c++: Refrain from using replace_placeholders in constexpr evaluation [PR94205]

This removes the use of replace_placeholders in cxx_eval_constant_expression
(which is causing the new test lambda-this6.C to ICE due to replace_placeholders
mutating the shared TARGET_EXPR_INITIAL tree which then trips up the
gimplifier).

In its place, this patch adds a 'parent' field to constexpr_ctx which is used to
store a pointer to an outer constexpr_ctx that refers to another object under
construction.  With this new field, we can beef up lookup_placeholder to resolve
PLACEHOLDER_EXPRs which refer to former objects under construction, which fixes
PR94205 without needing to do replace_placeholders.  Also we can now respect the
CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag when resolving PLACEHOLDER_EXPRs, and
doing so fixes the constexpr analogue of PR79937.

gcc/cp/ChangeLog:

	PR c++/94205
	PR c++/79937
	* constexpr.c (struct constexpr_ctx): New field 'parent'.
	(cxx_eval_bare_aggregate): Propagate CONSTRUCTOR_PLACEHOLDER_BOUNDARY
	flag from the original constructor to the reduced constructor.
	(lookup_placeholder): Prefer to return the outermost matching object
	by recursively calling lookup_placeholder on the 'parent' context,
	but don't cross CONSTRUCTOR_PLACEHOLDER_BOUNDARY constructors.
	(cxx_eval_constant_expression): Link the 'ctx' context to the 'new_ctx'
	context via 'new_ctx.parent' when being expanded without an explicit
	target.  Don't call replace_placeholders.
	(cxx_eval_outermost_constant_expr): Initialize 'ctx.parent' to NULL.

gcc/testsuite/ChangeLog:

	PR c++/94205
	PR c++/79937
	* g++.dg/cpp1y/pr79937-5.C: New test.
	* g++.dg/cpp1z/lambda-this6.C: New test.

gcc/cp/ChangeLog

@@ -1,5 +1,18 @@
 2020-04-04  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94205
+	PR c++/79937
+	* constexpr.c (struct constexpr_ctx): New field 'parent'.
+	(cxx_eval_bare_aggregate): Propagate CONSTRUCTOR_PLACEHOLDER_BOUNDARY
+	flag from the original constructor to the reduced constructor.
+	(lookup_placeholder): Prefer to return the outermost matching object
+	by recursively calling lookup_placeholder on the 'parent' context,
+	but don't cross CONSTRUCTOR_PLACEHOLDER_BOUNDARY constructors.
+	(cxx_eval_constant_expression): Link the 'ctx' context to the 'new_ctx'
+	context via 'new_ctx.parent' when being expanded without an explicit
+	target.  Don't call replace_placeholders.
+	(cxx_eval_outermost_constant_expr): Initialize 'ctx.parent' to NULL.
+
 	PR c++/94219
 	PR c++/94205
 	* constexpr.c (get_or_insert_ctor_field): Split out (while adding

gcc/cp/constexpr.c

@@ -1076,6 +1076,9 @@ struct constexpr_ctx {
   tree object;
   /* If inside SWITCH_EXPR.  */
   constexpr_switch_state *css_state;
+  /* The aggregate initialization context inside which this one is nested.  This
+     is used by lookup_placeholder to resolve PLACEHOLDER_EXPRs.  */
+  const constexpr_ctx *parent;
 
   /* Whether we should error on a non-constant expression or fail quietly.
      This flag needs to be here, but some of the others could move to global
@@ -3841,6 +3844,9 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree t,
   vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
   vec_alloc (*p, vec_safe_length (v));
 
+  if (CONSTRUCTOR_PLACEHOLDER_BOUNDARY (t))
+    CONSTRUCTOR_PLACEHOLDER_BOUNDARY (ctx->ctor) = 1;
+
   unsigned i;
   tree index, value;
   bool constant_p = true;
@@ -5303,6 +5309,12 @@ lookup_placeholder (const constexpr_ctx *ctx, bool lval, tree type)
   if (!ctx)
     return NULL_TREE;
 
