Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -287,6 +287,7 @@
 def NullabilityDeclSpec : DiagGroup<"nullability-declspec">;
 def NullabilityInferredOnNestedType : DiagGroup<"nullability-inferred-on-nested-type">;
 def NullableToNonNullConversion : DiagGroup<"nullable-to-nonnull-conversion">;
+def NullConstToNonnull : DiagGroup<"null-const-to-nonnull">;
 def NullabilityCompletenessOnArrays : DiagGroup<"nullability-completeness-on-arrays">;
 def NullabilityCompleteness : DiagGroup<"nullability-completeness",
                                         [NullabilityCompletenessOnArrays]>;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -8802,6 +8802,10 @@
   "type %1">,
   InGroup<NullableToNonNullConversion>, DefaultIgnore;
 
+def warn_null_const_to_nonnull : Warning<
+  "implicitly casting a null constant to non-nullable pointer type %0">,
+  InGroup<NullConstToNonnull>, DefaultIgnore;
+
 def err_nullability_cs_multilevel : Error<
   "nullability keyword %0 cannot be applied to multi-level pointer type %1">;
 def note_nullability_type_specifier : Note<
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -3729,6 +3729,11 @@
   void diagnoseNullableToNonnullConversion(QualType DstType, QualType SrcType,
                                            SourceLocation Loc);
 
+  /// Warn if we're implicitly casting from a null pointer constant to a
+  /// _Nonnull pointer type.
+  void diagnoseNullPtrToNonnullCast(QualType DstType, Expr *E,
+                                    SourceLocation Loc);
+
   ParsingDeclState PushParsingDeclaration(sema::DelayedDiagnosticPool &pool) {
     return DelayedDiagnostics.push(pool);
   }
Index: lib/Sema/Sema.cpp
===================================================================
--- lib/Sema/Sema.cpp
+++ lib/Sema/Sema.cpp
@@ -441,6 +441,7 @@
     }
   }
 
+  diagnoseNullPtrToNonnullCast(Ty, E, E->getExprLoc());
   return ImplicitCastExpr::Create(Context, Ty, Kind, E, BasePath, VK);
 }
 
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -9475,6 +9475,17 @@
   return false;
 }
 
+void Sema::diagnoseNullPtrToNonnullCast(QualType DstType, Expr *E,
+                                        SourceLocation Loc) {
+  if (!DstType->isAnyPointerType() || CurContext->isDependentContext())
+    return;
+
+  if (Optional<NullabilityKind> Kind = DstType->getNullability(Context))
+    if (*Kind == NullabilityKind::NonNull &&
+        E->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNotNull))
+      Diag(Loc, diag::warn_null_const_to_nonnull) << DstType;
+}
+
 /// \brief Diagnose pointers that are always non-null.
 /// \param E the expression containing the pointer
 /// \param NullKind NPCK_NotNull if E is a cast to bool, otherwise, E is
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -717,6 +717,7 @@
       E->getType().getObjCLifetime() == Qualifiers::OCL_Weak)
     Cleanup.setExprNeedsCleanups(true);
 
+  diagnoseNullPtrToNonnullCast(T, E, E->getExprLoc());
   ExprResult Res = ImplicitCastExpr::Create(Context, T, CK_LValueToRValue, E,
                                             nullptr, VK_RValue);
 
Index: test/Sema/null_const_to_nonnull.c
===================================================================
--- /dev/null
+++ test/Sema/null_const_to_nonnull.c
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fsyntax-only -Wnull-const-to-nonnull %s -verify
+
+void null_const_to_nonnull(int c) {
+  int * _Nonnull p0 = 0; // expected-warning{{implicitly casting a null constant to non-nullable pointer type 'int * _Nonnull'}}
+  int * _Nonnull p1;
+  int * _Nonnull p2 = c ? p1 : 0; // expected-warning{{implicitly casting a null constant to non-nullable pointer type 'int * _Nonnull'}}
+}