[Diagnostics] Warn for std::is_constant_evaluated in constexpr mode

[xbolva00]

Summary

constexpr int fn1() {

if constexpr (std::is_constant_evaluated()) // condition is always true!
  return 0;
else
  return 1;

}

constexpr int fn2() {

if (std::is_constant_evaluated())
  return 0;
else
  return 1;

}

Solves PR42977

[xbolva00]

clang/lib/Sema/SemaExprCXX.cpp

3695

Not ideal place. Missing !E case, etc...

Maybe in ActOnCompletedExpr?

[xbolva00]

Support !E.

[xbolva00]

New tests

[rsmith]

clang/include/clang/Basic/DiagnosticSemaKinds.td

8410

"constexpr mode" isn't a thing. Maybe just "in this context"?

I think this should have its own warning group, not be grouped under -Wtautological-compare.

clang/lib/Sema/SemaExprCXX.cpp

3694

As noted in PR42977, we should warn whenever we find a call to is_constant_evaluated in a manifestly constant evaluated expression, not only in the condition of a constexpr if.

This can be checked in ExprConstant.cpp when we evaluate a call to __builtin_is_constant_evaluated: if we're about to return true from __builtin_is_constant_evaluated, and there's either no calls on the call stack or exactly one call which is to std::is_constant_evaluated, then warn.

clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp

3

This second RUN line doesn't add any value; remove it?

[xbolva00]

clang/include/clang/Basic/DiagnosticSemaKinds.td

8410

I agree. just... gcc put it under -Wtautological-compare. So should I create a new subgroup of -Wtautological-compare? Or just a new group?

clang/lib/Sema/SemaExprCXX.cpp

3694

Yeah, thanks for the hint!

clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp

3

Right, I will remove it.

[rsmith]

clang/include/clang/Basic/DiagnosticSemaKinds.td

8410

I don't really imagine many people wanting to turn this warning (or -Wtautological-compare) off, so I don't think it matters all that much. But making this a subgroup of -Wtautological-compare is cheap and would preserve GCC compatibility, so let's do that.

[xbolva00]

• Merge branch 'master' of https://github.com/llvm/llvm-project

[xbolva00]

Removed old unused change.

[xbolva00]

clang/lib/AST/ExprConstant.cpp

10595 ↗

(On Diff #227192)

!Info.CheckingPotentialConstantExpression

Basically just to silence test case:
namespace std {
constexpr bool is_constant_evaluated() noexcept {

return __builtin_is_constant_evaluated();

}
}

In real world code, this will not warn, since call's loc is in system header. Leave it?

10600 ↗

(On Diff #227192)

Without this condition, it warns 3 times for

if constexpr (std::is_constant_evaluated() == false)

[rsmith]

clang/include/clang/Basic/DiagnosticASTKinds.td

333 ↗

(On Diff #227192)

"constexpr context" doesn't mean anything, and I'm not sure it has the right implications (eg, is the body of a constexpr function a constexpr context?).

The technically-correct thing would be "in a manifestly constant-evaluated expression".

334 ↗

(On Diff #227192)

Looks like you decided not to make this a subgroup of tautological-compare? (I'm OK with that, I just wanted to make sure this was intentional given the prior discussion.)

clang/lib/AST/ExprConstant.cpp

10595 ↗

(On Diff #227192)

The check seems right to me.

10597 ↗

(On Diff #227192)

The number of arguments check is redundant; we know the builtin has no parameters.

10600 ↗

(On Diff #227192)

Please add a "// FIXME: Find a better way to avoid duplicated diagnostics." or similar. This is relying somewhat on the implementation details of callers of the constant evaluator.

[xbolva00]

clang/include/clang/Basic/DiagnosticASTKinds.td

334 ↗

(On Diff #227192)

Yes, I decided to go with a new group since the warning is more powerful than gcc's version of this warning.

The technically-correct thing would be "in a manifestly constant-evaluated expression".

I will use this message - Thanks!

[xbolva00]

Fixed review comments.

[xbolva00]

Thanks you for the review and advices!

[davezarzycki]

This broke two stage builds of libc++. Can we revert this until a fix is ready?

