PR c++/91264 - detect modifying const objects in constexpr.

	* constexpr.c (modifying_const_object_error): New function.
	(cxx_eval_call_expression): Set TREE_READONLY on a CONSTRUCTOR of
	a const-qualified object after it's been fully constructed.
	(modifying_const_object_p): New function.
	(cxx_eval_store_expression): Detect modifying a const object
	during constant expression evaluation.
	(cxx_eval_increment_expression): Use a better location when building
	up the store.
	(cxx_eval_constant_expression) <case DECL_EXPR>: Mark a constant
	object's constructor TREE_READONLY.

	* g++.dg/cpp1y/constexpr-tracking-const1.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const2.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const3.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const4.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const5.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const6.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const7.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const8.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const9.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const10.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const11.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const12.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const13.C: New test.
	* g++.dg/cpp1y/constexpr-tracking-const14.C: New test.

From-SVN: r274671

1 files changed, 139 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 23f2a02..dbd0dc3 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1575,6 +1575,19 @@ clear_no_implicit_zero (tree ctor)
     }
 }
 
+/* Complain about a const object OBJ being modified in a constant expression.
+   EXPR is the MODIFY_EXPR expression performing the modification.  */
+
+static void
+modifying_const_object_error (tree expr, tree obj)
+{
+  location_t loc = cp_expr_loc_or_input_loc (expr);
+  auto_diagnostic_group d;
+  error_at (loc, "modifying a const object %qE is not allowed in "
+	    "a constant expression", TREE_OPERAND (expr, 0));
+  inform (location_of (obj), "originally declared %<const%> here");
+}
+
 /* Subroutine of cxx_eval_constant_expression.
    Evaluate the call expression tree T in the context of OLD_CALL expression
    evaluation.  */
@@ -1775,6 +1788,19 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 
   depth_ok = push_cx_call_context (t);
 
+  /* Remember the object we are constructing.  */
+  tree new_obj = NULL_TREE;
+  if (DECL_CONSTRUCTOR_P (fun))
+    {
+      /* In a constructor, it should be the first `this' argument.
+	 At this point it has already been evaluated in the call
+	 to cxx_bind_parameters_in_call.  */
+      new_obj = TREE_VEC_ELT (new_call.bindings, 0);
+      STRIP_NOPS (new_obj);
+      if (TREE_CODE (new_obj) == ADDR_EXPR)
+	new_obj = TREE_OPERAND (new_obj, 0);
+    }
+
   tree result = NULL_TREE;
 
   constexpr_call *entry = NULL;
@@ -1910,6 +1936,23 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 		}
 	    }
 
+	  /* At this point, the object's constructor will have run, so
+	     the object is no longer under construction, and its possible
+	     'const' semantics now apply.  Make a note of this fact by
+	     marking the CONSTRUCTOR TREE_READONLY.  */
+	  if (new_obj
+	      && CLASS_TYPE_P (TREE_TYPE (new_obj))
+	      && CP_TYPE_CONST_P (TREE_TYPE (new_obj)))
+	    {
+	      /* Subobjects might not be stored in ctx->values but we can
+		 get its CONSTRUCTOR by evaluating *this.  */
+	      tree e = cxx_eval_constant_expression (ctx, new_obj,
+						     /*lval*/false,
+						     non_constant_p,
+						     overflow_p);
+	      TREE_READONLY (e) = true;
+	    }
+
 	  /* Forget the saved values of the callee's SAVE_EXPRs.  */
 	  unsigned int i;
 	  tree save_expr;
@@ -3724,6 +3767,26 @@ maybe_simplify_trivial_copy (tree &target, tree &init)
     }
 }
 
+/* Return true if we are modifying something that is const during constant
+   expression evaluation.  CODE is the code of the statement, OBJ is the
+   object in question, MUTABLE_P is true if one of the subobjects were
+   declared mutable.  */
+
+static bool
+modifying_const_object_p (tree_code code, tree obj, bool mutable_p)
+{
+  /* If this is initialization, there's no problem.  */
+  if (code != MODIFY_EXPR)
+    return false;
+
+  /* [basic.type.qualifier] "A const object is an object of type
+     const T or a non-mutable subobject of a const object."  */
+  if (mutable_p)
+    return false;
+
+  return (TREE_READONLY (obj) || CP_TYPE_CONST_P (TREE_TYPE (obj)));
+}
+
 /* Evaluate an INIT_EXPR or MODIFY_EXPR.  */
 
