c++: Delegating constructor in constexpr init [PR94772]

In the first testcase below, the call to the target constructor foo{} from foo's
delegating constructor is encoded as the INIT_EXPR

  *(struct foo *) this = AGGR_INIT_EXPR <4, __ct_comp, D.2140, ...>;

During initialization of the variable 'bar', we prematurely set TREE_READONLY on
bar's CONSTRUCTOR in two places before the outer delegating constructor has
returned: first, at the end of cxx_eval_call_expression after evaluating the RHS
of the above INIT_EXPR, and second, at the end of cxx_eval_store_expression
after having finished evaluating the above INIT_EXPR.  This then prevents the
rest of the outer delegating constructor from mutating 'bar'.

This (hopefully minimally risky) patch makes cxx_eval_call_expression refrain
from setting TREE_READONLY when evaluating the target constructor of a
delegating constructor.  It also makes cxx_eval_store_expression refrain from
setting TREE_READONLY when the object being initialized is "*this', on the basis
that it should be the responsibility of the routine that set 'this' in the first
place to set the object's TREE_READONLY appropriately.

gcc/cp/ChangeLog:

	PR c++/94772
	* constexpr.c (cxx_eval_call_expression): Don't set new_obj if we're
	evaluating the target constructor of a delegating constructor.
	(cxx_eval_store_expression): Don't set TREE_READONLY if the LHS of the
	INIT_EXPR is '*this'.

gcc/testsuite/ChangeLog:

	PR c++/94772
	* g++.dg/cpp1y/constexpr-tracking-const23.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const24.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const25.C: New test.

1 files changed, 27 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 6b3e514..637cb74 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2371,6 +2371,21 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
       STRIP_NOPS (new_obj);
       if (TREE_CODE (new_obj) == ADDR_EXPR)
 	new_obj = TREE_OPERAND (new_obj, 0);
+
+      if (ctx->call && ctx->call->fundef
+	  && DECL_CONSTRUCTOR_P (ctx->call->fundef->decl))
+	{
+	  tree cur_obj = TREE_VEC_ELT (ctx->call->bindings, 0);
+	  STRIP_NOPS (cur_obj);
+	  if (TREE_CODE (cur_obj) == ADDR_EXPR)
+	    cur_obj = TREE_OPERAND (cur_obj, 0);
+	  if (new_obj == cur_obj)
+	    /* We're calling the target constructor of a delegating
+	       constructor, or accessing a base subobject through a
+	       NOP_EXPR as part of a call to a base constructor, so
+	       there is no new (sub)object.  */
+	    new_obj = NULL_TREE;
+	}
     }
 
   tree result = NULL_TREE;
@@ -4950,7 +4965,18 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   if (TREE_CODE (t) == INIT_EXPR
       && TREE_CODE (*valp) == CONSTRUCTOR
       && TYPE_READONLY (type))
-    TREE_READONLY (*valp) = true;
+    {
+      if (INDIRECT_REF_P (target)
+	  && (is_this_parameter
+	      (tree_strip_nop_conversions (TREE_OPERAND (target, 0)))))
+	/* We've just initialized '*this' (perhaps via the target
+	   constructor of a delegating constructor).  Leave it up to the
+	   caller that set 'this' to set TREE_READONLY appropriately.  */
+	gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
+			     (TREE_TYPE (target), type));
+      else
+	TREE_READONLY (*valp) = true;
+    }
 
   /* Update TREE_CONSTANT and TREE_SIDE_EFFECTS on enclosing
      CONSTRUCTORs, if any.  */