c++: Add support for __real__/__imag__ modifications in constant expressions [PR88174]

We claim we support P0415R1 (constexpr complex), but e.g.
 #include <complex>

constexpr bool
foo ()
{
  std::complex<double> a (1.0, 2.0);
  a += 3.0;
  a.real (6.0);
  return a.real () == 6.0 && a.imag () == 2.0;
}

static_assert (foo ());

fails with
test.C:12:20: error: non-constant condition for static assertion
   12 | static_assert (foo ());
      |                ~~~~^~
test.C:12:20:   in ‘constexpr’ expansion of ‘foo()’
test.C:8:10:   in ‘constexpr’ expansion of ‘a.std::complex<double>::real(6.0e+0)’
test.C:12:20: error: modification of ‘__real__ a.std::complex<double>::_M_value’ is not a constant expression

The problem is we don't handle REALPART_EXPR and IMAGPART_EXPR
in cxx_eval_store_expression.
The following patch attempts to support it (with a requirement
that those are the outermost expressions, ARRAY_REF/COMPONENT_REF
etc. are just not possible on the result of these, BIT_FIELD_REF
would be theoretically possible if trying to extract some bits
from one part of a complex int, but I don't see how it could appear
in the FE trees.

For these references, the code handles value being COMPLEX_CST,
COMPLEX_EXPR or CONSTRUCTOR_NO_CLEARING empty CONSTRUCTOR (what we use
to represent uninitialized values for C++20 and later) and the
code starts by rewriting it to COMPLEX_EXPR, so that we can freely
adjust the individual parts and later on possibly optimize it back
to COMPLEX_CST if both halves are constant.

2022-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/88174
	* constexpr.cc (cxx_eval_store_expression): Handle REALPART_EXPR
	and IMAGPART_EXPR.  Change ctors from releasing_vec to
	auto_vec<tree *>, adjust all uses.  For !preeval, update ctors
	vector.

	* g++.dg/cpp1y/constexpr-complex1.C: New test.

1 files changed, 80 insertions, 14 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5e0d339..c047fe4 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5732,6 +5732,20 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  }
 	  break;
 
+	case REALPART_EXPR:
+	  gcc_assert (probe == target);
+	  vec_safe_push (refs, probe);
+	  vec_safe_push (refs, TREE_TYPE (probe));
+	  probe = TREE_OPERAND (probe, 0);
+	  break;
+
+	case IMAGPART_EXPR:
+	  gcc_assert (probe == target);
+	  vec_safe_push (refs, probe);
+	  vec_safe_push (refs, TREE_TYPE (probe));
+	  probe = TREE_OPERAND (probe, 0);
+	  break;
+
 	default:
 	  if (evaluated)
 	    object = probe;
@@ -5770,7 +5784,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   type = TREE_TYPE (object);
   bool no_zero_init = true;
 
-  releasing_vec ctors, indexes;
+  auto_vec<tree *> ctors;
+  releasing_vec indexes;
   auto_vec<int> index_pos_hints;
   bool activated_union_member_p = false;
   bool empty_base = false;
@@ -5810,14 +5825,36 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  *valp = ary_ctor;
 	}
 
-      /* If the value of object is already zero-initialized, any new ctors for
-	 subobjects will also be zero-initialized.  */
-      no_zero_init = CONSTRUCTOR_NO_CLEARING (*valp);
-
       enum tree_code code = TREE_CODE (type);
       tree reftype = refs->pop();
       tree index = refs->pop();
 
+      if (code == COMPLEX_TYPE)
+	{
+	  if (TREE_CODE (*valp) == COMPLEX_CST)
+	    *valp = build2 (COMPLEX_EXPR, type, TREE_REALPART (*valp),
+			    TREE_IMAGPART (*valp));
+	  else if (TREE_CODE (*valp) == CONSTRUCTOR
+		   && CONSTRUCTOR_NELTS (*valp) == 0
+		   && CONSTRUCTOR_NO_CLEARING (*valp))
+	    {
+	      tree r = build_constructor (reftype, NULL);
+	      CONSTRUCTOR_NO_CLEARING (r) = 1;
+	      *valp = build2 (COMPLEX_EXPR, type, r, r);
+	    }
+	  gcc_assert (TREE_CODE (*valp) == COMPLEX_EXPR);
+	  ctors.safe_push (valp);
+	  vec_safe_push (indexes, index);
+	  valp = &TREE_OPERAND (*valp, TREE_CODE (index) == IMAGPART_EXPR);
+	  gcc_checking_assert (refs->is_empty ());
+	  type = reftype;
+	  break;
+	}
+
+      /* If the value of object is already zero-initialized, any new ctors for
+	 subobjects will also be zero-initialized.  */
+      no_zero_init = CONSTRUCTOR_NO_CLEARING (*valp);
+
       if (code == RECORD_TYPE && is_empty_field (index))
 	/* Don't build a sub-CONSTRUCTOR for an empty base or field, as they
 	   have no data and might have an offset lower than previously declared
@@ -5860,7 +5897,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  no_zero_init = true;
 	}
 
