[PATCH] c++: Make braced-init-list as template arg work with aggr init [PR95369]

[Marek Polacek via Gcc-patches]

Barry pointed out to me that our braced-init-list as a template-argument
extension doesn't work as expected when we aggregate-initialize.  Thus
fixed by calling digest_init in convert_nontype_argument so that we can
actually convert the CONSTRUCTOR.

I don't think we can call digest_init any earlier, and it needs to
happen before the call to build_converted_constant_expr.

Barry also noticed that we allow designated initializers for
non-aggregate types in the template-argument argument context, i.e. this

  struct S {
    unsigned a;
    unsigned b;
    constexpr S(unsigned _a, unsigned _b) noexcept: a{_a}, b{_b} { }
  };

  template<S s> struct X { };

  void f()
  {
    X<{.a = 1, .b = 2}> x;
  }

probably should not compile.  But I'm not too sure about it, and don't
know how I would fix it anyway, so I'm not dealing with it in this
patch.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

gcc/cp/ChangeLog:

        PR c++/95369
        * pt.c (convert_nontype_argument): In C++20, reshape and digest
        a braced-init-list if the type is an aggregate.

gcc/testsuite/ChangeLog:

        PR c++/95369
        * g++.dg/cpp2a/nontype-class38.C: New test.
---
 gcc/cp/pt.c                                  | 13 +++++++++
 gcc/testsuite/g++.dg/cpp2a/nontype-class38.C | 30 ++++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class38.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 90dafff3aa7..adb7593f77d 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -7133,6 +7133,19 @@ convert_nontype_argument (tree type, tree expr, 
tsubst_flags_t complain)
        return error_mark_node;
     }
 
+  /* For a { } template argument, like in X<{ 1, 2 }>, we need to digest
+     here so that build_converted_constant_expr below is able to convert
+     it to TYPE.  */
+  if (cxx_dialect >= cxx20
+      && BRACE_ENCLOSED_INITIALIZER_P (expr)
+      && CP_AGGREGATE_TYPE_P (type))
+    {
+      expr = reshape_init (type, expr, complain);
+      expr = digest_init (type, expr, complain);
+      if (expr == error_mark_node)
+       return error_mark_node;
+    }
+
   /* If we are in a template, EXPR may be non-dependent, but still
      have a syntactic, rather than semantic, form.  For example, EXPR
      might be a SCOPE_REF, rather than the VAR_DECL to which the
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C 
b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
new file mode 100644
index 00000000000..5b440fd1c9e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
@@ -0,0 +1,30 @@
+// PR c++/95369
+// { dg-do compile { target c++20 } }
+
+struct S {
+  int a;
+  int b;
+};
+
+struct W {
+  int i;
+  S s;
+};
+
+template <S p>
+void fnc()
+{
+}
+
+template<S s> struct X { };
+template<W w> struct Y { };
+
+void f()
+{
+  fnc<{ .a = 10, .b = 20 }>();
+  fnc<{ 10, 20 }>();
+  X<{ .a = 1, .b = 2 }> x;
+  X<{ 1, 2 }> x2;
+  // Brace elision is likely to be allowed.
+  Y<{ 1, 2, 3 }> x3;
+}

base-commit: 3d8d5ddb539a5254c7ef83414377f4c74c7701d4
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA