[clang] [Clang] use constant evaluation context for constexpr if conditions (PR #123667)

[Oleksandr T. via cfe-commits]

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@@ -2203,6 +2203,12 @@ Parser::ParseCXXCondition(StmtResult *InitStmt, SourceLocation Loc,
       return ParseCXXCondition(nullptr, Loc, CK, MissingOK);
     }
 
+    EnterExpressionEvaluationContext Eval(
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a-tarasyuk wrote:

This context will be applied only for `ConstexprIf`. It uses the `ShouldEnter` flag to control the pushing of the extra context, all other kinds will be ignored

https://github.com/llvm/llvm-project/blob/e1c1e74a6fd71dd889155100d4c0f5e3284f7a22/clang/include/clang/Sema/EnterExpressionEvaluationContext.h#L29-L33

https://github.com/llvm/llvm-project/pull/123667