diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -14598,7 +14598,8 @@
                                                  oldCapture));
       assert(blockScope->CaptureMap.count(newCapture));
     }
-    assert(oldBlock->capturesCXXThis() == blockScope->isCXXThisCaptured());
+    assert((!blockScope->isCXXThisCaptured() || oldBlock->capturesCXXThis()) &&
+           "this pointer isn't captured in the old block");
   }
 #endif
 
diff --git a/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp b/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp
--- a/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp
+++ b/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp
@@ -1,4 +1,4 @@
-// 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 -emit-llvm -fblocks -o - | FileCheck %s --implicit-check-not=should_not_be_used
 
 void should_be_used_1();
 void should_be_used_2();
@@ -32,3 +32,20 @@
 // CHECK: should_be_used_1
 // CHECK: should_be_used_2
 // CHECK: should_be_used_3
+
+namespace BlockThisCapture {
+  void foo();
+  struct S {
+    template <bool b>
+    void m() {
+      ^{ if constexpr(b) (void)this; else foo();  }();
+    }
+  };
+
+  void test() {
+    S().m<false>();
+  }
+}
+
+// CHECK-LABEL: define internal void @___ZN16BlockThisCapture1S1mILb0EEEvv_block_invoke(
+// CHECK: call void @_ZN16BlockThisCapture3fooEv(