Unnamed bitfields don't block constant evaluation of constexpr ctors

C++14 [dcl.constexpr]p4 states that in the body of a constexpr
constructor,

> every non-variant non-static data member and base class sub-object
  shall be initialized

However, [class.bit]p2 notes that

> Unnamed bit-fields are not members and cannot be initialized.

Therefore, we should make sure to filter them out of the check that
all fields are initialized.

Fixing this makes the constant evaluator a bit smarter, and
specifically allows constexpr constructors to avoid tripping
-Wglobal-constructors when the type contains unnamed bitfields.

Reviewed at https://reviews.llvm.org/D39035.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316408 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 38 insertions, 0 deletions

diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index c25b3b6322..c09c99386c 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -1818,6 +1818,9 @@ static bool CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc,
       }
     }
     for (const auto *I : RD->fields()) {
+      if (I->isUnnamedBitfield())
+        continue;
+
       if (!CheckConstantExpression(Info, DiagLoc, I->getType(),
                                    Value.getStructField(I->getFieldIndex())))
         return false;
diff --git a/test/SemaCXX/constant-expression-cxx11.cpp b/test/SemaCXX/constant-expression-cxx11.cpp
index 68b82c7d96..51dd6199e6 100644
--- a/test/SemaCXX/constant-expression-cxx11.cpp
+++ b/test/SemaCXX/constant-expression-cxx11.cpp
@@ -1931,6 +1931,22 @@ namespace Bitfields {
     };
     static_assert(X::f(3) == -1, "3 should truncate to -1");
   }
+
+  struct HasUnnamedBitfield {
+    unsigned a;
+    unsigned : 20;
+    unsigned b;
+
+    constexpr HasUnnamedBitfield() : a(), b() {}
+    constexpr HasUnnamedBitfield(unsigned a, unsigned b) : a(a), b(b) {}
+  };
+
+  void testUnnamedBitfield() {
+    const HasUnnamedBitfield zero{};
+    int a = 1 / zero.b; // expected-warning {{division by zero is undefined}}
+    const HasUnnamedBitfield oneZero{1, 0};
+    int b = 1 / oneZero.b; // expected-warning {{division by zero is undefined}}
+  }
 }
 
 namespace ZeroSizeTypes {
diff --git a/test/SemaCXX/warn-global-constructors.cpp b/test/SemaCXX/warn-global-constructors.cpp
index 856826414a..430f239a3e 100644
--- a/test/SemaCXX/warn-global-constructors.cpp
+++ b/test/SemaCXX/warn-global-constructors.cpp
@@ -126,3 +126,22 @@ namespace pr20420 {
 void *array_storage[1];
 const int &global_reference = *(int *)array_storage;
 }
+
+namespace bitfields {
+  struct HasUnnamedBitfield {
+    unsigned a;
+    unsigned : 20;
+    unsigned b;
+
+    constexpr HasUnnamedBitfield() : a(), b() {}
+    constexpr HasUnnamedBitfield(unsigned a, unsigned b) : a(a), b(b) {}
+    explicit HasUnnamedBitfield(unsigned a) {}
+  };
+
+  const HasUnnamedBitfield zeroConst{};
+  HasUnnamedBitfield zeroMutable{};
+  const HasUnnamedBitfield explicitConst{1, 2};
+  HasUnnamedBitfield explicitMutable{1, 2};
+  const HasUnnamedBitfield nonConstexprConst{1}; // expected-warning {{global constructor}}
+  HasUnnamedBitfield nonConstexprMutable{1}; // expected-warning {{global constructor}}
+}