Fix crash with modules and constexpr destructor (#69076)

With modules, serialization might omit the outer ExprWithCleanups
as it calls ParmVarDecl::getDefaultArg(). Complementary to fixing
this in a separate change, make the code more robust by adding a
FullExpressionRAII and avoid the llvm_unreachable in the added test
clang/test/Modules/pr68702.cpp.

Closes #68702

Author: hahnjo

clang/docs/ReleaseNotes.rst

@@ -958,6 +958,8 @@ Miscellaneous Clang Crashes Fixed
 - Fixed a crash when a lambda marked as ``static`` referenced a captured
   variable in an expression.
   `Issue 74608 <https://github.com/llvm/llvm-project/issues/74608>`_
+- Fixed a crash with modules and a ``constexpr`` destructor.
+  `Issue 68702 <https://github.com/llvm/llvm-project/issues/68702>`_
 
 
 OpenACC Specific Changes

clang/lib/AST/ExprConstant.cpp

@@ -15754,10 +15754,22 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
     LValue LVal;
     LVal.set(VD);
 
-    if (!EvaluateInPlace(Value, Info, LVal, this,
-                         /*AllowNonLiteralTypes=*/true) ||
-        EStatus.HasSideEffects)
-      return false;
+    {
+      // C++23 [intro.execution]/p5
+      // A full-expression is ... an init-declarator ([dcl.decl]) or a
+      // mem-initializer.
+      // So we need to make sure temporary objects are destroyed after having
+      // evaluated the expression (per C++23 [class.temporary]/p4).
+      //
+      // FIXME: Otherwise this may break test/Modules/pr68702.cpp because the
+      // serialization code calls ParmVarDecl::getDefaultArg() which strips the
+      // outermost FullExpr, such as ExprWithCleanups.
+      FullExpressionRAII Scope(Info);
+      if (!EvaluateInPlace(Value, Info, LVal, this,
+                           /*AllowNonLiteralTypes=*/true) ||
+          EStatus.HasSideEffects)
+        return false;
+    }
 
     // At this point, any lifetime-extended temporaries are completely
     // initialized.

clang/test/Modules/pr68702.cpp

@@ -0,0 +1,65 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %t/main.cpp -o %t/main.o
+
+//--- V.h
+#ifndef V_H
+#define V_H
+
+class A {
+public:
+  constexpr A() { }
+  constexpr ~A() { }
+};
+
+template <typename T>
+class V {
+public:
+  V() = default;
+
+  constexpr V(int n, const A& a = A()) {}
+};
+
+#endif
+
+//--- inst1.h
+#include "V.h"
+
+static void inst1() {
+  V<int> v;
+}
+
+//--- inst2.h
+#include "V.h"
+
+static void inst2() {
+  V<int> v(100);
+}
+
+//--- module.modulemap
+module "M" {
+  export *
+  module "V.h" {
+    export *
+    header "V.h"
+  }
+  module "inst1.h" {
+    export *
+    header "inst1.h"
+  }
+}
+
+module "inst2.h" {
+  export *
+  header "inst2.h"
+}
+
+//--- main.cpp
+#include "V.h"
+#include "inst2.h"
+
+static void m() {
+  static V<int> v(100);
+}