[PATCH] c++: Fix ICE with __builtin_bit_cast [PR98469]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

On the following testcase we ICE during constexpr evaluation (for warnings),
because the IL has ADDR_EXPR of BIT_CAST_EXPR and ADDR_EXPR case asserts
the result is not a CONSTRUCTOR.
I've tried to force a temporary for those in call.c next to:
        if (convs->need_temporary_p
            || TREE_CODE (expr) == CONSTRUCTOR
            || TREE_CODE (expr) == VA_ARG_EXPR)
but that resulted in a lot of ICEs, so this patch just punts on lval
evaluation of BIT_CAST_EXPR instead, normally __builtin_bit_cast is called
from std::bit_cast which is constexpr and therefore the BIT_CAST_EXPR
isn't evaluated there during parsing or tsubst and when evaluating the call
to std::bit_cast the NRV optimized return is assigned to some temporary or
variable and so BIT_CAST_EXPR is not evaluated as lval.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2020-12-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98469
	* constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>:
	Punt if lval is true.

	* g++.dg/cpp2a/bit-cast8.C: New test.
	* g++.dg/cpp2a/bit-cast9.C: New test.

--- gcc/cp/constexpr.c.jj	2020-12-23 22:44:05.398093175 +0100
+++ gcc/cp/constexpr.c	2020-12-29 10:32:44.865030881 +0100
@@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons
       return t;
 
     case BIT_CAST_EXPR:
+      if (lval)
+	{
+	  if (!ctx->quiet)
+	    error_at (EXPR_LOCATION (t),
+		      "address of a call to %qs is not a constant expression",
+		      "__builtin_bit_cast");
+	  *non_constant_p = true;
+	  return t;
+	}
       r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p);
       break;
 
--- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj	2020-12-29 10:35:31.547140723 +0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C	2020-12-29 10:34:26.431879120 +0100
@@ -0,0 +1,11 @@
+// PR c++/98469
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wall" }
+
+struct S { int s; };
+
+S
+foo ()
+{
+  return __builtin_bit_cast (S, 0);
+}
--- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj	2020-12-29 10:35:35.018101365 +0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C	2020-12-29 10:35:05.905431494 +0100
@@ -0,0 +1,15 @@
+// PR c++/98469
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wall" }
+
+template<typename T, typename F>
+constexpr T
+bit_cast (const F &f) noexcept
+{
+  return __builtin_bit_cast (T, f);
+}
+struct S { int s; };
+constexpr int foo (const S &x) { return x.s; }
+constexpr int bar () { return foo (bit_cast<S> (0)); }
+constexpr int x = bar ();
+static_assert (!x);

	Jakub