[C PATCH] Don't leak C_MAYBE_CONST_EXPRs into fold() (PR c/68412)

[Marek Polacek]

Since C++ delayed folding, warn_tautological_cmp needs to fold its arguments.
But sometimes this function gets C_MAYBE_CONST_EXPR from the C FE, and fold()
duly ICEs.

I was torn if I should just return from warn_tautological_cmp and not warn
when it gets C_MAYBE_CONST_EXPR as an argument, or if I should strip
C_MAYBE_CONST_EXPR before calling warn_tautological_cmp.  I decided for the
latter since I think warn_tautological_cmp probably should warn on code such
as
  int i = 10;
  bool b = ({ i; }) == ({ i; });
(ugh).

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

2015-11-18  Marek Polacek  <pola...@redhat.com>

        PR c/68412
        * c-typeck.c (parser_build_binary_op): Strip C_MAYBE_CONST_EXPR
        when passing arguments to warn_tautological_compare.

        * gcc.dg/pr68412.c: New test.
        * gcc.dg/pr68412-2.c: New test.

diff --git gcc/c/c-typeck.c gcc/c/c-typeck.c
index c18c307..48c1a98 100644
--- gcc/c/c-typeck.c
+++ gcc/c/c-typeck.c
@@ -3512,7 +3512,11 @@ parser_build_binary_op (location_t location, enum 
tree_code code,
                           code1, arg1.value, code2, arg2.value);
 
   if (warn_tautological_compare)
-    warn_tautological_cmp (location, code, arg1.value, arg2.value);
+    warn_tautological_cmp (location, code,
+                          /* This function will try to fold both
+                             args, so don't leak C_MAYBE_CONST_EXPR.  */
+                          remove_c_maybe_const_expr (arg1.value),
+                          remove_c_maybe_const_expr (arg2.value));
 
   if (warn_logical_not_paren
       && TREE_CODE_CLASS (code) == tcc_comparison
diff --git gcc/testsuite/gcc.dg/pr68412-2.c gcc/testsuite/gcc.dg/pr68412-2.c
index e69de29..be1dcfa 100644
--- gcc/testsuite/gcc.dg/pr68412-2.c
+++ gcc/testsuite/gcc.dg/pr68412-2.c
@@ -0,0 +1,15 @@
+/* PR c/68412 */
+/* { dg-do compile } */
+/* { dg-options "-Wall -Wextra" } */
+
+int
+fn1 (int i)
+{
+  return ({ i; }) == ({ i; }); /* { dg-warning "self-comparison always 
evaluates to true" } */
+}
+
+int
+fn2 (int i)
+{
+  return ({ i + 1; }) != ({ i + 1; }); /* { dg-warning "self-comparison always 
evaluates to false" } */
+}
diff --git gcc/testsuite/gcc.dg/pr68412.c gcc/testsuite/gcc.dg/pr68412.c
index e69de29..825eb63 100644
--- gcc/testsuite/gcc.dg/pr68412.c
+++ gcc/testsuite/gcc.dg/pr68412.c
@@ -0,0 +1,41 @@
+/* PR c/68412 */
+/* { dg-do compile } */
+/* { dg-options "-Wall -Wextra" } */
+
+#define M (sizeof (int) * __CHAR_BIT__)
+
+int
+fn1 (int i)
+{
+  return i == (-1 << 8); /* { dg-warning "left shift of negative value" } */
+}
+
+int
+fn2 (int i)
+{
+  return i == (1 << M); /* { dg-warning "left shift count" } */
+}
+
+int
+fn3 (int i)
+{
+  return i == 10 << (M - 1); /* { dg-warning "requires" } */
+}
+
+int
+fn4 (int i)
+{
+  return i == 1 << -1; /* { dg-warning "left shift count" } */
+}
+
+int
+fn5 (int i)
+{
+  return i == 1 >> M; /* { dg-warning "right shift count" } */
+}
+
+int
+fn6 (int i)
+{
+  return i == 1 >> -1; /* { dg-warning "right shift count" } */
+}

        Marek