[clang] [clang][ExprConst] Can't be past an invalid LValue designator (PR #84293)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/84293

For the test case in C, both `LV.getLValueOffset()` and `Ctx.getTypeSizeInChars(Ty)` are zero, so we return `true` from `isOnePastTheEndOfCompleteObject()` and ultimately diagnose this as being one past the end, but the diagnostic doesn't make sense.

>From e393a6a7ae875eab1154762b1d5bb673adfb9f2b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Thu, 7 Mar 2024 10:14:03 +0100
Subject: [PATCH] [clang][ExprConst] Can't be past an invalid LValue designator

---
 clang/lib/AST/ExprConstant.cpp        |  7 ++++++-
 clang/test/Sema/const-eval.c          |  3 +--
 clang/test/Sema/constexpr-void-cast.c | 14 ++++++++++++++
 3 files changed, 21 insertions(+), 3 deletions(-)
 create mode 100644 clang/test/Sema/constexpr-void-cast.c

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index d8ca35740fbc35..bc8de9d08542cd 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9211,7 +9211,8 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
            Info.getLangOpts().CPlusPlus26)) {
         // Permitted.
       } else {
-        if (SubExpr->getType()->isVoidPointerType()) {
+        if (SubExpr->getType()->isVoidPointerType() &&
+            Info.getLangOpts().CPlusPlus) {
           if (HasValidResult)
             CCEDiag(E, diag::note_constexpr_invalid_void_star_cast)
                 << SubExpr->getType() << Info.getLangOpts().CPlusPlus26
@@ -12899,6 +12900,10 @@ static bool isOnePastTheEndOfCompleteObject(const ASTContext &Ctx,
   if (Ty->isIncompleteType())
     return true;
 
+  // Can't be past the end of an invalid object.
+  if (LV.getLValueDesignator().Invalid)
+    return false;
+
   // We're a past-the-end pointer if we point to the byte after the object,
   // no matter what our type or path is.
   auto Size = Ctx.getTypeSizeInChars(Ty);
diff --git a/clang/test/Sema/const-eval.c b/clang/test/Sema/const-eval.c
index 2e38d5e23c208a..e358aceaad5a43 100644
--- a/clang/test/Sema/const-eval.c
+++ b/clang/test/Sema/const-eval.c
@@ -134,8 +134,7 @@ void PR21945(void) { int i = (({}), 0l); }
 
 void PR24622(void);
 struct PR24622 {} pr24622;
-EVAL_EXPR(52, &pr24622 == (void *)&PR24622); // expected-error {{not an integer constant expression}}
-                                             // expected-note at -1 {{past the end}}
+EVAL_EXPR(52, &pr24622 == (void *)&PR24622);
 
 // We evaluate these by providing 2s' complement semantics in constant
 // expressions, like we do for integers.
diff --git a/clang/test/Sema/constexpr-void-cast.c b/clang/test/Sema/constexpr-void-cast.c
new file mode 100644
index 00000000000000..c5caa3b9e58feb
--- /dev/null
+++ b/clang/test/Sema/constexpr-void-cast.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -x c -fsyntax-only %s -verify=c
+// RUN: %clang_cc1 -x c -fsyntax-only %s -pedantic -verify=c-pedantic
+//
+// RUN: %clang_cc1 -x c++ -fsyntax-only %s -verify=cxx
+// RUN: %clang_cc1 -x c++ -fsyntax-only %s -pedantic -verify=cxx-pedantic
+
+// c-no-diagnostics
+// cxx-no-diagnostics
+
+void f(void);
+struct S {char c;} s;
+_Static_assert(&s != (void *)&f, ""); // c-pedantic-warning {{not an integer constant expression}} \
+                                      // c-pedantic-note {{this conversion is not allowed in a constant expression}} \
+                                      // cxx-pedantic-warning {{'_Static_assert' is a C11 extension}}