diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1487,6 +1487,8 @@
 // C++ declarations
 def err_static_assert_expression_is_not_constant : Error<
   "static_assert expression is not an integral constant expression">;
+def err_constexpr_if_condition_expression_is_not_constant : Error<
+  "constexpr if condition is not a constant expression convertible to bool">;
 def err_static_assert_failed : Error<"static_assert failed%select{ %1|}0">;
 def err_static_assert_requirement_failed : Error<
   "static_assert failed due to requirement '%0'%select{ %2|}1">;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -3911,7 +3911,7 @@
 
 /// CheckCXXBooleanCondition - Returns true if a conversion to bool is invalid.
 ExprResult Sema::CheckCXXBooleanCondition(Expr *CondExpr, bool IsConstexpr) {
-  // C++ 6.4p4:
+  // C++11 6.4p4:
   // The value of a condition that is an initialized declaration in a statement
   // other than a switch statement is the value of the declared variable
   // implicitly converted to type bool. If that conversion is ill-formed, the
@@ -3919,12 +3919,27 @@
   // The value of a condition that is an expression is the value of the
   // expression, implicitly converted to bool.
   //
+  // C++2b 8.5.2p2
+  // If the if statement is of the form if constexpr, the value of the condition
+  // is contextually converted to bool and the converted expression shall be
+  // a constant expression
+  //
   // FIXME: Return this value to the caller so they don't need to recompute it.
-  llvm::APSInt Value(/*BitWidth*/1);
-  return (IsConstexpr && !CondExpr->isValueDependent())
-             ? CheckConvertedConstantExpression(CondExpr, Context.BoolTy, Value,
-                                                CCEK_ConstexprIf)
-             : PerformContextuallyConvertToBool(CondExpr);
+
+  if (IsConstexpr && !LangOpts.CPlusPlus2b && !CondExpr->isValueDependent()) {
+    llvm::APSInt Value(/*BitWidth*/ 1);
+    return CheckConvertedConstantExpression(CondExpr, Context.BoolTy, Value,
+                                            CCEK_ConstexprIf);
+  }
+  ExprResult E = PerformContextuallyConvertToBool(CondExpr);
+  if (!IsConstexpr || !E.isUsable() || E.get()->isValueDependent())
+    return E;
+
+  llvm::APSInt Cond;
+  E = VerifyIntegerConstantExpression(
+      E.get(), &Cond,
+      diag::err_constexpr_if_condition_expression_is_not_constant);
+  return E;
 }
 
 /// Helper function to determine whether this is the (deprecated) C++
diff --git a/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp b/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
--- a/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
+++ b/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
@@ -1,5 +1,7 @@
-// RUN: %clang_cc1 -std=c++1z -verify %s
-// RUN: %clang_cc1 -std=c++1z -verify %s -DUNDEFINED
+// RUN: %clang_cc1 -std=c++1z -verify=expected,cxx17 %s
+// RUN: %clang_cc1 -std=c++1z -verify=expected,cxx17 %s -DUNDEFINED
+// RUN: %clang_cc1 -std=c++2b -verify=expected,cxx2b %s
+// RUN: %clang_cc1 -std=c++2b -verify=expected,cxx2b %s -DUNDEFINED
 
 #ifdef UNDEFINED
 // "used but not defined" errors don't get produced if we have more interesting
@@ -38,18 +40,43 @@
 }
 #else
 namespace ccce {
+
+  struct S {
+  } s;
   void f() {
     if (5) {}
-    if constexpr (5) {} // expected-error {{cannot be narrowed}}
+    if constexpr (5) { // cxx17-error {{cannot be narrowed}}
+    }
   }
   template<int N> void g() {
-    if constexpr (N) {} // expected-error {{cannot be narrowed}}
+    if constexpr (N) { // cxx17-error {{cannot be narrowed}}
+    }
   }
-  template void g<5>(); // expected-note {{instantiation of}}
+  template void g<5>(); //cxx17-note  {{instantiation of}}
   void h() {
-    if constexpr (4.3) {} // expected-error{{conversion from 'double' to 'bool' is not allowed in a converted constant expression}}
+    if constexpr (4.3) { //  cxx17-error {{conversion from 'double' to 'bool' is not allowed in a converted constant expression}}
+      // cxx2b-warning@-1 {{implicit conversion from 'double' to 'bool' changes value}}
+    }
     constexpr void *p = nullptr;
-    if constexpr (p) {} // expected-error{{conversion from 'void *const' to 'bool' is not allowed in a converted constant expression}}
+    if constexpr (p) { // cxx17-error {{conversion from 'void *const' to 'bool' is not allowed in a converted constant expression}}
+    }
+  }
+
+  void not_constant(int b, S s) { //  expected-note 2{{declared here}}
+    if constexpr (bool(b)) {      // expected-error {{constexpr if condition is not a constant expression}} expected-note {{cannot be used in a constant expression}}
+    }
+    if constexpr (b) { // expected-error {{constexpr if condition is not a constant expression}} expected-note {{cannot be used in a constant expression}}
+    }
+    if constexpr (s) {
+      // cxx17-error@-1 {{value of type 'ccce::S' is not implicitly convertible to 'bool'}}
+      // cxx2b-error@-2 {{value of type 'ccce::S' is not contextually convertible to 'bool'}}
+    }
+
+    constexpr S constexprS;
+    if constexpr (constexprS) {
+      //cxx17-error@-1 {{value of type 'const ccce::S' is not implicitly convertible to 'bool'}}
+      //cxx2b-error@-2 {{value of type 'const ccce::S' is not contextually convertible to 'bool'}}
+    }
   }
 }
 
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -1296,7 +1296,7 @@
     <tr>
       <td>Narrowing contextual conversions to bool</td>
       <td><a href="https://wg21.link/P1401R5">P1401R5</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 13</td>
     </tr>
     <tr>
       <td>Trimming whitespaces before line splicing</td>