[PATCH] D59038: [8.0 Regression] Fix handling of `__builtin_constant_p` inside the enable_if attribute.

[Eric Fiselier via Phabricator via cfe-commits]

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;


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