 static tree
@@ -3773,6 +3836,9 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
   /* Find the underlying variable.  */
   releasing_vec refs;
   tree object = NULL_TREE;
+  /* If we're modifying a const object, save it.  */
+  tree const_object_being_modified = NULL_TREE;
+  bool mutable_p = false;
   for (tree probe = target; object == NULL_TREE; )
     {
       switch (TREE_CODE (probe))
@@ -3783,6 +3849,12 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  {
 	    tree ob = TREE_OPERAND (probe, 0);
 	    tree elt = TREE_OPERAND (probe, 1);
+	    if (DECL_P (elt) && DECL_MUTABLE_P (elt))
+	      mutable_p = true;
+	    if (evaluated
+		&& modifying_const_object_p (TREE_CODE (t), probe, mutable_p)
+		&& const_object_being_modified == NULL_TREE)
+	      const_object_being_modified = probe;
 	    if (TREE_CODE (probe) == ARRAY_REF)
 	      {
 		elt = eval_and_check_array_index (ctx, probe, false,
@@ -3811,6 +3883,10 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	}
     }
 
+  if (modifying_const_object_p (TREE_CODE (t), object, mutable_p)
+      && const_object_being_modified == NULL_TREE)
+    const_object_being_modified = object;
+
   /* And then find/build up our initializer for the path to the subobject
      we're initializing.  */
   tree *valp;
@@ -3950,6 +4026,62 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
       valp = &cep->value;
     }
 
+  /* Detect modifying a constant object in constexpr evaluation.
+     We have found a const object that is being modified.  Figure out
+     if we need to issue an error.  Consider
+
+     struct A {
+       int n;
+       constexpr A() : n(1) { n = 2; } // #1
+     };
+     struct B {
+       const A a;
+       constexpr B() { a.n = 3; } // #2
+     };
+    constexpr B b{};
+
+    #1 is OK, since we're modifying an object under construction, but
+    #2 is wrong, since "a" is const and has been fully constructed.
+    To track it, we use the TREE_READONLY bit in the object's CONSTRUCTOR
+    which means that the object is read-only.  For the example above, the
+    *ctors stack at the point of #2 will look like:
+
+      ctors[0] = {.a={.n=2}}  TREE_READONLY = 0
+      ctors[1] = {.n=2}       TREE_READONLY = 1
+
+    and we're modifying "b.a", so we search the stack and see if the
+    constructor for "b.a" has already run.  */
+  if (const_object_being_modified)
+    {
+      bool fail = false;
+      if (!CLASS_TYPE_P (TREE_TYPE (const_object_being_modified)))
+	fail = true;
+      else
+	{
+	  /* [class.ctor]p5 "A constructor can be invoked for a const,
+	     volatile, or const volatile object.  const and volatile
+	     semantics are not applied on an object under construction.
+	     They come into effect when the constructor for the most
+	     derived object ends."  */
+	  tree elt;
+	  unsigned int i;
+	  FOR_EACH_VEC_ELT (*ctors, i, elt)
+	    if (same_type_ignoring_top_level_qualifiers_p
+		(TREE_TYPE (const_object_being_modified), TREE_TYPE (elt)))
+	      {
+		fail = TREE_READONLY (elt);
+		break;
+	      }
+	}
+      if (fail)
+	{
+	  if (!ctx->quiet)
+	    modifying_const_object_error (t, const_object_being_modified);
+	  *non_constant_p = true;
+	  return t;
+	}
+    }
+
   if (!preeval)
     {
       /* Create a new CONSTRUCTOR in case evaluation of the initializer
@@ -4063,7 +4195,8 @@ cxx_eval_increment_expression (const constexpr_ctx *ctx, tree t,
     VERIFY_CONSTANT (mod);
 
   /* Storing the modified value.  */
-  tree store = build2 (MODIFY_EXPR, type, op, mod);
+  tree store = build2_loc (cp_expr_loc_or_loc (t, input_location),
+			   MODIFY_EXPR, type, op, mod);
   cxx_eval_constant_expression (ctx, store,
 				true, non_constant_p, overflow_p);
   ggc_free (store);
@@ -4650,6 +4783,11 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 						 non_constant_p, overflow_p);
 	    /* Don't share a CONSTRUCTOR that might be changed.  */
 	    init = unshare_constructor (init);
+	    /* Remember that a constant object's constructor has already
+	       run.  */
+	    if (CLASS_TYPE_P (TREE_TYPE (r))
+		&& CP_TYPE_CONST_P (TREE_TYPE (r)))
+	      TREE_READONLY (init) = true;
 	    ctx->values->put (r, init);
 	  }
 	else if (ctx == &new_ctx)