[C++ PATCH] Fix ICE with return in statement expression in constexpr.c (PR c++/84192)

[Jakub Jelinek]

Hi!

pop_stmt_list, if there is just a single stmt inside statement expression
moves the stmt out of the STATEMENT_LIST (and I think cp_fold does too).
We only initialize jump_target to non-NULL in cxx_eval_statement_list
or for calls, so before we have a chance to diagnose the error of using
an expression with void type, we ICE trying to dereference NULL jump_target.

This can't happen with BREAK_STMT nor CONTINUE_STMT, because they are not
potential constant expressions, and I think can only happen when ctx->quiet
is true, otherwise it should have been diagnosed already before.
If a RETURN_EXPR (or in theory break/continue) appears in a (potential) constant
expression we want to evaluate, not doing anything with jump_target if we
aren't inside a statement list makes sense to me, there is no following
statement to bypass.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-02-16  Marek Polacek  <pola...@redhat.com>
            Jakub Jelinek  <ja...@redhat.com>

        PR c++/84192
        * constexpr.c (cxx_eval_constant_expression) <case RETURN_EXPR>: Don't
        set *jump_target to anything if jump_target is NULL.

        * g++.dg/cpp1y/constexpr-84192.C: New test.

--- gcc/cp/constexpr.c.jj       2018-02-12 19:17:37.937216029 +0100
+++ gcc/cp/constexpr.c  2018-02-15 16:10:56.630572360 +0100
@@ -4254,7 +4254,13 @@ cxx_eval_constant_expression (const cons
        r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0),
                                          lval,
                                          non_constant_p, overflow_p);
-      *jump_target = t;
+      if (jump_target)
+       *jump_target = t;
+      else
+       /* Can happen with ({ return true; }) && false; passed to
+          maybe_constant_value.  There is nothing to jump over in this
+          case, and the bug will be diagnosed later.  */
+       gcc_assert (ctx->quiet);
       break;
 
     case SAVE_EXPR:
--- gcc/testsuite/g++.dg/cpp1y/constexpr-84192.C.jj     2018-02-15 
16:00:58.242588914 +0100
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-84192.C        2018-02-15 
16:01:30.219585291 +0100
@@ -0,0 +1,41 @@
+// PR c++/84192
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+bool
+f1 ()
+{ 
+  return ({ return true; }) && false;  // { dg-error "could not convert" }
+}
+
+void
+f2 ()
+{ 
+  for (;;)
+    constexpr bool b = ({ break; false; }) && false;   // { dg-error 
"statement is not a constant expression" }
+}
+
+constexpr bool
+f3 (int n)
+{
+  bool b = false;
+  for (int i = 0; i < n; i++)
+    b = ({ break; });  // { dg-error "void value not ignored as it ought to 
be" }
+  return b;
+}
+
+constexpr bool b = f3 (4);
+
+bool
+f4 ()
+{
+  constexpr bool b = ({ return true; }) && false;      // { dg-error "could 
not convert" }
+  return false;
+}
+
+constexpr bool
+f5 (int x)
+{
+  constexpr bool b = ({ switch (x) case 0: true; }) && false;  // { dg-error 
"could not convert" }
+  return false;
+}

        Jakub