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.

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 2852093..d1fc7e1 100644 (file)
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7397,12 +7397,19 @@ 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 (file)
index 0000000..c30e3f2
--- /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" }
+}