c++: Track lifetimes in constant evaluation [PR70331, PR96630, PR98675]

This adds rudimentary lifetime tracking in C++ constexpr contexts,
allowing the compiler to report errors with using values after their
backing has gone out of scope. We don't yet handle other ways of ending
lifetimes (e.g. explicit destructor calls).

	PR c++/70331
	PR c++/96630
	PR c++/98675

gcc/cp/ChangeLog:

	* constexpr.cc (constexpr_global_ctx::put_value): Mark value as
        in lifetime.
        (constexpr_global_ctx::remove_value): Mark value as expired.
        (cxx_eval_call_expression): Remove comment that is no longer
        applicable.
	(non_const_var_error): Add check for expired values.
	(cxx_eval_constant_expression): Add checks for expired values.
        Forget local variables at end of bind expressions. Forget
        temporaries at end of cleanup points.
	* module.cc (trees_out::core_bools): Write out the new flag.
	(trees_in::core_bools): Read in the new flag.

gcc/ChangeLog:

	* tree-core.h (struct tree_decl_common): New flag to check if
        value lifetime has expired.
	* tree.h (DECL_EXPIRED): Access the new flag.
	* print-tree.cc (print_node): Print the new flag.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ice20.C: Update error raised by test.
	* g++.dg/cpp1y/constexpr-lifetime1.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime2.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime3.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime4.C: New test.
	* g++.dg/cpp1y/constexpr-lifetime5.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

gcc/cp/constexpr.cc

@@ -1185,10 +1185,17 @@ class constexpr_global_ctx {
   void put_value (tree t, tree v)
   {
     bool already_in_map = values.put (t, v);
+    if (!already_in_map && DECL_P (t))
+      DECL_EXPIRED (t) = false;
     if (!already_in_map && modifiable)
       modifiable->add (t);
   }
-  void remove_value (tree t) { values.remove (t); }
+  void remove_value (tree t)
+  {
+    if (DECL_P (t))
+      DECL_EXPIRED (t) = true;
+    values.remove (t);
+  }
 };
 
 /* Helper class for constexpr_global_ctx.  In some cases we want to avoid
@@ -3157,10 +3164,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	  for (tree save_expr : save_exprs)
 	    ctx->global->remove_value (save_expr);
 
