double const (&arr)[N][M]); // { dg-warning "never defined" }
+  constexpr Matrix(std::array<std::array<double, M>, N> const &arr);
+};
+int main() {
+  constexpr Matrix<2, 3>
+    mat {{ {1.0, 2.0, 3.0}, {4.0, 5.0, 6.0} }}; // { dg-error "before its definition" }
+}
-- 
2.11.4.GIT