FAIL: libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp (55845 of 59103)
******************** TEST 'libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp' FAILED ********************
Command: ['/p/tllvm/bin/clang++', '-o', '/dev/null', '-x', 'c++', '/home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp', '-c', '-v', '-ftemplate-depth=270', '-fsyntax-only', '-Xclang', '-verify', '-Xclang', '-verify-ignore-unexpected=note', '-ferror-limit=1024', '-Werror=thread-safety', '-std=c++2a', '-include', '/home/dave/s/lp/libcxx/test/support/nasty_macros.h', '-nostdinc++', '-I/home/dave/s/lp/libcxx/include', '-I/tmp/_update_lc/t/projects/libcxx/include/c++build', '-D__STDC_FORMAT_MACROS', '-D__STDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS', '-I/home/dave/s/lp/libcxx/test/support', '-DLIBCXX_FILESYSTEM_STATIC_TEST_ROOT="/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT="/tmp/_update_lc/t/projects/libcxx/test/filesystem/Output/dynamic_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER="/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py"', '-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER', '-Wall', '-Wextra', '-Werror', '-Wuser-defined-warnings', '-Wshadow', '-Wno-unused-command-line-argument', '-Wno-attributes', '-Wno-pessimizing-move', '-Wno-c++11-extensions', '-Wno-user-defined-literals', '-Wno-noexcept-type', '-Wsign-compare', '-Wunused-variable', '-Wunused-parameter', '-Wunreachable-code', '-Wno-error=user-defined-warnings', '-c']
Exit Code: 1
Standard Error:
--
clang version 10.0.0 (/home/dave/s/lp/clang 7e1a3076419d4d453d71143a1e81409ea1db177c)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /p/tllvm/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
 "/p/tllvm/bin/clang-10" -cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -disable-free -disable-llvm-verifier -discard-value-names -main-file-name is_constant_evaluated.fail.cpp -mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -nostdinc++ -resource-dir /p/tllvm/lib64/clang/10.0.0 -include /home/dave/s/lp/libcxx/test/support/nasty_macros.h -I /home/dave/s/lp/libcxx/include -I /tmp/_update_lc/t/projects/libcxx/include/c++build -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -D __STDC_CONSTANT_MACROS -I /home/dave/s/lp/libcxx/test/support -D "LIBCXX_FILESYSTEM_STATIC_TEST_ROOT=\"/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT=\"/tmp/_update_lc/t/projects/libcxx/test/filesystem/Output/dynamic_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER=\"/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py\"" -D _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -internal-isystem /usr/local/include -internal-isystem /p/tllvm/lib64/clang/10.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -Werror=thread-safety -Wall -Wextra -Werror -Wuser-defined-warnings -Wshadow -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-c++11-extensions -Wno-user-defined-literals -Wno-noexcept-type -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-error=user-defined-warnings -std=c++2a -fdeprecated-macro -fdebug-compilation-dir /tmp/_update_lc/t -ftemplate-depth 270 -ferror-limit 1024 -fmessage-length 0 -fgnuc-version=4.2.1 -fno-implicit-modules -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -verify -verify-ignore-unexpected=note -faddrsig -x c++ /home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp
clang -cc1 version 10.0.0 based upon LLVM 10.0.0svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/dave/s/lp/libcxx/include
 /tmp/_update_lc/t/projects/libcxx/include/c++build
 /home/dave/s/lp/libcxx/test/support
 /usr/local/include
 /p/tllvm/lib64/clang/10.0.0/include
 /usr/include
End of search list.
error: 'error' diagnostics seen but not expected: 
  File /home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp Line 26: 'std::is_constant_evaluated' will always evaluate to 'true' in a manifestly constant-evaluated expression
1 error generated.
--

Expected compilation using verify to pass!

