Index: include/clang/Basic/AttrDocs.td
===================================================================
--- include/clang/Basic/AttrDocs.td
+++ include/clang/Basic/AttrDocs.td
@@ -283,6 +283,32 @@
     ptr = &i; // OK: 'TrueConstant' is a truthy constant
     ptr = &j; // error: 'FalseConstant' is a constant, but not truthy
   }
+
+Because ``enable_if`` evaluation happens during overload resolution,
+``enable_if`` may give unintuitive results when used with templates, depending
+on when overloads are resolved. In the example below, clang will emit a
+diagnostic about no viable overloads for ``foo`` in ``bar``, but not in ``baz``:
+
+.. code-block:: c++
+
+  double foo(int i) __attribute__((enable_if(i > 0, "")));
+  void *foo(int i) __attribute__((enable_if(i <= 0, "")));
+  template <int I>
+  auto bar() { return foo(I); }
+
+  template <typename T>
+  auto baz() { return foo(T::number); }
+
+  struct WithNumber { constexpr static int number = 1; };
+  void callThem() {
+    bar<sizeof(WithNumber)>();
+    baz<WithNumber>();
+  }
+
+This is because, in ``bar``, ``foo`` is resolved prior to template
+instantiation, so the value for ``I`` isn't known (thus, both ``enable_if``
+conditions for ``foo`` fail). However, in ``baz``, ``foo`` is resolved during
+template instantiation, so the value for ``T::number`` is known.
   }];
 }
 
Index: lib/Sema/SemaOverload.cpp
===================================================================
--- lib/Sema/SemaOverload.cpp
+++ lib/Sema/SemaOverload.cpp
@@ -5984,7 +5984,6 @@
   SFINAETrap Trap(*this);
   SmallVector<Expr *, 16> ConvertedArgs;
   bool InitializationFailed = false;
-  bool ContainsValueDependentExpr = false;
 
   // Convert the arguments.
   for (unsigned I = 0, E = Args.size(); I != E; ++I) {
@@ -6006,7 +6005,6 @@
       break;
     }
 
-    ContainsValueDependentExpr |= R.get()->isValueDependent();
     ConvertedArgs.push_back(R.get());
   }
 
@@ -6027,7 +6025,6 @@
         InitializationFailed = true;
         break;
       }
-      ContainsValueDependentExpr |= R.get()->isValueDependent();
       ConvertedArgs.push_back(R.get());
     }
 
@@ -6037,18 +6034,14 @@
 
   for (auto *EIA : EnableIfAttrs) {
     APValue Result;
-    if (EIA->getCond()->isValueDependent()) {
-      // Don't even try now, we'll examine it after instantiation.
-      continue;
-    }
-
+    // FIXME: This doesn't consider value-dependent cases, because doing so is
+    // very difficult. Ideally, we should handle them more gracefully.
     if (!EIA->getCond()->EvaluateWithSubstitution(
-            Result, Context, Function, llvm::makeArrayRef(ConvertedArgs))) {
-      if (!ContainsValueDependentExpr)
-        return EIA;
-    } else if (!Result.isInt() || !Result.getInt().getBoolValue()) {
+            Result, Context, Function, llvm::makeArrayRef(ConvertedArgs)))
+      return EIA;
+
+    if (!Result.isInt() || !Result.getInt().getBoolValue())
       return EIA;
-    }
   }
   return nullptr;
 }
Index: test/SemaCXX/enable_if.cpp
===================================================================
--- test/SemaCXX/enable_if.cpp
+++ test/SemaCXX/enable_if.cpp
@@ -116,9 +116,9 @@
   void g() { f(); }
 };
 
-int fn3(bool b) __attribute__((enable_if(b, "")));
+int fn3(bool b) __attribute__((enable_if(b, ""))); // FIXME: This test should net 0 error messages.
 template <class T> void test3() {
-  fn3(sizeof(T) == 1);
+  fn3(sizeof(T) == 1); // expected-error{{no matching function for call to 'fn3'}} expected-note@-2{{candidate disabled}}
 }
 
 template <typename T>
@@ -138,7 +138,7 @@
 void h(int);
 template <typename T> void outer() {
   void local_function() __attribute__((enable_if(::h(T()), "")));
-  local_function();
+  local_function(); // expected-error{{no matching function for call to 'local_function'}} expected-note@-1{{candidate disabled}}
 };
 
 namespace PR20988 {
@@ -158,9 +158,9 @@
     fn2(expr);  // expected-error{{no matching function for call to 'fn2'}}
   }
 
-  int fn3(bool b) __attribute__((enable_if(b, "")));
+  int fn3(bool b) __attribute__((enable_if(b, ""))); // FIXME: This test should net 0 error messages.
   template <class T> void test3() {
-    fn3(sizeof(T) == 1);
+    fn3(sizeof(T) == 1); // expected-error{{no matching function for call to 'fn3'}} expected-note@-2{{candidate disabled}}
   }
 }
 
@@ -386,3 +386,34 @@
   f.bar(1, 2); // expected-error{{too many arguments}}
 }
 }
+
+// Ideally, we should be able to handle value-dependent expressions sanely.
+// Sadly, that isn't the case at the moment.
+namespace dependent {
+int error(int N) __attribute__((enable_if(N, ""))); // expected-note{{candidate disabled}}
+int error(int N) __attribute__((enable_if(!N, ""))); // expected-note{{candidate disabled}}
+template <int N> int callUnavailable() {
+  return error(N); // expected-error{{no matching function for call to 'error'}}
+}
+
+constexpr int noError(int N) __attribute__((enable_if(N, ""))) { return -1; }
+constexpr int noError(int N) __attribute__((enable_if(!N, ""))) { return -1; }
+constexpr int noError(int N) { return 0; }
+
+template <int N>
+constexpr int callNoError() { return noError(N); }
+static_assert(callNoError<0>() == 0, "");
+static_assert(callNoError<1>() == 0, "");
+
+template <int N> constexpr int templated() __attribute__((enable_if(N, ""))) {
+  return 1;
+}
+
+constexpr int A = templated<0>(); // expected-error{{no matching function for call to 'templated'}} expected-note@-4{{candidate disabled}}
+static_assert(templated<1>() == 1, "");
+
+template <int N> constexpr int callTemplated() { return templated<N>(); }
+
+constexpr int B = callTemplated<0>(); // expected-error{{initialized by a constant expression}} expected-error@-2{{no matching function for call to 'templated'}} expected-note{{in instantiation of function template}} expected-note@-9{{candidate disabled}}
+static_assert(callTemplated<1>() == 1, "");
+}