C++ PATCH for c++/85113, ICE with constexpr and __builtin_constant_p.

[Jason Merrill]

My recent patch to make sure that we've given an error for failed
template argument conversion caught this latent bug: when we evaluate
__builtin_constant_p within a constexpr function, we don't fold it
until later, at constexpr evaluation time when we have values for the
arguments.  Which makes sense for the maybe_constant_value case.  But
here we require a constant value, but are silently calling it
non-constant.  So only do the deferral if we're being quiet about
non-constant expressions.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.

commit be9bfa78348d2f5aec7209e126cf77ec89d60fc3
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue Apr 3 13:59:29 2018 -0400

            PR c++/85113 - ICE with constexpr and __builtin_constant_p.
    
            * constexpr.c (cxx_eval_builtin_function_call): Only defer
            __builtin_constant_p if ctx->quiet.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index bebd9f5b5d0..201f27d8e66 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1171,7 +1171,10 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
   /* Don't fold __builtin_constant_p within a constexpr function.  */
   bool bi_const_p = (DECL_FUNCTION_CODE (fun) == BUILT_IN_CONSTANT_P);
 
+  /* If we aren't requiring a constant expression, defer __builtin_constant_p
+     in a constexpr function until we have values for the parameters.  */
   if (bi_const_p
+      && ctx->quiet
       && current_function_decl
       && DECL_DECLARED_CONSTEXPR_P (current_function_decl))
     {
diff --git a/gcc/testsuite/g++.dg/ext/builtin12.C b/gcc/testsuite/g++.dg/ext/builtin12.C
new file mode 100644
index 00000000000..1d6bb75cd77
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/builtin12.C
@@ -0,0 +1,10 @@
+// PR c++/85113
+// { dg-do compile { target c++14 } }
+
+template<bool> struct A {};
+
+constexpr int foo()
+{
+  A<__builtin_constant_p(0)> a{};
+  return 0;
+}