diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -2777,6 +2777,13 @@
            FD->getIdentifier() && FD->getIdentifier()->isStr("move");
   }
 
+  bool isCallToStdIsConstantEvaluated() const {
+    const FunctionDecl *FD = getDirectCallee();
+    return getNumArgs() == 0 && FD && FD->isInStdNamespace() &&
+           FD->getIdentifier() &&
+           FD->getIdentifier()->isStr("is_constant_evaluated");
+  }
+
   static bool classof(const Stmt *T) {
     return T->getStmtClass() >= firstCallExprConstant &&
            T->getStmtClass() <= lastCallExprConstant;
diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
--- a/clang/include/clang/Basic/DiagnosticASTKinds.td
+++ b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -329,6 +329,10 @@
 def note_unimplemented_constexpr_lambda_feature_ast : Note<
     "unimplemented constexpr lambda feature: %0 (coming soon!)">;
 
+def warn_is_constant_evaluated_always_true_constexpr : Warning<
+  "'%0' will always evaluate to 'true' in constexpr context">,
+  InGroup<DiagGroup<"constant-evaluated">>;
+
 // inline asm related.
 let CategoryName = "Inline Assembly Issue" in {
   def err_asm_invalid_escape : Error<
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -10590,8 +10590,23 @@
     return false;
   }
 
-  case Builtin::BI__builtin_is_constant_evaluated:
+  case Builtin::BI__builtin_is_constant_evaluated: {
+    const auto *Callee = Info.CurrentCall->getCallee();
+    if (Info.InConstantContext && !Info.CheckingPotentialConstantExpression &&
+        (Info.CallStackDepth == 1 ||
+         (Info.CallStackDepth == 2 && E->getNumArgs() == 0 &&
+          Callee->isInStdNamespace() && Callee->getIdentifier() &&
+          Callee->getIdentifier()->isStr("is_constant_evaluated")))) {
+      if (Info.EvalStatus.Diag)
+        Info.report((Info.CallStackDepth == 1) ? E->getExprLoc()
+                                               : Info.CurrentCall->CallLoc,
+                    diag::warn_is_constant_evaluated_always_true_constexpr)
+            << (Info.CallStackDepth == 1 ? "__builtin_is_constant_evaluated"
+                                         : "std::is_constant_evaluated");
+    }
+
     return Success(Info.InConstantContext, E);
+  }
 
   case Builtin::BI__builtin_ctz:
   case Builtin::BI__builtin_ctzl:
diff --git a/clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp b/clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp
@@ -0,0 +1,57 @@
+// RUN: %clang_cc1 -std=c++2a -fsyntax-only -verify %s
+
+namespace std {
+constexpr bool is_constant_evaluated() noexcept {
+  return __builtin_is_constant_evaluated();
+}
+}
+
+constexpr int fn1() {
+  if constexpr (std::is_constant_evaluated()) // expected-warning {{'std::is_constant_evaluated' will always evaluate to 'true' in constexpr context}}
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int fn2() {
+  if constexpr (!std::is_constant_evaluated()) // expected-warning {{'std::is_constant_evaluated' will always evaluate to 'true' in constexpr context}}
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int fn3() {
+  if constexpr (std::is_constant_evaluated() == false) // expected-warning {{'std::is_constant_evaluated' will always evaluate to 'true' in constexpr context}}
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int fn4() {
+  if constexpr (__builtin_is_constant_evaluated() == true) // expected-warning {{'__builtin_is_constant_evaluated' will always evaluate to 'true' in constexpr context}}
+    return 0;
+  else
+    return 1;
+}
+
+
+constexpr int fn5() {
+  if constexpr (__builtin_is_constant_evaluated()) // expected-warning {{'__builtin_is_constant_evaluated' will always evaluate to 'true' in constexpr context}}
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int nowarn1() {
+  if (std::is_constant_evaluated())
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int nowarn2() {
+  if (!__builtin_is_constant_evaluated())
+    return 0;
+  else
+    return 1;
+}