-	  /* Remove the parms/result from the values map.  Is it worth
-	     bothering to do this when the map itself is only live for
-	     one constexpr evaluation?  If so, maybe also clear out
-	     other vars from call, maybe in BIND_EXPR handling?  */
+	  /* Remove the parms/result from the values map.  */
 	  ctx->global->remove_value (res);
 	  for (tree parm = parms; parm; parm = TREE_CHAIN (parm))
 	    ctx->global->remove_value (parm);
@@ -5708,6 +5712,13 @@ non_const_var_error (location_t loc, tree r, bool fundef_p)
 	inform (DECL_SOURCE_LOCATION (r), "allocated here");
       return;
     }
+  if (DECL_EXPIRED (r))
+    {
+      if (constexpr_error (loc, fundef_p, "accessing object outside its "
+			   "lifetime"))
+	inform (DECL_SOURCE_LOCATION (r), "declared here");
+      return;
+    }
   if (!constexpr_error (loc, fundef_p, "the value of %qD is not usable in "
 			"a constant expression", r))
     return;
@@ -6995,7 +7006,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       else
 	{
 	  if (!ctx->quiet)
-	    error ("%qE is not a constant expression", t);
+	    non_const_var_error (loc, t, /*fundef_p*/false);
 	  *non_constant_p = true;
 	}
       break;
@@ -7048,6 +7059,13 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  r = build_constructor (TREE_TYPE (t), NULL);
 	  TREE_CONSTANT (r) = true;
 	}
+      else if (DECL_EXPIRED (t))
+	{
+	  if (!ctx->quiet)
+	    non_const_var_error (loc, r, /*fundef_p*/false);
+	  *non_constant_p = true;
+	  break;
+	}
       else if (ctx->strict)
 	r = decl_really_constant_value (t, /*unshare_p=*/false);
       else
@@ -7093,7 +7111,15 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       else
 	{
 	  if (!ctx->quiet)
-	    error ("%qE is not a constant expression", t);
+	    {
+	      if (DECL_EXPIRED (r))
+		{
+		  error_at (loc, "accessing object outside its lifetime");
+		  inform (DECL_SOURCE_LOCATION (r), "declared here");
+		}
+	      else
+		error_at (loc, "%qE is not a constant expression", t);
+	    }
 	  *non_constant_p = true;
 	}
       break;
@@ -7315,17 +7341,28 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	auto_vec<tree, 2> cleanups;
 	vec<tree> *prev_cleanups = ctx->global->cleanups;
 	ctx->global->cleanups = &cleanups;
-	r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0),
+
+	auto_vec<tree, 10> save_exprs;
+	constexpr_ctx new_ctx = *ctx;
+	new_ctx.save_exprs = &save_exprs;
+
+	r = cxx_eval_constant_expression (&new_ctx, TREE_OPERAND (t, 0),
 					  lval,
 					  non_constant_p, overflow_p,
 					  jump_target);
+
 	ctx->global->cleanups = prev_cleanups;
 	unsigned int i;
 	tree cleanup;
 	/* Evaluate the cleanups.  */
 	FOR_EACH_VEC_ELT_REVERSE (cleanups, i, cleanup)
-	  cxx_eval_constant_expression (ctx, cleanup, vc_discard,
+	  cxx_eval_constant_expression (&new_ctx, cleanup, vc_discard,
 					non_constant_p, overflow_p);
+
+	/* Forget SAVE_EXPRs and TARGET_EXPRs created by this
+	   full-expression.  */
+	for (tree save_expr : save_exprs)
+	  ctx->global->remove_value (save_expr);
       }
       break;
 
@@ -7831,10 +7868,13 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 				      non_constant_p, overflow_p, jump_target);
 
     case BIND_EXPR:
-      return cxx_eval_constant_expression (ctx, BIND_EXPR_BODY (t),
-					   lval,
-					   non_constant_p, overflow_p,
-					   jump_target);
+      r = cxx_eval_constant_expression (ctx, BIND_EXPR_BODY (t),
+					lval,
+					non_constant_p, overflow_p,
+					jump_target);
+      for (tree decl = BIND_EXPR_VARS (t); decl; decl = DECL_CHAIN (decl))
+	ctx->global->remove_value (decl);
+      return r;
 
     case PREINCREMENT_EXPR:
     case POSTINCREMENT_EXPR:

gcc/cp/module.cc

@@ -5435,6 +5435,7 @@ trees_out::core_bools (tree t)
       WB (t->decl_common.decl_read_flag);
       WB (t->decl_common.decl_nonshareable_flag);
       WB (t->decl_common.decl_not_flexarray);
+      WB (t->decl_common.decl_expired_flag);
     }
 
   if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))
@@ -5580,6 +5581,7 @@ trees_in::core_bools (tree t)
       RB (t->decl_common.decl_read_flag);
       RB (t->decl_common.decl_nonshareable_flag);
       RB (t->decl_common.decl_not_flexarray);
+      RB (t->decl_common.decl_expired_flag);
     }
 
   if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))

gcc/print-tree.cc

@@ -495,6 +495,10 @@ print_node (FILE *file, const char *prefix, tree node, int indent,
 	  && DECL_BY_REFERENCE (node))
 	fputs (" passed-by-reference", file);
 
+      if ((code == VAR_DECL || code == PARM_DECL || code == RESULT_DECL)
+	  && DECL_EXPIRED (node))
+	fputs (" expired", file);
+
       if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS)  && DECL_DEFER_OUTPUT (node))
 	fputs (" defer-output", file);
 

gcc/testsuite/g++.dg/cpp0x/constexpr-ice20.C

@@ -4,4 +4,4 @@
 typedef bool (*Function)(int);
 constexpr bool check(int x, Function p) { return p(x); }  // { dg-message "in .constexpr. expansion of" }
 