+  /* Prefer the outermost matching object, but don't cross
+     CONSTRUCTOR_PLACEHOLDER_BOUNDARY constructors.  */
+  if (ctx->ctor && !CONSTRUCTOR_PLACEHOLDER_BOUNDARY (ctx->ctor))
+    if (tree outer_ob = lookup_placeholder (ctx->parent, lval, type))
+      return outer_ob;
+
   /* We could use ctx->object unconditionally, but using ctx->ctor when we
      can is a minor optimization.  */
   if (!lval && ctx->ctor && same_type_p (TREE_TYPE (ctx->ctor), type))
@@ -5606,19 +5618,16 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    r = *p;
 	    break;
 	  }
-	tree init = TARGET_EXPR_INITIAL (t);
 	if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
 	  {
-	    if (ctx->object)
-	      /* If the initializer contains any PLACEHOLDER_EXPR, we need to
-		 resolve them before we create a new CONSTRUCTOR for the
-		 temporary.  */
-	      init = replace_placeholders (init, ctx->object);
-
 	    /* We're being expanded without an explicit target, so start
 	       initializing a new object; expansion with an explicit target
 	       strips the TARGET_EXPR before we get here.  */
 	    new_ctx = *ctx;
+	    /* Link CTX to NEW_CTX so that lookup_placeholder can resolve
+	       any PLACEHOLDER_EXPR within the initializer that refers to the
+	       former object under construction.  */
+	    new_ctx.parent = ctx;
 	    new_ctx.ctor = build_constructor (type, NULL);
 	    CONSTRUCTOR_NO_CLEARING (new_ctx.ctor) = true;
 	    new_ctx.object = slot;
@@ -6472,7 +6481,7 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
   bool overflow_p = false;
 
   constexpr_global_ctx global_ctx;
-  constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL,
+  constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL, NULL,
 			allow_non_constant, strict,
 			manifestly_const_eval || !allow_non_constant,
 			uid_sensitive };

gcc/testsuite/ChangeLog

@@ -1,5 +1,10 @@
 2020-04-04  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94205
+	PR c++/79937
+	* g++.dg/cpp1y/pr79937-5.C: New test.
+	* g++.dg/cpp1z/lambda-this6.C: New test.
+
 	PR c++/94219
 	PR c++/94205
 	* g++.dg/cpp1y/constexpr-nsdmi3.C: New test.

gcc/testsuite/g++.dg/cpp1y/pr79937-5.C

@@ -0,0 +1,42 @@
+// PR c++/79937
+// This is a constexpr adaptation of pr79937-3.C and pr79937-4.C.
+// { dg-do compile { target c++14 } }
+
+struct X {
+  unsigned i;
+  unsigned n = i;
+};
+
+constexpr X bar(X x) {
+  return x;
+}
+
+struct Y
+{
+  static constexpr Y bar(Y y) { return y; }
+  unsigned i;
+  unsigned n = bar(Y{2,i}).n;
+};
+
+constexpr X x { 1, bar(X{2}).n };
+static_assert(x.n == 2, "");
+
+constexpr Y y { 1 };
+static_assert(y.n == 1, "");
+
+struct Z {
+  unsigned i;
+  unsigned n = i;
+  unsigned m = i;
+};
+
+constexpr Z
+baz (Z z)
+{
+  if (z.i != 1 || z.n != 2 || z.m != 1)
+    __builtin_abort ();
+  return z;
+}
+
+constexpr Z z = baz (Z {1, Z {2}.n});
+static_assert(z.i == 1 && z.n == 2 && z.m == 1, "");

gcc/testsuite/g++.dg/cpp1z/lambda-this6.C

@@ -0,0 +1,12 @@
+// { dg-do compile { target c++17 } }
+
+struct S
+{
+  int a = [this] { return 6; } ();
+};
+
+S
+foo()
+{
+  return {};
+}