Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -204,6 +204,7 @@
 def DanglingElse: DiagGroup<"dangling-else">;
 def DanglingField : DiagGroup<"dangling-field">;
 def DistributedObjectModifiers : DiagGroup<"distributed-object-modifiers">;
+def ExpansionToUndefined : DiagGroup<"expansion-to-undefined">;
 def FlagEnum : DiagGroup<"flag-enum">;
 def IncrementBool : DiagGroup<"increment-bool", [DeprecatedIncrementBool]>;
 def InfiniteRecursion : DiagGroup<"infinite-recursion">;
Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -658,6 +658,13 @@
 def note_header_guard : Note<
   "%0 is defined here; did you mean %1?">;
 
+def warn_defined_in_object_type_macro : Warning<
+  "macro expansion producing 'defined' has undefined behavior">,
+  InGroup<ExpansionToUndefined>;
+def warn_defined_in_function_type_macro : Extension<
+  "macro expansion producing 'defined' has undefined behavior">,
+  InGroup<ExpansionToUndefined>;
+
 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 +82,7 @@
 static bool EvaluateDefined(PPValue &Result, Token &PeekTok, DefinedTracker &DT,
                             bool ValueLive, Preprocessor &PP) {
   SourceLocation beginLoc(PeekTok.getLocation());
+
   Result.setBegin(beginLoc);
 
   // Get the next token, don't expand it.
@@ -140,6 +141,51 @@
     PP.LexNonComment(PeekTok);
   }
 
+  // [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()) {
+    bool IsFunctionTypeMacro =
+        PP.getSourceManager()
+            .getSLocEntry(PP.getSourceManager().getFileID(beginLoc))
+            .getExpansion()
+            .isFunctionMacroExpansion();
+    // For object-type macros, it's easy to replace
+    //   #define FOO defined(BAR)
+    // with
+    //   #if defined(BAR)
+    //   #define FOO 1
+    //   #else
+    //   #define FOO 0
+    //   #endif
+    // and doing so makes sense since compilers handle this differently in
+    // practice (see example further up).  But for function-type macros,
+    // there is no good way to write
+    //   # define FOO(x) (defined(M_ ## x) && M_ ## x)
+    // in a different way, and compilers seem to agree on how to behave here.
+    // So warn by default on object-type macros, but only warn in -pedantic
+    // mode on function-type macros.
+    if (IsFunctionTypeMacro)
+      PP.Diag(beginLoc, diag::warn_defined_in_function_type_macro);
+    else
+      PP.Diag(beginLoc, diag::warn_defined_in_object_type_macro);
+  }
+
   // Invoke the 'defined' callback.
   if (PPCallbacks *Callbacks = PP.getPPCallbacks()) {
     Callbacks->Defined(macroToken, Macro,
@@ -177,8 +223,8 @@
   if (IdentifierInfo *II = PeekTok.getIdentifierInfo()) {
     // Handle "defined X" and "defined(X)".
     if (II->isStr("defined"))
-      return(EvaluateDefined(Result, PeekTok, DT, ValueLive, PP));
-    
+      return EvaluateDefined(Result, PeekTok, DT, ValueLive, PP);
+
     // If this identifier isn't 'defined' or one of the special
     // preprocessor keywords and it wasn't macro expanded, it turns
     // into a simple 0, unless it is the C++ keyword "true", in which case it
Index: test/Lexer/cxx-features.cpp
===================================================================
--- test/Lexer/cxx-features.cpp
+++ test/Lexer/cxx-features.cpp
@@ -6,6 +6,7 @@
 
 // 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 +1,28 @@
-// RUN: %clang_cc1 %s -E -CC -pedantic -verify
-// expected-no-diagnostics
+// RUN: %clang_cc1 %s -E -CC -verify
+// RUN: %clang_cc1 %s -E -CC -DPEDANTIC -pedantic -verify
 
 #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
+
+#define m_foo
+#define TEST(a) (defined(m_##a) && a)
+
+#if defined(PEDANTIC)
+// expected-warning@+4{{macro expansion producing 'defined' has undefined behavior}}
+#endif
+
+// This shouldn't warn by default, only with pedantic:
+#if TEST(foo)
+#endif
+
+
+// Only one diagnostic for this case:
+#define INVALID defined(
+#if INVALID // expected-error{{macro name missing}}
+#endif