r355743 - [8.0 Regression] Fix handling of `__builtin_constant_p` inside template arguments, enumerators, case statements, and the enable_if attribute.

[Hans Wennborg via cfe-commits]

Merged to release_80 in r355898.

On Fri, Mar 8, 2019 at 11:05 PM Eric Fiselier via cfe-commits
<cfe-commits at lists.llvm.org> wrote:
>
> Author: ericwf
> Date: Fri Mar  8 14:06:48 2019
> New Revision: 355743
>
> URL: http://llvm.org/viewvc/llvm-project?rev=355743&view=rev
> Log:
> [8.0 Regression] Fix handling of `__builtin_constant_p` inside template arguments, enumerators, case statements, and the enable_if attribute.
>
> Summary:
> 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
>
> Reviewers: rsmith, hans, sbenza
>
> Reviewed By: rsmith
>
> Subscribers: kristina, cfe-commits
>
> Tags: #clang
>
> Differential Revision: https://reviews.llvm.org/D59038
>
> Modified:
>     cfe/trunk/lib/AST/ExprConstant.cpp
>     cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp
>     cfe/trunk/test/SemaCXX/enable_if.cpp
>
> Modified: cfe/trunk/lib/AST/ExprConstant.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/ExprConstant.cpp?rev=355743&r1=355742&r2=355743&view=diff
> ==============================================================================
> --- cfe/trunk/lib/AST/ExprConstant.cpp (original)
> +++ cfe/trunk/lib/AST/ExprConstant.cpp Fri Mar  8 14:06:48 2019
> @@ -11131,6 +11131,7 @@ bool Expr::EvaluateAsConstantExpr(EvalRe
>                                    const ASTContext &Ctx) const {
>    EvalInfo::EvaluationMode EM = EvalInfo::EM_ConstantExpression;
>    EvalInfo Info(Ctx, Result, EM);
> +  Info.InConstantContext = true;
>    if (!::Evaluate(Result.Val, Info, this))
>      return false;
>
> @@ -11771,6 +11772,7 @@ bool Expr::EvaluateWithSubstitution(APVa
>                                      const Expr *This) const {
>    Expr::EvalStatus Status;
>    EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantExpressionUnevaluated);
> +  Info.InConstantContext = true;
>
>    LValue ThisVal;
>    const LValue *ThisPtr = nullptr;
> @@ -11854,6 +11856,7 @@ bool Expr::isPotentialConstantExprUneval
>
>    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;
>
> Modified: cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp?rev=355743&r1=355742&r2=355743&view=diff
> ==============================================================================
> --- cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp (original)
> +++ cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp Fri Mar  8 14:06:48 2019
> @@ -1135,3 +1135,27 @@ constexpr bool indirect_builtin_constant
>    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();
> +
> +template <bool n = indirect_builtin_constant_p("a")>
> +int test_in_template_param() { return 0; }
> +int n3 = test_in_template_param();
> +
> +void test_in_case(int n) {
> +  switch (n) {
> +    case indirect_builtin_constant_p("abc"):
> +    break;
> +  }
> +}
> +enum InEnum1 {
> +  ONE = indirect_builtin_constant_p("abc")
> +};
> +enum InEnum2 : int {
> +  TWO = indirect_builtin_constant_p("abc")
> +};
> +enum class InEnum3 {
> +  THREE = indirect_builtin_constant_p("abc")
> +};
>
> Modified: cfe/trunk/test/SemaCXX/enable_if.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/enable_if.cpp?rev=355743&r1=355742&r2=355743&view=diff
> ==============================================================================
> --- cfe/trunk/test/SemaCXX/enable_if.cpp (original)
> +++ cfe/trunk/test/SemaCXX/enable_if.cpp Fri Mar  8 14:06:48 2019
> @@ -514,3 +514,11 @@ namespace TypeOfFn {
>
>    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
>
>
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