--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init1.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,99 @@
+// PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.
+// { dg-do compile { target c++2a } }
+// Test basic use.
+
+struct S {
+  int i;
+  constexpr S(bool b) {
+    if (b)
+      i = 42;
+  }
+};
+constexpr S s1(true);
+constexpr S s2(false); // { dg-error "not a constant expression" }
+
+constexpr int
+fn1 (int x)
+{
+  int a;
+  a = 5;
+  return x + a;
+}
+
+static_assert (fn1 (2) == 7);
+
+constexpr int
+fn2 (int x)
+{
+  const int a; // { dg-error "uninitialized .const a." }
+  constexpr int b; // { dg-error "uninitialized .const b." }
+  return x;
+}
+
+constexpr int
+fn3 (int x)
+{
+  int a; // { dg-message ".int a. is not const" }
+  return x + a; // { dg-error "the value of .a. is not usable in a constant expression" }
+}
+
+constexpr int a = fn3 (5); // { dg-message "in .constexpr. expansion of" }
+
+constexpr int
+fn4 ()
+{
+  struct S { int a = -5; int b; } s;
+  return s.a;
+}
+
+static_assert (fn4 () == -5);
+
+constexpr int
+fn5 ()
+{
+  struct S { int a = 9; int b; } s;
+  return s.b;
+}
+
+constexpr int b = fn5 (); // { dg-error "accessing uninitialized member" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
+
+constexpr int
+fn6 ()
+{
+  int a;
+  return 42;
+}
+
+static_assert (fn6 () == 42);
+
+constexpr int
+fn7 (bool b)
+{
+  int a; // { dg-message ".int a. is not const" }
+  if (b)
+    a = 42;
+  return a;
+}
+
+static_assert (fn7 (true) == 42);
+static_assert (fn7 (false) == 42); // { dg-error "non-constant condition|the value of .a. is not usable" }
+// { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }
+
+constexpr int
+fn8 (int n)
+{
+  int r;
+  switch (n)
+    {
+    case 1:
+    r = n;
+    return r;
+    case 42:
+    r = n;
+    return r;
+    }
+}
+
+static_assert (fn8 (1) == 1);
+static_assert (fn8 (42) == 42);