--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/clang/test/Sema/tautological-unsigned-enum-zero-compare.cpp	Thu Feb 13 15:10:13 2020 +0900
@@ -0,0 +1,148 @@
+// RUN: %clang_cc1 -std=c++11 -triple=x86_64-pc-linux-gnu -fsyntax-only \
+// RUN:            -Wtautological-unsigned-enum-zero-compare \
+// RUN:            -verify=unsigned,unsigned-signed %s
+// RUN: %clang_cc1 -std=c++11 -triple=x86_64-pc-win32 -fsyntax-only \
+// RUN:            -Wtautological-unsigned-enum-zero-compare \
+// RUN:            -verify=unsigned-signed %s
+// RUN: %clang_cc1 -std=c++11 -triple=x86_64-pc-win32 -fsyntax-only \
+// RUN:            -verify=silence %s
+
+// silence-no-diagnostics
+
+int main() {
+  // On Windows, all enumerations have a fixed underlying type, which is 'int'
+  // if not otherwise specified, so A is identical to C on Windows. Otherwise,
+  // we follow the C++ rules, which say that the only valid values of A are 0
+  // and 1.
+  enum A { A_foo = 0, A_bar, };
+  enum A a;
+
+  enum B : unsigned { B_foo = 0, B_bar, };
+  enum B b;
+
+  enum C : signed { C_foo = 0, C_bar, };
+  enum C c;
+
+  if (a < 0) // unsigned-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+  if (0 >= a)
+    return 0;
+  if (a > 0)
+    return 0;
+  if (0 <= a) // unsigned-warning {{comparison of 0 <= unsigned enum expression is always true}}
+    return 0;
+  if (a <= 0)
+    return 0;
+  if (0 > a) // unsigned-warning {{comparison of 0 > unsigned enum expression is always false}}
+    return 0;
+  if (a >= 0) // unsigned-warning {{comparison of unsigned enum expression >= 0 is always true}}
+    return 0;
+  if (0 < a)
+    return 0;
+
+  // FIXME: As below, the issue here is that the enumeration is promoted to
+  // unsigned.
+  if (a < 0U) // unsigned-signed-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+  if (0U >= a)
+    return 0;
+  if (a > 0U)
+    return 0;
+  if (0U <= a) // unsigned-signed-warning {{comparison of 0 <= unsigned enum expression is always true}}
+    return 0;
+  if (a <= 0U)
+    return 0;
+  if (0U > a) // unsigned-signed-warning {{comparison of 0 > unsigned enum expression is always false}}
+    return 0;
+  if (a >= 0U) // unsigned-signed-warning {{comparison of unsigned enum expression >= 0 is always true}}
+    return 0;
+  if (0U < a)
+    return 0;
+
+  if (b < 0) // unsigned-signed-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+  if (0 >= b)
+    return 0;
+  if (b > 0)
+    return 0;
+  if (0 <= b) // unsigned-signed-warning {{comparison of 0 <= unsigned enum expression is always true}}
+    return 0;
+  if (b <= 0)
+    return 0;
+  if (0 > b) // unsigned-signed-warning {{comparison of 0 > unsigned enum expression is always false}}
+    return 0;
+  if (b >= 0) // unsigned-signed-warning {{comparison of unsigned enum expression >= 0 is always true}}
+    return 0;
+  if (0 < b)
+    return 0;
+
+  if (b < 0U) // unsigned-signed-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+  if (0U >= b)
+    return 0;
+  if (b > 0U)
+    return 0;
+  if (0U <= b) // unsigned-signed-warning {{comparison of 0 <= unsigned enum expression is always true}}
+    return 0;
+  if (b <= 0U)
+    return 0;
+  if (0U > b) // unsigned-signed-warning {{comparison of 0 > unsigned enum expression is always false}}
+    return 0;
+  if (b >= 0U) // unsigned-signed-warning {{comparison of unsigned enum expression >= 0 is always true}}
+    return 0;
+  if (0U < b)
+    return 0;
+
+  if (c < 0)
+    return 0;
+  if (0 >= c)
+    return 0;
+  if (c > 0)
+    return 0;
+  if (0 <= c)
+    return 0;
+  if (c <= 0)
+    return 0;
+  if (0 > c)
+    return 0;
+  if (c >= 0)
+    return 0;
+  if (0 < c)
+    return 0;
+
+  // FIXME: These diagnostics are terrible. The issue here is that the signed
+  // enumeration value was promoted to an unsigned type.
+  if (c < 0U) // unsigned-signed-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+  if (0U >= c)
+    return 0;
+  if (c > 0U)
+    return 0;
+  if (0U <= c) // unsigned-signed-warning {{comparison of 0 <= unsigned enum expression is always true}}
+    return 0;
+  if (c <= 0U)
+    return 0;
+  if (0U > c) // unsigned-signed-warning {{comparison of 0 > unsigned enum expression is always false}}
+    return 0;
+  if (c >= 0U) // unsigned-signed-warning {{comparison of unsigned enum expression >= 0 is always true}}
+    return 0;
+  if (0U < c)
+    return 0;
+
+  return 1;
+}
+
+namespace crash_enum_zero_width {
+int test() {
+  enum A : unsigned {
+    A_foo = 0
+  };
+  enum A a;
+
+  // used to crash in llvm::APSInt::getMaxValue()
+  if (a < 0) // unsigned-signed-warning {{comparison of unsigned enum expression < 0 is always false}}
+    return 0;
+
+  return 1;
+}
+} // namespace crash_enum_zero_width