********************
FAIL: libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp (55846 of 59103)
******************** TEST 'libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp' FAILED ********************
Command: ['/p/tllvm/bin/clang++', '-o', '/tmp/_update_lc/t/projects/libcxx/test/std/utilities/meta/meta.const.eval/Output/is_constant_evaluated.pass.cpp.o', '-x', 'c++', '/home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp', '-c', '-v', '-ftemplate-depth=270', '-Werror=thread-safety', '-std=c++2a', '-include', '/home/dave/s/lp/libcxx/test/support/nasty_macros.h', '-nostdinc++', '-I/home/dave/s/lp/libcxx/include', '-I/tmp/_update_lc/t/projects/libcxx/include/c++build', '-D__STDC_FORMAT_MACROS', '-D__STDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS', '-I/home/dave/s/lp/libcxx/test/support', '-DLIBCXX_FILESYSTEM_STATIC_TEST_ROOT="/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT="/tmp/_update_lc/t/projects/libcxx/test/filesystem/Output/dynamic_env"', '-DLIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER="/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py"', '-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER', '-Wall', '-Wextra', '-Werror', '-Wuser-defined-warnings', '-Wshadow', '-Wno-unused-command-line-argument', '-Wno-attributes', '-Wno-pessimizing-move', '-Wno-c++11-extensions', '-Wno-user-defined-literals', '-Wno-noexcept-type', '-Wsign-compare', '-Wunused-variable', '-Wunused-parameter', '-Wunreachable-code', '-c']
Exit Code: 1
Standard Error:
--
clang version 10.0.0 (/home/dave/s/lp/clang 7e1a3076419d4d453d71143a1e81409ea1db177c)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /p/tllvm/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/9
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
 "/p/tllvm/bin/clang-10" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name is_constant_evaluated.pass.cpp -mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -nostdinc++ -resource-dir /p/tllvm/lib64/clang/10.0.0 -include /home/dave/s/lp/libcxx/test/support/nasty_macros.h -I /home/dave/s/lp/libcxx/include -I /tmp/_update_lc/t/projects/libcxx/include/c++build -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -D __STDC_CONSTANT_MACROS -I /home/dave/s/lp/libcxx/test/support -D "LIBCXX_FILESYSTEM_STATIC_TEST_ROOT=\"/home/dave/s/lp/libcxx/test/std/input.output/filesystems/Inputs/static_test_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_ROOT=\"/tmp/_update_lc/t/projects/libcxx/test/filesystem/Output/dynamic_env\"" -D "LIBCXX_FILESYSTEM_DYNAMIC_TEST_HELPER=\"/usr/bin/python /home/dave/s/lp/libcxx/test/support/filesystem_dynamic_test_helper.py\"" -D _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -internal-isystem /usr/local/include -internal-isystem /p/tllvm/lib64/clang/10.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -Werror=thread-safety -Wall -Wextra -Werror -Wuser-defined-warnings -Wshadow -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-c++11-extensions -Wno-user-defined-literals -Wno-noexcept-type -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -std=c++2a -fdeprecated-macro -fdebug-compilation-dir /tmp/_update_lc/t/projects/libcxx/test/std/utilities/meta/meta.const.eval -ftemplate-depth 270 -ferror-limit 19 -fmessage-length 0 -fgnuc-version=4.2.1 -fno-implicit-modules -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -faddrsig -o /tmp/_update_lc/t/projects/libcxx/test/std/utilities/meta/meta.const.eval/Output/is_constant_evaluated.pass.cpp.o -x c++ /home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp
clang -cc1 version 10.0.0 based upon LLVM 10.0.0svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/dave/s/lp/libcxx/include
 /tmp/_update_lc/t/projects/libcxx/include/c++build
 /home/dave/s/lp/libcxx/test/support
 /usr/local/include
 /p/tllvm/lib64/clang/10.0.0/include
 /usr/include
End of search list.
/home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp:35:24: error: 'std::is_constant_evaluated' will always evaluate to 'true' in a manifestly constant-evaluated expression [-Werror,-Wconstant-evaluated]
    constexpr bool p = std::is_constant_evaluated();
                       ^
/home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp:41:19: error: 'std::is_constant_evaluated' will always evaluate to 'true' in a manifestly constant-evaluated expression [-Werror,-Wconstant-evaluated]
    static_assert(std::is_constant_evaluated(), "");
                  ^
/home/dave/s/lp/libcxx/test/std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp:44:33: error: 'std::is_constant_evaluated' will always evaluate to 'true' in a manifestly constant-evaluated expression [-Werror,-Wconstant-evaluated]
    ASSERT_SAME_TYPE(InTemplate<std::is_constant_evaluated()>, InTemplate<true>);
                                ^
3 errors generated.
--

Compilation failed unexpectedly!
********************
Testing Time: 111.41s
********************
Failing Tests (2):
    libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.fail.cpp
    libc++ :: std/utilities/meta/meta.const.eval/is_constant_evaluated.pass.cpp

  Expected Passes    : 43344
  Expected Failures  : 103
  Unsupported Tests  : 15654
  Unexpected Failures: 2

[xbolva00]

Line 918

Add:
self.cxx.addWarningFlagIfSupported('-Wno-constant-evaluated')

to libcxx/utils/libcxx/test/config.py

Can you please try?

cc @EricWF

[davezarzycki]

Hi @xbolva00 – Thanks. That does make the libcxx test suite pass.

