[PATCH] c++, v2: Fix constexpr evaluation of parameters passed by invisible reference [PR111284]

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Apr 15, 2024 at 02:19:36PM +0200, Jakub Jelinek wrote:
> They weren't the same, one was trying to evaluate the convert_from_reference
> with vc_glvalue, the other evaluates it without that and with vc_prvalue.
> Now, I guess the
> +     /* Undo convert_for_arg_passing work here.  */
> +     if (TYPE_REF_P (TREE_TYPE (x))
> +         && !same_type_p (type, TREE_TYPE (TREE_TYPE (x))))
> +       x = cp_fold_convert (build_reference_type (type), x);
> part could be thrown away, given the other !same_type_p check (that one is
> needed because adjust_temp_type can't deal with that), at least
> when I remove that
> GXX_TESTSUITE_STDS=98,11,14,17,20,23,26 make check-g++ RUNTESTFLAGS="dg.exp='constexpr-dtor* pr114426.C constexpr-111284.C constexpr-lifetime7.C'"
> still passes.

I've now tested that version and it passed bootstrap/regtest on x86_64-linux
and i686-linux.  But as I said earlier, trying to tweak the patch further
doesn't work on the constexpr-dtor{5,6}.C tests.

2024-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/111284
	* constexpr.cc (cxx_bind_parameters_in_call): For PARM_DECLs with
	TREE_ADDRESSABLE types use vc_glvalue rather than vc_prvalue for
	cxx_eval_constant_expression and if it doesn't have the same
	type as it should, cast the reference type to reference to type
	before convert_from_reference and instead of adjust_temp_type
	take address of the arg, cast to reference to type and then
	convert_from_reference.
	(cxx_eval_constant_expression) <case PARM_DECL>: For lval case
	on parameters with TREE_ADDRESSABLE types lookup result in
	ctx->globals if possible.  Otherwise if lookup in ctx->globals
	was successful for parameter with TREE_ADDRESSABLE type,
	recurse with vc_prvalue on the returned value.

	* g++.dg/cpp1z/constexpr-111284.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime7.C: Expect one error on a different
	line.

--- gcc/cp/constexpr.cc.jj	2024-02-13 10:29:57.979155641 +0100
+++ gcc/cp/constexpr.cc	2024-03-07 19:35:01.032412221 +0100
@@ -1877,13 +1877,18 @@ cxx_bind_parameters_in_call (const const
 	  x = build_address (x);
 	}
       if (TREE_ADDRESSABLE (type))
-	/* Undo convert_for_arg_passing work here.  */
-	x = convert_from_reference (x);
-      /* Normally we would strip a TARGET_EXPR in an initialization context
-	 such as this, but here we do the elision differently: we keep the
-	 TARGET_EXPR, and use its CONSTRUCTOR as the value of the parm.  */
-      arg = cxx_eval_constant_expression (ctx, x, vc_prvalue,
-					  non_constant_p, overflow_p);
+	{
+	  /* Undo convert_for_arg_passing work here.  */
+	  x = convert_from_reference (x);
+	  arg = cxx_eval_constant_expression (ctx, x, vc_glvalue,
+					      non_constant_p, overflow_p);
+	}
+      else
+	/* Normally we would strip a TARGET_EXPR in an initialization context
+	   such as this, but here we do the elision differently: we keep the
+	   TARGET_EXPR, and use its CONSTRUCTOR as the value of the parm.  */
+	arg = cxx_eval_constant_expression (ctx, x, vc_prvalue,
+					    non_constant_p, overflow_p);
       /* Check we aren't dereferencing a null pointer when calling a non-static
 	 member function, which is undefined behaviour.  */
       if (i == 0 && DECL_OBJECT_MEMBER_FUNCTION_P (fun)
@@ -1909,7 +1914,16 @@ cxx_bind_parameters_in_call (const const
 	{
 	  /* Make sure the binding has the same type as the parm.  But
 	     only for constant args.  */
-	  if (!TYPE_REF_P (type))
+	  if (TREE_ADDRESSABLE (type))
+	    {
+	      if (!same_type_p (type, TREE_TYPE (arg)))
+		{
+		  arg = build_fold_addr_expr (arg);
+		  arg = cp_fold_convert (build_reference_type (type), arg);
+		  arg = convert_from_reference (arg);
+		}
+	    }
+	  else if (!TYPE_REF_P (type))
 	    arg = adjust_temp_type (type, arg);
 	  if (!TREE_CONSTANT (arg))
 	    *non_constant_args = true;
@@ -7499,9 +7513,19 @@ cxx_eval_constant_expression (const cons
 
     case PARM_DECL:
       if (lval && !TYPE_REF_P (TREE_TYPE (t)))
-	/* glvalue use.  */;
+	{
+	  /* glvalue use.  */
+	  if (TREE_ADDRESSABLE (TREE_TYPE (t)))
+	    if (tree v = ctx->global->get_value (t))
+	      r = v;
+	}
       else if (tree v = ctx->global->get_value (t))
-	r = v;
+	{
+	  r = v;
+	  if (TREE_ADDRESSABLE (TREE_TYPE (t)))
+	    r = cxx_eval_constant_expression (ctx, r, vc_prvalue,
+					      non_constant_p, overflow_p);
+	}
       else if (lval)
 	/* Defer in case this is only used for its type.  */;
       else if (ctx->global->is_outside_lifetime (t))
--- gcc/testsuite/g++.dg/cpp1z/constexpr-111284.C.jj	2024-03-07 16:27:48.113651999 +0100
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-111284.C	2024-03-07 16:26:49.565466606 +0100
@@ -0,0 +1,19 @@
+// PR c++/111284
+// { dg-do compile { target c++17 } }
+
+struct S {
+  S () = default;
+  constexpr S (const S &) noexcept : s{this} {}
+  constexpr S & operator= (const S &) noexcept { return *this; }
+  constexpr bool foo () const noexcept { return s == this; }
+  S *s = this;
+};
+
+constexpr bool
+bar (S x) noexcept
+{
+  return x.foo ();
+}
+
+static_assert (bar (S {}), "");
+static_assert ([] (S x) { return x.foo (); } (S {}), "");
--- gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime7.C.jj	2023-12-13 19:09:33.252657826 +0100
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime7.C	2024-03-07 19:49:49.342334090 +0100
@@ -87,7 +87,7 @@ constexpr bool n1 = test_access<NonTrivi
 constexpr bool n2 = test_modification<NonTrivial>();  // { dg-message "in .constexpr." "" { target c++20 } }
 constexpr bool n3 = test_scope<NonTrivial>();         // { dg-message "in .constexpr." "" { target c++20 } }
 constexpr bool n4 = test_destroy_temp<NonTrivial>();  // { dg-message "in .constexpr." "" { target c++20 } }
-constexpr bool n5 = test_parameter(NonTrivial{});     // { dg-error "destroying" "" { target c++20 } }
+constexpr bool n5 = test_parameter(NonTrivial{});     // { dg-message "in .constexpr." "" { target c++20 } }
 constexpr bool n6 = test_bindings<NonTrivial>();
 #endif
 


	Jakub