EricWF created this revision. EricWF added reviewers: rsmith, hans, sbenza. Herald added a subscriber: kristina. Herald added a project: clang.
The following code is accepted by Clang 7 and prior but rejected by the upcoming 8 release and in trunk [1] // error {{never produces a constant expression}} void foo(const char* s) __attribute__((enable_if(__builtin_constant_p(*s) == false, "trap"))) {} void test() { foo("abc"); } Prior to Clang 8, the call to `__builtin_constant_p` was a constant expression returning false. Currently, it's not a valid constant expression. The bug is caused because we failed to set `InConstantContext` when attempting to evaluate unevaluated constant expressions. [1] https://godbolt.org/z/ksAjmq Repository: rC Clang https://reviews.llvm.org/D59038 Files: lib/AST/ExprConstant.cpp test/SemaCXX/constant-expression-cxx1y.cpp test/SemaCXX/enable_if.cpp Index: test/SemaCXX/enable_if.cpp =================================================================== --- test/SemaCXX/enable_if.cpp +++ test/SemaCXX/enable_if.cpp @@ -514,3 +514,11 @@ static_assert(is_same<__typeof__(foo)*, decltype(&foo)>::value, ""); } + +namespace InConstantContext { +void foo(const char *s) __attribute__((enable_if(((void)__builtin_constant_p(*s), true), "trap"))) {} + +void test() { + InConstantContext::foo("abc"); +} +} // namespace InConstantContext Index: test/SemaCXX/constant-expression-cxx1y.cpp =================================================================== --- test/SemaCXX/constant-expression-cxx1y.cpp +++ test/SemaCXX/constant-expression-cxx1y.cpp @@ -1135,3 +1135,7 @@ return __builtin_constant_p(*__s); } constexpr bool n = indirect_builtin_constant_p("a"); + +__attribute__((enable_if(indirect_builtin_constant_p("a") == n, "OK"))) +int test_in_enable_if() { return 0; } +int n2 = test_in_enable_if(); Index: lib/AST/ExprConstant.cpp =================================================================== --- lib/AST/ExprConstant.cpp +++ lib/AST/ExprConstant.cpp @@ -11767,6 +11767,7 @@ const Expr *This) const { Expr::EvalStatus Status; EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantExpressionUnevaluated); + Info.InConstantContext = true; LValue ThisVal; const LValue *ThisPtr = nullptr; @@ -11850,6 +11851,7 @@ EvalInfo Info(FD->getASTContext(), Status, EvalInfo::EM_PotentialConstantExpressionUnevaluated); + Info.InConstantContext = true; // Fabricate a call stack frame to give the arguments a plausible cover story. ArrayRef<const Expr*> Args;
Index: test/SemaCXX/enable_if.cpp =================================================================== --- test/SemaCXX/enable_if.cpp +++ test/SemaCXX/enable_if.cpp @@ -514,3 +514,11 @@ static_assert(is_same<__typeof__(foo)*, decltype(&foo)>::value, ""); } + +namespace InConstantContext { +void foo(const char *s) __attribute__((enable_if(((void)__builtin_constant_p(*s), true), "trap"))) {} + +void test() { + InConstantContext::foo("abc"); +} +} // namespace InConstantContext Index: test/SemaCXX/constant-expression-cxx1y.cpp =================================================================== --- test/SemaCXX/constant-expression-cxx1y.cpp +++ test/SemaCXX/constant-expression-cxx1y.cpp @@ -1135,3 +1135,7 @@ return __builtin_constant_p(*__s); } constexpr bool n = indirect_builtin_constant_p("a"); + +__attribute__((enable_if(indirect_builtin_constant_p("a") == n, "OK"))) +int test_in_enable_if() { return 0; } +int n2 = test_in_enable_if(); Index: lib/AST/ExprConstant.cpp =================================================================== --- lib/AST/ExprConstant.cpp +++ lib/AST/ExprConstant.cpp @@ -11767,6 +11767,7 @@ const Expr *This) const { Expr::EvalStatus Status; EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantExpressionUnevaluated); + Info.InConstantContext = true; LValue ThisVal; const LValue *ThisPtr = nullptr; @@ -11850,6 +11851,7 @@ EvalInfo Info(FD->getASTContext(), Status, EvalInfo::EM_PotentialConstantExpressionUnevaluated); + Info.InConstantContext = true; // Fabricate a call stack frame to give the arguments a plausible cover story. ArrayRef<const Expr*> Args;
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