[clang] Consider aggregate bases when checking if an InitListExpr is constant (PR #80519)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Reid Kleckner (rnk)

<details>
<summary>Changes</summary>

This code was correct as written prior to C++17, which allowed bases to appear in the initializer list.

Clang currently requires compound literal initializers at file scope to be constants, which is how I tested this behavior change, but I am open to other testing ideas.

This fixes at least one part of issue #<!-- -->80510 .

---
Full diff: https://github.com/llvm/llvm-project/pull/80519.diff


2 Files Affected:

- (modified) clang/lib/AST/Expr.cpp (+12) 
- (modified) clang/test/SemaCXX/compound-literal.cpp (+20) 


``````````diff
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index d665a08deb47e..8852fadf79b9a 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -3342,6 +3342,18 @@ bool Expr::isConstantInitializer(ASTContext &Ctx, bool IsForRef,
     if (ILE->getType()->isRecordType()) {
       unsigned ElementNo = 0;
       RecordDecl *RD = ILE->getType()->castAs<RecordType>()->getDecl();
+
+      // Check bases for C++17 aggregate initializers.
+      if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+        for (unsigned i = 0, e = CXXRD->getNumBases(); i < e; i++) {
+          if (ElementNo < ILE->getNumInits()) {
+            const Expr *Elt = ILE->getInit(ElementNo++);
+            if (!Elt->isConstantInitializer(Ctx, false, Culprit))
+              return false;
+          }
+        }
+      }
+
       for (const auto *Field : RD->fields()) {
         // If this is a union, skip all the fields that aren't being initialized.
         if (RD->isUnion() && ILE->getInitializedFieldInUnion() != Field)
diff --git a/clang/test/SemaCXX/compound-literal.cpp b/clang/test/SemaCXX/compound-literal.cpp
index 5957099de53af..81f8b41ff0313 100644
--- a/clang/test/SemaCXX/compound-literal.cpp
+++ b/clang/test/SemaCXX/compound-literal.cpp
@@ -3,6 +3,7 @@
 // RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify -ast-dump %s > %t-11
 // RUN: FileCheck --input-file=%t-11 %s
 // RUN: FileCheck --input-file=%t-11 %s --check-prefix=CHECK-CXX11
+// RUN: %clang_cc1 -verify -std=c++17 %s
 
 // http://llvm.org/PR7905
 namespace PR7905 {
@@ -108,3 +109,22 @@ int computed_with_lambda = [] {
   return result;
 }();
 #endif
+
+#if __cplusplus >= 201703L
+namespace DynamicFileScopeLiteral {
+// This covers the case where we have a file-scope compound literal with a
+// non-constant initializer in C++. Previously, we had a bug where Clang forgot
+// to consider initializer list elements for bases.
+struct Empty {};
+struct Foo : Empty {
+  int x;
+  int y;
+};
+int f();
+Foo o = (Foo){
+  {},
+  1,
+  f() // expected-error {{initializer element is not a compile-time constant}}
+};
+}
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/80519