[clang] Fix C23 constexpr crashes

[Fznamznon]

Before using a constexpr variable that is not properly initialized check that it is valid.

Fixes #109095
Fixes #112516

[llvmbot]

@llvm/pr-subscribers-clang

Author: Mariya Podchishchaeva (Fznamznon)

Changes

Before using a constexpr variable that is not properly initialized check that it is valid.

Fixes #109095 Fixes #112516

---

Full diff: https://github.com/llvm/llvm-project/pull/112708.diff

2 Files Affected:

• (modified) clang/lib/AST/Decl.cpp (+4-2)
• (modified) clang/test/Sema/constexpr.c (+17)

diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index f083ffff87a8ec..c642fcf8785151 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -2512,7 +2512,8 @@ bool VarDecl::isUsableInConstantExpressions(const ASTContext &Context) const {
   if (!DefVD->mightBeUsableInConstantExpressions(Context))
     return false;
   //   ... and its initializer is a constant initializer.
-  if (Context.getLangOpts().CPlusPlus && !DefVD->hasConstantInitialization())
+  if ((Context.getLangOpts().CPlusPlus || getLangOpts().C23) &&
+      !DefVD->hasConstantInitialization())
     return false;
   // C++98 [expr.const]p1:
   //   An integral constant-expression can involve only [...] const variables
@@ -2620,7 +2621,8 @@ bool VarDecl::hasICEInitializer(const ASTContext &Context) const {
 
 bool VarDecl::hasConstantInitialization() const {
   // In C, all globals (and only globals) have constant initialization.
-  if (hasGlobalStorage() && !getASTContext().getLangOpts().CPlusPlus)
+  if (hasGlobalStorage() && !getASTContext().getLangOpts().CPlusPlus &&
+      !isConstexpr())
     return true;
 
   // In C++, it depends on whether the evaluation at the point of definition
diff --git a/clang/test/Sema/constexpr.c b/clang/test/Sema/constexpr.c
index eaa000b3b97758..3dcb0b3a7d95fd 100644
--- a/clang/test/Sema/constexpr.c
+++ b/clang/test/Sema/constexpr.c
@@ -374,3 +374,20 @@ void constexprif() {
 void constevalif() {
   if consteval (300) {} //expected-error {{expected '(' after 'if'}}
 }
+
+struct S11 {
+  int len;
+};
+void ghissue112516() {
+  struct S11 *s11 = 0;
+  constexpr int num = s11->len; // expected-error {{constexpr variable 'num' must be initialized by a constant expression}}
+  void *Arr[num];
+}
+
+void ghissue109095() {
+  constexpr char c[] = { 'a' };
+  constexpr int i = c[1]; // expected-error {{constexpr variable 'i' must be initialized by a constant expression}}\
+                          // expected-note {{declared here}}
+  _Static_assert(i == c[0]); // expected-error {{static assertion expression is not an integral constant expression}}\
+                             // expected-note {{initializer of 'i' is not a constant expression}}
+}

[AaronBallman]

These changes seem reasonable to me, and likely worth backporting (CC @tbaederr for a second set of eyes just to be sure)

[tbaederr]

Agreed, LGTM. The first comment in VarDecl::hasConstantInitialization() could use some love, since it's no longer true for global constexpr variables.

[shafik]

LGTM

[Fznamznon]

/cherry-pick abfba7d

[llvmbot]

Failed to cherry-pick: abfba7d

https://github.com/llvm/llvm-project/actions/runs/11400407910

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[Fznamznon]

Release branch cherry-pick #112855