[Sema] Relax a failing assertion in TransformBlockExpr

The assertion fails when the expression causing the this pointer to be
captured is part of a constexpr if statement's branch and the branch
gets discarded when the enclosing method is instantiated.

Note that the test case is added to CodeGen instead of Sema since the
translation unit has to be free of errors in order for the assertion to
be checked.

Differential Revision: https://reviews.llvm.org/D144016

Author: ahatanaka

clang/docs/ReleaseNotes.rst

@@ -141,6 +141,9 @@ Bug Fixes in This Version
   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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/TreeTransform.h

@@ -14598,7 +14598,12 @@ TreeTransform<Derived>::TransformBlockExpr(BlockExpr *E) {
                                                  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
 

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 @@ foo: should_not_be_used();
 // 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(