Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -656,6 +656,10 @@
 def note_header_guard : Note<
   "%0 is defined here; did you mean %1?">;
 
+def warn_defined_in_macro : Warning<
+  "macro expansion producing 'defined' has undefined behavior">,
+  InGroup<DiagGroup<"expansion-to-defined">>;
+
 let CategoryName = "Nullability Issue" in {
 
 def err_pp_assume_nonnull_syntax : Error<"expected 'begin' or 'end'">;
Index: lib/Lex/PPExpressions.cpp
===================================================================
--- lib/Lex/PPExpressions.cpp
+++ lib/Lex/PPExpressions.cpp
@@ -82,6 +82,28 @@
 static bool EvaluateDefined(PPValue &Result, Token &PeekTok, DefinedTracker &DT,
                             bool ValueLive, Preprocessor &PP) {
   SourceLocation beginLoc(PeekTok.getLocation());
+
+  // [cpp.cond]p4:
+  //   Prior to evaluation, macro invocations in the list of preprocessing
+  //   tokens that will become the controlling constant expression are replaced
+  //   (except for those macro names modified by the 'defined' unary operator),
+  //   just as in normal text. If the token 'defined' is generated as a result
+  //   of this replacement process or use of the 'defined' unary operator does
+  //   not match one of the two specified forms prior to macro replacement, the
+  //   behavior is undefined.
+  // This isn't an idle threat, consider this program:
+  //   #define FOO
+  //   #define BAR defined(FOO)
+  //   #if BAR
+  //   ...
+  //   #else
+  //   ...
+  //   #endif
+  // clang and gcc will pick the #if branch while Visual Studio will take the
+  // #else branch.  Emit a warning about this undefined behavior.
+  if (beginLoc.isMacroID())
+    PP.Diag(beginLoc, diag::warn_defined_in_macro);
+
   Result.setBegin(beginLoc);
 
   // Get the next token, don't expand it.
Index: test/Lexer/cxx-features.cpp
===================================================================
--- test/Lexer/cxx-features.cpp
+++ test/Lexer/cxx-features.cpp
@@ -1,11 +1,12 @@
-// RUN: %clang_cc1 -std=c++98 -verify %s
-// RUN: %clang_cc1 -std=c++11 -verify %s
-// RUN: %clang_cc1 -std=c++1y -fsized-deallocation -verify %s
-// RUN: %clang_cc1 -std=c++1y -fsized-deallocation -fconcepts-ts -DCONCEPTS_TS=1 -verify %s
-// RUN: %clang_cc1 -fcoroutines -DCOROUTINES -verify %s
+// RUN: %clang_cc1 -Wno-expansion-to-defined -std=c++98 -verify %s
+// RUN: %clang_cc1 -Wno-expansion-to-defined -std=c++11 -verify %s
+// RUN: %clang_cc1 -Wno-expansion-to-defined -std=c++1y -fsized-deallocation -verify %s
+// RUN: %clang_cc1 -Wno-expansion-to-defined -std=c++1y -fsized-deallocation -fconcepts-ts -DCONCEPTS_TS=1 -verify %s
+// RUN: %clang_cc1 -Wno-expansion-to-defined -fcoroutines -DCOROUTINES -verify %s
 
 // expected-no-diagnostics
 
+// FIXME using `defined` in a macro has undefined behavior.
 #if __cplusplus < 201103L
 #define check(macro, cxx98, cxx11, cxx1y) cxx98 == 0 ? defined(__cpp_##macro) : __cpp_##macro != cxx98
 #elif __cplusplus < 201304L
Index: test/Preprocessor/expr_define_expansion.c
===================================================================
--- test/Preprocessor/expr_define_expansion.c
+++ test/Preprocessor/expr_define_expansion.c
@@ -1,6 +1,10 @@
 // RUN: %clang_cc1 %s -E -CC -pedantic -verify
-// expected-no-diagnostics
 
 #define FOO && 1
 #if defined FOO FOO
 #endif
+
+#define A
+#define B defined(A)
+#if B // expected-warning{{macro expansion producing 'defined' has undefined behavior}}
+#endif