Fix handling of overflow in C casts in integer constant expressions (PR c/93241).

Bug 93241 reports a case where certain C expressions involving casts,
that would not be valid in an evaluated part of an integer constant
expression (because of e.g. involving integer overflow), are wrongly
rejected in an unevaluated part of an integer constant expression even
though all the operands and operations are ones that are valid in that
context.  This is a rejects-valid regression in GCC 4.5 and later
relative to 4.4 (for some testcases; the one in the bug uses
_Static_assert which isn't supported in those older releases).

The rule in the C front end is that an expression with those
properties (valid in an unevaluated part of an integer constant
expression but not an evaluated part) must be represented either as an
INTEGER_CST with TREE_OVERFLOW set or as a C_MAYBE_CONST_EXPR with
C_MAYBE_CONST_EXPR_INT_OPERANDS set.  This patch fixes build_c_cast to
check for that case and call note_integer_operands as needed.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	PR c/93241
gcc/c:
	* c-typeck.c (build_c_cast): Check for expressions with integer
	operands that can occur in an unevaluated part of an integer
	constant expression and call note_integer_operands as needed.

gcc/testsuite:
	* gcc.dg/c11-static-assert-10.c, gcc.dg/c99-const-expr-15.c: New
	tests.

gcc/c/ChangeLog

@@ -1,3 +1,10 @@
+2020-01-13  Joseph Myers  <joseph@codesourcery.com>
+
+	PR c/93241
+	* c-typeck.c (build_c_cast): Check for expressions with integer
+	operands that can occur in an unevaluated part of an integer
+	constant expression and call note_integer_operands as needed.
+
 2019-01-08  Richard Biener  <rguenther@suse.de>
 
 	PR middle-end/93199

gcc/c/c-typeck.c

@@ -5709,6 +5709,8 @@ build_c_cast (location_t loc, tree type, tree expr)
 {
   tree value;
 
+  bool int_operands = EXPR_INT_CONST_OPERANDS (expr);
+
   if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
     expr = TREE_OPERAND (expr, 0);
 
@@ -5943,6 +5945,14 @@ build_c_cast (location_t loc, tree type, tree expr)
 	       || TREE_CODE (expr) == COMPLEX_CST)))
       value = build1 (NOP_EXPR, type, value);
 
+  /* If the expression has integer operands and so can occur in an
+     unevaluated part of an integer constant expression, ensure the
+     return value reflects this.  */
+  if (int_operands
+      && INTEGRAL_TYPE_P (type)
+      && !EXPR_INT_CONST_OPERANDS (value))
+    value = note_integer_operands (value);
+
   protected_set_expr_location (value, loc);
   return value;
 }

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-01-13  Joseph Myers  <joseph@codesourcery.com>
+
+	PR c/93241
+	* gcc.dg/c11-static-assert-10.c, gcc.dg/c99-const-expr-15.c: New
+	tests.
+
 2020-01-13  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/93213

gcc/testsuite/gcc.dg/c11-static-assert-10.c

@@ -0,0 +1,9 @@
+/* Test for constant expressions: casts with integer overflow.  PR
+   c/93241.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#include <limits.h>
+
+_Static_assert (0 ? (_Bool) (INT_MAX + 1) : 1, "");
+_Static_assert (0 ? (short) ((INT_MAX + 1) != 0) : 1, "");

gcc/testsuite/gcc.dg/c99-const-expr-15.c

@@ -0,0 +1,9 @@
+/* Test for constant expressions: casts with integer overflow.  PR
+   c/93241.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c99 -pedantic-errors" } */
+
+#include <limits.h>
+
+struct s { int a : (0 ? (_Bool) (INT_MAX + 1) : 1); };
+struct t { int a : (0 ? (short) ((INT_MAX + 1) != 0) : 1); };