c++: ICE with GOTO_EXPR [PR118928]

In this PR we crash in cxx_eval_constant_expression/GOTO_EXPR on:

  gcc_assert (cxx_dialect >= cxx23);

The code obviously doesn't expect to see a goto pre-C++23.  But we can
get here with the new prvalue optimization.  In this test we found
ourselves in synthesize_method for X::X().  This function calls:

 a) finish_function, which does cp_genericize -> ... -> genericize_c_loops,
    which creates the GOTO_EXPR;
 b) expand_or_defer_fn -> maybe_clone_body -> ... -> cp_fold_function
    where we reach the new maybe_constant_init call and crash on the
    goto.

Since we can validly get to that assert, I think we should just remove
it.  I don't see other similar asserts like this one.

	PR c++/118928

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression) <case GOTO_EXPR>: Remove
	an assert.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-prvalue5.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

2 files changed, 24 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 59dd066..c68666c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8691,7 +8691,6 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	*jump_target = TREE_OPERAND (t, 0);
       else
 	{
-	  gcc_assert (cxx_dialect >= cxx23);
 	  if (!ctx->quiet)
 	    error_at (loc, "%<goto%> is not a constant expression");
 	  *non_constant_p = true;
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-prvalue5.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-prvalue5.C
new file mode 100644
index 0000000..1f847bb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-prvalue5.C
@@ -0,0 +1,24 @@
+// PR c++/118928
+// { dg-do compile { target c++11 } }
+// { dg-options "-O" }
+
+using size_t = decltype(sizeof(0));
+
+namespace std {
+template <typename T> struct initializer_list {
+  const T *_M_array;
+  size_t _M_len;
+};
+struct S {
+  constexpr S(const char *); // { dg-warning "used but never defined" }
+};
+struct vector {
+  constexpr vector(initializer_list<S>) {}
+};
+}
+struct Y {
+    std::vector v;
+};
+struct X {
+  Y y{{""}};
+} x;