Index: lib/Sema/SemaOverload.cpp
===================================================================
--- lib/Sema/SemaOverload.cpp
+++ lib/Sema/SemaOverload.cpp
@@ -5140,12 +5140,18 @@
   //  implicitly converted to type T, where the converted
   //  expression is a constant expression and the implicit conversion
   //  sequence contains only [... list of conversions ...].
+  // C++1z [stmt.if]p2:
+  //  If the if statement is of the form if constexpr, the value of the
+  //  condition shall be a contextually converted constant expression of type
+  //  bool.
   ImplicitConversionSequence ICS =
-    TryCopyInitialization(S, From, T,
-                          /*SuppressUserConversions=*/false,
-                          /*InOverloadResolution=*/false,
-                          /*AllowObjcWritebackConversion=*/false,
-                          /*AllowExplicit=*/false);
+      CCE == Sema::CCEK_ConstexprIf
+          ? TryContextuallyConvertToBool(S, From)
+          : TryCopyInitialization(S, From, T,
+                                  /*SuppressUserConversions=*/false,
+                                  /*InOverloadResolution=*/false,
+                                  /*AllowObjcWritebackConversion=*/false,
+                                  /*AllowExplicit=*/false);
   StandardConversionSequence *SCS = nullptr;
   switch (ICS.getKind()) {
   case ImplicitConversionSequence::StandardConversion:
Index: test/CodeGenCXX/cxx1z-constexpr-if.cpp
===================================================================
--- test/CodeGenCXX/cxx1z-constexpr-if.cpp
+++ test/CodeGenCXX/cxx1z-constexpr-if.cpp
@@ -1,8 +1,17 @@
 // RUN: %clang_cc1 -std=c++1z %s -emit-llvm -o - | FileCheck %s --implicit-check-not=should_not_be_used
+// RUN: %clang_cc1 -std=c++1z %s -DCHECK_CONVERSION -verify %s
 
 void should_be_used_1();
 void should_be_used_2();
+void should_be_used_3();
 void should_not_be_used();
+
+struct A {
+  constexpr explicit operator bool() const {
+    return true;
+  }
+};
+
 void f() {
   if constexpr (false)
     should_not_be_used();
@@ -15,7 +24,17 @@
     goto foo;
 foo: should_not_be_used();
   }
+  if constexpr (A())
+    should_be_used_3();
+  else
+    should_not_be_used();
+#ifdef CHECK_CONVERSION
+  if constexpr (4.3) ; // expected-error{{conversion from 'double' to 'bool' is not allowed in a converted constant expression}}
+  constexpr void *p = nullptr;
+  if constexpr (p) ; // expected-error{{conversion from 'void *const' to 'bool' is not allowed in a converted constant expression}}
+#endif
 }
 
 // CHECK: should_be_used_1
 // CHECK: should_be_used_2
+// CHECK: should_be_used_3