PR c++/91705 - constexpr evaluation rejects ++/-- on floats.

	* constexpr.c (cxx_eval_increment_expression): Call fold_simple on
	the offset.

	* g++.dg/cpp1y/constexpr-incr2.C: New test.

From-SVN: r275613

gcc/cp/ChangeLog

+2019-09-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91705 - constexpr evaluation rejects ++/-- on floats.
+	* constexpr.c (cxx_eval_increment_expression): Call fold_simple on
+	the offset.
+
 2019-09-10  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (has_designator_problem): Use cp_expr_loc_or_input_loc

gcc/cp/constexpr.c

@@ -4164,6 +4164,10 @@ cxx_eval_increment_expression (const constexpr_ctx *ctx, tree t,
   tree offset = TREE_OPERAND (t, 1);
   gcc_assert (TREE_CONSTANT (offset));
 
+  /* OFFSET is constant, but perhaps not constant enough.  We need to
+     e.g. bash FLOAT_EXPRs to REAL_CSTs.  */
+  offset = fold_simple (offset);
+
   /* The operand as an lvalue.  */
   op = cxx_eval_constant_expression (ctx, op, true,
 				     non_constant_p, overflow_p);

gcc/testsuite/ChangeLog

+2019-09-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91705 - constexpr evaluation rejects ++/-- on floats.
+	* g++.dg/cpp1y/constexpr-incr2.C: New test.
+
 2019-09-10  David Edelsohn  <dje.gcc@gmail.com>
 
 	* gfortran.dg/default_format_1.f90: Remove XFAIL AIX.

gcc/testsuite/g++.dg/cpp1y/constexpr-incr2.C

+// PR c++/91705 - constexpr evaluation rejects ++/-- on floats.
+// { dg-do compile { target c++14 } }
+
+#define SA(X) static_assert((X),#X)
+
+template <class T>
+constexpr T fn1(T t)
+{
+  return ++t;
+}
+
+constexpr float fn2(float t)
+{
+  return ++t;
+}
+
+template <class T>
+constexpr T fn3(T t)
+{
+  return --t;
+}
+
+constexpr float fn4(float t)
+{
+  return --t;
+}
+
+template <class T>
+constexpr T fn5(T t)
+{
+  return t++;
+}
+
+constexpr float fn6(float t)
+{
+  return t++;
+}
+
+template <class T>
+constexpr T fn7(T t)
+{
+  return t--;
+}
+
+constexpr float fn8(float t)
+{
+  return t--;
+}
+
+constexpr double r1 = fn1(2.0f);
+SA(r1 == 3);
+constexpr double r2 = fn2(2.0f);
+SA(r2 == 3);
+constexpr double r3 = fn3(2.0f);
+SA(r3 == 1);
+constexpr double r4 = fn4(2.0f);
+SA(r4 == 1);
+
+constexpr double r5 = fn5(2.0f);
+SA(r5 == 2);
+constexpr double r6 = fn6(2.0f);
+SA(r6 == 2);
+constexpr double r7 = fn7(2.0f);
+SA(r7 == 2);
+constexpr double r8 = fn8(2.0f);
+SA(r8 == 2);