[xbolva00]

Thanks! Commited in rGdba83965722b540f6baf43163210943c41e1378a

clang/include/clang/AST/Expr.h

public:
   bool isBuiltinAssumeFalse(const ASTContext &Ctx) const;
 
   bool isCallToStdMove() const {
     const FunctionDecl *FD = getDirectCallee();
     return getNumArgs() == 1 && FD && FD->isInStdNamespace() &&
            FD->getIdentifier() && FD->getIdentifier()->isStr("move");
   }
 
+  bool isCallToStdIsConstantEvaluated() const {
+    const FunctionDecl *FD = getDirectCallee();
+    return getNumArgs() == 0 && FD && FD->isInStdNamespace() &&
+           FD->getIdentifier() &&
+           FD->getIdentifier()->isStr("is_constant_evaluated");
+  }
+
   static bool classof(const Stmt *T) {
     return T->getStmtClass() >= firstCallExprConstant &&
            T->getStmtClass() <= lastCallExprConstant;
   }
 
   // Iterators
   child_range children() {
     return child_range(getTrailingStmts(), getTrailingStmts() + PREARGS_START +

clang/include/clang/Basic/DiagnosticSemaKinds.td

 
 // For non-floating point, expressions of the form x == x or x != x
 // should result in a warning, since these always evaluate to a constant.
 // Array comparisons have similar warnings
 def warn_comparison_always : Warning<
   "%select{self-|array }0comparison always evaluates to "
   "%select{a constant|true|false|'std::strong_ordering::equal'}1">,
   InGroup<TautologicalCompare>;
+def warn_std_is_constant_evaluated_always_true_constexpr : Warning<
+  "'std::is_constant_evaluated' will always evaluate to "
+  "'true' in constexpr mode">, InGroup<TautologicalCompare>;
 def warn_comparison_bitwise_always : Warning<
   "bitwise comparison always evaluates to %select{false|true}0">,
   InGroup<TautologicalBitwiseCompare>, DefaultIgnore;
 def warn_comparison_bitwise_or : Warning<
   "bitwise or with non-zero value always evaluates to true">,
   InGroup<TautologicalBitwiseCompare>, DefaultIgnore;
 def warn_tautological_overlap_comparison : Warning<
   "overlapping comparisons always evaluate to %select{false|true}0">,

clang/lib/Sema/SemaExprCXX.cpp

ExprResult Sema::CheckConditionVariable(VarDecl *ConditionVar,
 
   case ConditionKind::Switch:
     return CheckSwitchCondition(StmtLoc, Condition.get());
   }
 
   llvm_unreachable("unexpected condition kind");
 }
 
+static void checkStdIsConstantEvaluated(Sema &S, Expr *BoolExpr,
+                                        bool IsConstexpr) {
+  if (!IsConstexpr)
+    return;
+  BoolExpr = BoolExpr->IgnoreParens();
+  CallExpr *Call = nullptr;
+  UnaryOperator *UO = dyn_cast<UnaryOperator>(BoolExpr);
+  if (UO && UO->getOpcode() == UO_LNot)
+    BoolExpr = UO->getSubExpr();
+
+  Call = dyn_cast<CallExpr>(BoolExpr);
+  if (Call && Call->isCallToStdIsConstantEvaluated())
+    S.Diag(Call->getBeginLoc(),
+         diag::warn_std_is_constant_evaluated_always_true_constexpr);
+}
+
 /// CheckCXXBooleanCondition - Returns true if a conversion to bool is invalid.
 ExprResult Sema::CheckCXXBooleanCondition(Expr *CondExpr, bool IsConstexpr) {
   // C++ 6.4p4:
   // The value of a condition that is an initialized declaration in a statement
   // other than a switch statement is the value of the declared variable
   // implicitly converted to type bool. If that conversion is ill-formed, the
   // program is ill-formed.
   // The value of a condition that is an expression is the value of the
   // expression, implicitly converted to bool.
   //
   // FIXME: Return this value to the caller so they don't need to recompute it.
+  checkStdIsConstantEvaluated(*this, CondExpr, IsConstexpr);
   llvm::APSInt Value(/*BitWidth*/1);
   return (IsConstexpr && !CondExpr->isValueDependent())
              ? CheckConvertedConstantExpression(CondExpr, Context.BoolTy, Value,
                                                 CCEK_ConstexprIf)
              : PerformContextuallyConvertToBool(CondExpr);
 }
 
 /// Helper function to determine whether this is the (deprecated) C++
 /// conversion from a string literal to a pointer to non-const char or

clang/test/SemaCXX/warn-std-is-constant-evaluated-constexpr.cpp

+// RUN: %clang_cc1 -std=c++2a -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++2a -fsyntax-only -verify -Wtautological-compare %s
+
+namespace std {
+constexpr bool is_constant_evaluated() noexcept {
+  return __builtin_is_constant_evaluated();
+}
+}
+
+constexpr int fn1() {
+  if constexpr (std::is_constant_evaluated()) // expected-warning {{'std::is_constant_evaluated' will always evaluate to 'true' in constexpr mode}}
+    return 0;
+  else
+    return 1;
+}
+
+constexpr int fn2() {
+  if (std::is_constant_evaluated())
+    return 0;
+  else
+    return 1;
+}