-      vec_safe_push (ctors, *valp);
+      ctors.safe_push (valp);
       vec_safe_push (indexes, index);
 
       constructor_elt *cep
@@ -5922,11 +5959,11 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	     semantics are not applied on an object under construction.
 	     They come into effect when the constructor for the most
 	     derived object ends."  */
-	  for (tree elt : *ctors)
+	  for (tree *elt : ctors)
 	    if (same_type_ignoring_top_level_qualifiers_p
-		(TREE_TYPE (const_object_being_modified), TREE_TYPE (elt)))
+		(TREE_TYPE (const_object_being_modified), TREE_TYPE (*elt)))
 	      {
-		fail = TREE_READONLY (elt);
+		fail = TREE_READONLY (*elt);
 		break;
 	      }
 	}
@@ -5967,6 +6004,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
       valp = ctx->global->values.get (object);
       for (unsigned i = 0; i < vec_safe_length (indexes); i++)
 	{
+	  ctors[i] = valp;
 	  constructor_elt *cep
 	    = get_or_insert_ctor_field (*valp, indexes[i], index_pos_hints[i]);
 	  valp = &cep->value;
@@ -6029,17 +6067,45 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
      CONSTRUCTORs, if any.  */
   bool c = TREE_CONSTANT (init);
   bool s = TREE_SIDE_EFFECTS (init);
+  if (!indexes->is_empty ())
+    {
+      tree last = indexes->last ();
+      if (TREE_CODE (last) == REALPART_EXPR
+	  || TREE_CODE (last) == IMAGPART_EXPR)
+	{
+	  /* And canonicalize COMPLEX_EXPR into COMPLEX_CST if
+	     possible.  */
+	  tree *cexpr = ctors.last ();
+	  if (tree c = const_binop (COMPLEX_EXPR, TREE_TYPE (*cexpr),
+				    TREE_OPERAND (*cexpr, 0),
+				    TREE_OPERAND (*cexpr, 1)))
+	    *cexpr = c;
+	  else
+	    {
+	      TREE_CONSTANT (*cexpr)
+		= (TREE_CONSTANT (TREE_OPERAND (*cexpr, 0))
+		   & TREE_CONSTANT (TREE_OPERAND (*cexpr, 1)));
+	      TREE_SIDE_EFFECTS (*cexpr)
+		= (TREE_SIDE_EFFECTS (TREE_OPERAND (*cexpr, 0))
+		   | TREE_SIDE_EFFECTS (TREE_OPERAND (*cexpr, 1)));
+	    }
+	  c = TREE_CONSTANT (*cexpr);
+	  s = TREE_SIDE_EFFECTS (*cexpr);
+	}
+    }
   if (!c || s || activated_union_member_p)
-    for (tree elt : *ctors)
+    for (tree *elt : ctors)
       {
+	if (TREE_CODE (*elt) != CONSTRUCTOR)
+	  continue;
 	if (!c)
-	  TREE_CONSTANT (elt) = false;
+	  TREE_CONSTANT (*elt) = false;
 	if (s)
-	  TREE_SIDE_EFFECTS (elt) = true;
+	  TREE_SIDE_EFFECTS (*elt) = true;
 	/* Clear CONSTRUCTOR_NO_CLEARING since we've activated a member of
 	   this union.  */
-	if (TREE_CODE (TREE_TYPE (elt)) == UNION_TYPE)
-	  CONSTRUCTOR_NO_CLEARING (elt) = false;
+	if (TREE_CODE (TREE_TYPE (*elt)) == UNION_TYPE)
+	  CONSTRUCTOR_NO_CLEARING (*elt) = false;
       }
 
   if (lval)