--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,67 @@
+// { dg-do compile { target c++2a } }
+
+// Tests constrained decltype(auto).
+
+template<typename T>
+concept Type = true;
+
+template<typename T>
+concept Int = __is_same_as(T, int);
+
+template<typename T, typename U>
+concept SameAs = __is_same_as(T, U);
+
+template<typename T, typename U>
+  requires SameAs<T, U>
+constexpr bool check = true;
+
+int z = 0;
+const int cz = 0;
+
+Type decltype(auto) x1 = 0;
+static_assert(check<decltype(x1), int>);
+Type decltype(auto) x2 = z;
+static_assert(check<decltype(x2), int>);
+Type decltype(auto) x3 = (z);
+static_assert(check<decltype(x3), int&>);
+Type decltype(auto) x4 = cz;
+static_assert(check<decltype(x4), const int>);
+Type decltype(auto) x5 = (cz);
+static_assert(check<decltype(x5), const int&>);
+
+Type decltype(auto) f1() { return 0; }
+static_assert(check<decltype(f1()), int>);
+Type decltype(auto) f2() { return z; }
+static_assert(check<decltype(f2()), int>);
+Type decltype(auto) f3() { return (z); }
+static_assert(check<decltype(f3()), int&>);
+Type decltype(auto) f4() { return cz; }
+static_assert(check<decltype(f4()), int>); // Top-level const is removed?
+Type decltype(auto) f5() { return (cz); }
+static_assert(check<decltype(f5()), const int&>);
+
+bool b = true;
+const bool cb = true;
+
+Int decltype(auto) b1 = true; // { dg-error "deduced initializer" }
+Int decltype(auto) b2 = (b);  // { dg-error "deduced initializer" }
+Int decltype(auto) b3 = (cb); // { dg-error "deduced initializer" }
+
+Int decltype(auto) g1() { } // { dg-error "deduced return type" }
+Int decltype(auto) g2() { return; } // { dg-error "deduced return type" }
+Int decltype(auto) g3() { return true; } // { dg-error "deduced return type" }
+int g4(Type decltype(auto) x) { return 0; } // { dg-error "cannot declare" }
+int g5(decltype(auto) x) { return 0; } // { dg-error "cannot declare" }
+
+template<Type decltype(auto) X, typename T>
+  requires SameAs<decltype(X), T>
+constexpr bool deduced_as = true;
+
+constexpr int Z = 10;
+
+static_assert(deduced_as<0, int>);
+static_assert(deduced_as<0, int&>); // { dg-error "invalid variable template" }
+static_assert(deduced_as<Z, const int>);
+static_assert(deduced_as<(Z), const int>); // { dg-error "invalid variable template" }
+static_assert(deduced_as<(Z), const int&>);
+