-static_assert(check(2, check), "");  // { dg-error "conversion|constant|in .constexpr. expansion of" }
+static_assert(check(2, check), "");  // { dg-error "conversion|constant|lifetime|in .constexpr. expansion of" }

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime1.C

@@ -0,0 +1,13 @@
+// PR c++/96630
+// { dg-do compile { target c++14 } }
+
+struct S {
+  int x = 0;
+  constexpr const int& get() const { return x; }
+};
+
+constexpr const int& test() {
+  auto local = S{};  // { dg-message "note: declared here" }
+  return local.get();
+}
+constexpr int x = test();  // { dg-error "accessing object outside its lifetime" }

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime2.C

@@ -0,0 +1,20 @@
+// PR c++/98675
+// { dg-do compile { target c++14 } }
+
+struct S {
+  int x = 0;
+  constexpr const int& get() const { return x; }
+};
+
+constexpr int error() {
+  const auto& local = S{}.get();  // { dg-message "note: declared here" }
+  return local;
+}
+constexpr int x = error();  // { dg-error "accessing object outside its lifetime" }
+
+constexpr int ok() {
+  // temporary should only be destroyed after end of full-expression
+  auto local = S{}.get();
+  return local;
+}
+constexpr int y = ok();

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime3.C

@@ -0,0 +1,13 @@
+// PR c++/70331
+// { dg-do compile { target c++14 } }
+
+constexpr int f(int i) {
+  int *p = &i;
+  if (i == 0) {
+    int j = 123;  // { dg-message "note: declared here" }
+    p = &j;
+  }
+  return *p;
+}
+
+constexpr int i = f(0);  // { dg-error "accessing object outside its lifetime" }

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime4.C

@@ -0,0 +1,11 @@
+// { dg-do compile { target c++14 } }
+
+constexpr const double& test() {
+  const double& local = 3.0;  // { dg-message "note: declared here" }
+  return local;
+}
+
+static_assert(test() == 3.0, "");  // { dg-error "constant|accessing object outside its lifetime" }
+
+// no deference, shouldn't error
+static_assert((test(), true), "");

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C

@@ -0,0 +1,11 @@
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wno-return-local-addr" }
+
+constexpr const int& id(int x) { return x; }
+
+constexpr bool test() {
+  const int& y = id(3);
+  return y == 3;
+}
+
+constexpr bool x = test();  // { dg-error "" }

gcc/tree-core.h

@@ -1834,7 +1834,10 @@ struct GTY(()) tree_decl_common {
   /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY.  */
   unsigned int decl_not_flexarray : 1;
 
-  /* 13 bits unused.  */
+  /* In VAR_DECL, PARM_DECL, or RESULT_DECL, this is DECL_EXPIRED.  */
+  unsigned int decl_expired_flag : 1;
+
+  /* 12 bits unused.  */
 
   /* UID for points-to sets, stable over copying from inlining.  */
   unsigned int pt_uid;

gcc/tree.h

@@ -2697,6 +2697,12 @@ extern tree vector_element_bits_tree (const_tree);
    ??? Need to figure out some way to check this isn't a PARM_DECL.  */
 #define DECL_INITIAL(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.initial)
 
+/* Used in a VAR_DECL, PARM_DECL, or RESULT_DECL to indicate whether
+   this declaration is currently in lifetime for constant evaluation
+   purposes.  */
+#define DECL_EXPIRED(NODE) \
+  (DECL_COMMON_CHECK(NODE)->decl_common.decl_expired_flag)
+
 /* Holds the size of the datum, in bits, as a tree expression.
    Need not be constant and may be null.  May be less than TYPE_SIZE
    for a C++ FIELD_DECL representing a base class subobject with its