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

[jongmyeong-choi]

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

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Jongmyeong Choi (jongmyeong-choi)

Changes

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 --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@-1 {{explicit(bool) is a C++20 extension}}
+  
+  explicit(false) S(float);
+  // expected-warning@-1 {{explicit(bool) is a C++20 extension}}
+};
\ No newline at end of file

[AaronBallman]

Thank you for the fix!

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[mizvekov]

LGTM sans small nit.

FYI the change is correct as far as if we want to allow this as an extension.

But we otherwise don't produce an extension warning here.

Usually when we allow something as an extension to earlier standards, this warning is added, which users can also either ignore it or turn it into an error, and is helpful for people who want strict standard conformance (ie want to make sure this code will compile in other compilers).

[shafik]

This need a release note in clang/docs/ReleaseNotes.rst. This goes back to clang-10.

[cor3ntin]

Thank. Do you need us to merge that for you?

[jongmyeong-choi]

Thank. Do you need us to merge that for you?

Oh.. could you please do that? I don't know what to do because the workflows awaiting.

[erichkeane]

Looks like the documentation failure is related to some OMP documentation, so just merging this.

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