[gcc r10-10760] c++: constexpr ref to array of array [PR102307]

[gcc r10-10760] c++: constexpr ref to array of array [PR102307]

[Jason Merrill]

https://gcc.gnu.org/g:5022f4af0d1e2f3fdf2bb159d4372ee3cf34b052

commit r10-10760-g5022f4af0d1e2f3fdf2bb159d4372ee3cf34b052
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Apr 26 11:15:04 2022 -0400

    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.c (check_initializer): Use build_cplus_new in case of
            constexpr failure.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1z/constexpr-array2.C: New test.

Diff:
---
 gcc/cp/decl.c                                 | 17 ++++++++++++-----
 gcc/testsuite/g++.dg/cpp1z/constexpr-array2.C | 12 ++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 6089cd7e52d..38003a5a70f 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -6876,12 +6876,19 @@ 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" }
+}