[clang][ExprConst] Can't be past an invalid LValue designator

[tbaederr]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/84293.diff

3 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+6-1)
• (modified) clang/test/Sema/const-eval.c (+1-2)
• (added) clang/test/Sema/constexpr-void-cast.c (+14)

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@-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}}

[AaronBallman]

Precommit CI seems to have found relevant failures

[tbaederr]

Ping

[AaronBallman]

LGTM