[llvm-branch-commits] [clang] release/20.x: [Clang][AST] Fix HandleLValueBase to deal with references (#140105) (PR #140246)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: A. Jiang (frederick-vs-ja)

<details>
<summary>Changes</summary>

Since P2280R4 Unknown references and pointers was implemented, HandleLValueBase now has to deal with referneces:

D.MostDerivedType->getAsCXXRecordDecl()

will return a nullptr if D.MostDerivedType is a ReferenceType. The fix is to use getNonReferenceType() to obtain the Pointee Type if we have a reference.

(cherry picked from commit 136f2ba2a7bca015ef831c91fb0db5e5e31b7632)

---
Full diff: https://github.com/llvm/llvm-project/pull/140246.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/AST/ExprConstant.cpp (+5-1) 
- (modified) clang/test/SemaCXX/constant-expression-p2280r4.cpp (+21) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 47ef2f80ac3f2..2f43dc4021fd8 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -909,6 +909,8 @@ Bug Fixes in This Version
   being deleted has a potentially throwing destructor (#GH118660).
 - Clang now outputs correct values when #embed data contains bytes with negative
   signed char values (#GH102798).
+- Fix crash due to unknown references and pointer implementation and handling of
+  base classes. (GH139452)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 23602362eaa79..e0746f4532245 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -3311,7 +3311,11 @@ static bool HandleLValueBase(EvalInfo &Info, const Expr *E, LValue &Obj,
     return false;
 
   // Extract most-derived object and corresponding type.
-  DerivedDecl = D.MostDerivedType->getAsCXXRecordDecl();
+  // FIXME: After implementing P2280R4 it became possible to get references
+  // here. We do MostDerivedType->getAsCXXRecordDecl() in several other
+  // locations and if we see crashes in those locations in the future
+  // it may make more sense to move this fix into Lvalue::set.
+  DerivedDecl = D.MostDerivedType.getNonReferenceType()->getAsCXXRecordDecl();
   if (!CastToDerivedClass(Info, E, Obj, DerivedDecl, D.MostDerivedPathLength))
     return false;
 
diff --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index 6c9a87267109c..87beeb4d3dc84 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -179,3 +179,24 @@ namespace extern_reference_used_as_unknown {
   int y;
   constinit int& g = (x,y); // expected-warning {{left operand of comma operator has no effect}}
 }
+
+namespace GH139452 {
+struct Dummy {
+  explicit operator bool() const noexcept { return true; }
+};
+
+struct Base { int error; };
+struct Derived : virtual Base { };
+
+template <class R>
+constexpr R get_value() {
+    const auto& derived_val = Derived{};
+    if (derived_val.error != 0)
+        /* nothing */;
+    return R{};
+}
+
+int f() {
+    return !get_value<Dummy>(); // contextually convert the function call result to bool
+}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/140246