diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -141,6 +141,9 @@
   driver mode and emit an error which suggests using ``/TC`` or ``/TP``
   ``clang-cl`` options instead.
   (`#59307 <https://github.com/llvm/llvm-project/issues/59307>`_)
+- Fix assert that fails when the expression causing the this pointer to be
+  captured by a block is part of a constexpr if statement's branch and
+  instantiation of the enclosing method causes the branch to be discarded.
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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 +14598,12 @@
                                                  oldCapture));
       assert(blockScope->CaptureMap.count(newCapture));
     }
-    assert(oldBlock->capturesCXXThis() == blockScope->isCXXThisCaptured());
+
+    // The this pointer may not be captured by the instantiated block, even when
+    // it's captured by the original block, if the expression causing the
+    // capture is in the discarded branch of a constexpr if statement.
+    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 -triple x86_64-apple-darwin10 -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(