[clang] [clang] Fix assertion failure with explicit(bool) in pre-C++11 modes (PR #152985)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Jongmyeong Choi (jongmyeong-choi)

<details>
<summary>Changes</summary>

Allow CCEKind::ExplicitBool in BuildConvertedConstantExpression for pre-C++11 contexts, similar to the existing TempArgStrict exception. This enables explicit(bool) to work as a C++20 extension in earlier language modes without triggering assertion failures.

Fixes #<!-- -->152729

---
Full diff: https://github.com/llvm/llvm-project/pull/152985.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaOverload.cpp (+3-1) 
- (added) clang/test/Parser/explicit-bool-pre-cxx17.cpp (+14) 


``````````diff
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 5dd5b495480d9..700c330f02427 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -6268,7 +6268,9 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
                                                    QualType T, CCEKind CCE,
                                                    NamedDecl *Dest,
                                                    APValue &PreNarrowingValue) {
-  assert((S.getLangOpts().CPlusPlus11 || CCE == CCEKind::TempArgStrict) &&
+  bool isCCEAllowedPreCXX11 =
+    (CCE == CCEKind::TempArgStrict || CCE == CCEKind::ExplicitBool);
+  assert((S.getLangOpts().CPlusPlus11 || isCCEAllowedPreCXX11) &&
          "converted constant expression outside C++11 or TTP matching");
 
   if (checkPlaceholderForOverload(S, From))
diff --git a/clang/test/Parser/explicit-bool-pre-cxx17.cpp b/clang/test/Parser/explicit-bool-pre-cxx17.cpp
new file mode 100644
index 0000000000000..0a704f3ef85cd
--- /dev/null
+++ b/clang/test/Parser/explicit-bool-pre-cxx17.cpp
@@ -0,0 +1,14 @@
+// Regression test for assertion failure when explicit(bool) is used in pre-C++20
+// Fixes GitHub issue #152729
+// RUN: %clang_cc1 -std=c++03 -verify %s
+// RUN: %clang_cc1 -std=c++11 -verify %s
+// RUN: %clang_cc1 -std=c++14 -verify %s
+// RUN: %clang_cc1 -std=c++17 -verify %s
+
+struct S {
+  explicit(true) S(int);
+  // expected-warning at -1 {{explicit(bool) is a C++20 extension}}
+  
+  explicit(false) S(float);
+  // expected-warning at -1 {{explicit(bool) is a C++20 extension}}
+};
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/152985