Index: clang/include/clang/AST/Stmt.h
===================================================================
--- clang/include/clang/AST/Stmt.h
+++ clang/include/clang/AST/Stmt.h
@@ -2080,6 +2080,7 @@
   /// If this is an 'if constexpr', determine which substatement will be taken.
   /// Otherwise, or if the condition is value-dependent, returns None.
   Optional<const Stmt*> getNondiscardedCase(const ASTContext &Ctx) const;
+  Optional<Stmt *> getNondiscardedCase(const ASTContext &Ctx);
 
   bool isObjCAvailabilityCheck() const;
 
Index: clang/lib/AST/Stmt.cpp
===================================================================
--- clang/lib/AST/Stmt.cpp
+++ clang/lib/AST/Stmt.cpp
@@ -989,12 +989,20 @@
   return isa<ObjCAvailabilityCheckExpr>(getCond());
 }
 
-Optional<const Stmt*> IfStmt::getNondiscardedCase(const ASTContext &Ctx) const {
+Optional<Stmt *> IfStmt::getNondiscardedCase(const ASTContext &Ctx) {
   if (!isConstexpr() || getCond()->isValueDependent())
     return None;
   return !getCond()->EvaluateKnownConstInt(Ctx) ? getElse() : getThen();
 }
 
+Optional<const Stmt *>
+IfStmt::getNondiscardedCase(const ASTContext &Ctx) const {
+  if (Optional<Stmt *> Result =
+          const_cast<IfStmt *>(this)->getNondiscardedCase(Ctx))
+    return *Result;
+  return None;
+}
+
 ForStmt::ForStmt(const ASTContext &C, Stmt *Init, Expr *Cond, VarDecl *condVar,
                  Expr *Inc, Stmt *Body, SourceLocation FL, SourceLocation LP,
                  SourceLocation RP)
Index: clang/lib/Sema/Sema.cpp
===================================================================
--- clang/lib/Sema/Sema.cpp
+++ clang/lib/Sema/Sema.cpp
@@ -1666,6 +1666,18 @@
       }
     }
   }
+
+  void VisitIfStmt(IfStmt *If) {
+    // Only evaluate the non-discarded side of a constexpr-if.
+    if (Optional<Stmt *> SubStmt = If->getNondiscardedCase(S.Context)) {
+      // The non-discarded statement could can be a non-existent 'else', so make
+      // sure we don't try to visit it.
+      if (*SubStmt)
+        Inherited::Visit(*SubStmt);
+    } else {
+      Inherited::VisitStmt(If);
+    }
+  }
 };
 } // namespace
 
Index: clang/test/SemaCUDA/deferred-diags.cu
===================================================================
--- clang/test/SemaCUDA/deferred-diags.cu
+++ clang/test/SemaCUDA/deferred-diags.cu
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fcxx-exceptions -fcuda-is-device -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fcxx-exceptions -fcuda-is-device -fsyntax-only -std=c++17 -verify %s
 
 #include "Inputs/cuda.h"
 
@@ -8,29 +8,55 @@
   // expected-error@-1 2{{cannot use 'throw' in __host__ __device__ function}}
 }
 
+inline __host__ __device__ void hasInvalid2() {
+  throw NULL;
+  // expected-error@-1 2{{cannot use 'throw' in __host__ __device__ function}}
+}
+
+inline __host__ __device__ void hasInvalidDiscarded() {
+  // This is only used in the discarded statements below, so this should not diagnose.
+  throw NULL;
+}
+
 static __device__ void use0() {
   hasInvalid(); // expected-note {{called by 'use0'}}
   hasInvalid(); // expected-note {{called by 'use0'}}
+
+  if constexpr (true) {
+    hasInvalid2(); // expected-note {{called by 'use0'}}
+  } else {
+    hasInvalidDiscarded();
+  }
+
+  if constexpr (false) {
+    hasInvalidDiscarded();
+  } else {
+    hasInvalid2(); // expected-note {{called by 'use0'}}
+  }
+
+  if constexpr (false) {
+    hasInvalidDiscarded();
+  }
 }
 
 // To avoid excessive diagnostic messages, deferred diagnostics are only
 // emitted the first time a function is called.
 static __device__ void use1() {
-  use0(); // expected-note 2{{called by 'use1'}}
+  use0(); // expected-note 4{{called by 'use1'}}
   use0();
 }
 
 static __device__ void use2() {
-  use1(); // expected-note 2{{called by 'use2'}}
+  use1(); // expected-note 4{{called by 'use2'}}
   use1();
 }
 
 static __device__ void use3() {
-  use2(); // expected-note 2{{called by 'use3'}}
+  use2(); // expected-note 4{{called by 'use3'}}
   use2();
 }
 
 __global__ void use4() {
-  use3(); // expected-note 2{{called by 'use4'}}
+  use3(); // expected-note 4{{called by 'use4'}}
   use3();
 }