[clang-tidy] Ignore unevaluated context in cppcoreguidelines-pro-type-vararg

[PiotrZSL]

Summary

Ignore decltype, sizeof, alignof in this check.

Fixes: #30542

clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeVarargCheck.cpp

 //===--- ProTypeVarargCheck.cpp - clang-tidy-------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "ProTypeVarargCheck.h"
+#include "../utils/Matchers.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/PPCallbacks.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/Token.h"
 

 };
 } // namespace
 
 void ProTypeVarargCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(VAArgExpr().bind("va_use"), this);
 
   Finder->addMatcher(
       callExpr(callee(functionDecl(isVariadic(),
-                                   unless(hasAnyName(AllowedVariadics)))))
+                                   unless(hasAnyName(AllowedVariadics)))),
+               unless(hasAncestor(expr(matchers::hasUnevaluatedContext()))),
+               unless(hasAncestor(typeLoc())))
           .bind("callvararg"),
       this);
 
   Finder->addMatcher(
       varDecl(unless(parmVarDecl()),
               hasType(qualType(
                   anyOf(isVAList(), decayedType(hasOriginalType(isVAList()))))))
           .bind("va_list"),

clang-tools-extra/docs/ReleaseNotes.rst

- Improved :doc:`cppcoreguidelines-avoid-non-const-global-variables
   <clang-tidy/checks/cppcoreguidelines/avoid-non-const-global-variables>` check
   to ignore ``static`` variables declared within the scope of
   ``class``/``struct``.
 
 - Improved :doc:`cppcoreguidelines-prefer-member-initializer
   <clang-tidy/checks/cppcoreguidelines/prefer-member-initializer>` check to
   ignore delegate constructors.
 
+- Improved :doc:`cppcoreguidelines-pro-type-vararg
+  <clang-tidy/checks/cppcoreguidelines/pro-type-vararg>` check to ignore
+  false-positives in unevaluated context (e.g., ``decltype``, ``sizeof``, ...).
+
 - Improved :doc:`llvm-namespace-comment
   <clang-tidy/checks/llvm/namespace-comment>` check to provide fixes for
   ``inline`` namespaces in the same format as :program:`clang-format`.
 
 - Improved :doc:`misc-include-cleaner
   <clang-tidy/checks/misc/include-cleaner>` check by adding option
   `DeduplicateFindings` to output one finding per symbol occurence.
 

clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/pro-type-vararg.rst

 .. title:: clang-tidy - cppcoreguidelines-pro-type-vararg
 
 cppcoreguidelines-pro-type-vararg
 =================================
 
 This check flags all calls to c-style vararg functions and all use of
 ``va_arg``.
 
 To allow for SFINAE use of vararg functions, a call is not flagged if a literal
-0 is passed as the only vararg argument.
+0 is passed as the only vararg argument or function is used in unevaluated
+context.
 
 Passing to varargs assumes the correct type will be read. This is fragile
 because it cannot generally be enforced to be safe in the language and so relies
 on programmer discipline to get it right.
 
 This rule is part of the `Type safety (Type.8)
 <https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Pro-type-varargs>`_
 profile from the C++ Core Guidelines.

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/pro-type-vararg.cpp

 
 // Some implementations of __builtin_va_list and __builtin_ms_va_list desugared
 // as 'char *' or 'void *'. This test checks whether we are handling this case
 // correctly and not generating false positives.
 void no_false_positive_desugar_va_list(char *in) {
   char *tmp1 = in;
   void *tmp2 = in;
 }
+
+namespace PR30542 {
+  struct X;
+  template <typename T>
+  char IsNullConstant(X*);
+  template <typename T>
+  char (&IsNullConstant(...))[2];
+
+  static_assert(sizeof(IsNullConstant<int>(0)) == 1, "");
+  static_assert(sizeof(IsNullConstant<int>(17)) == 2, "");
+
+  using Type = decltype(IsNullConstant<int>(17));
+}