[LLVMbugs] [Bug 19572] New: __is_trivial() gives wrong value when Clang generates code from AST file

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19572

            Bug ID: 19572
           Summary: __is_trivial() gives wrong value when Clang generates
                    code from AST file
           Product: clang
           Version: 3.4
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: wluo at coverity.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 12448
  --> http://llvm.org/bugs/attachment.cgi?id=12448&action=edit
A standalone reproducer.

I've attached a small standalone reproducer.

Clang 3.2 compiles it with no issue, but Clang 3.4 triggers the static_assert.
The problem exists only if I use -emit-ast and then compile from AST. If I skip
the -emit-ast step, things appear to work.

=== Clang 3.2 ===
$ ../clang32/bin/clang -c -std=c++11 test.cpp

$ ../clang32/bin/clang -emit-ast -std=c++11 test.cpp

$ ../clang32/bin/clang -c -std=c++11 test.ast


=== Clang 3.4 ===
$ ../clang34/bin/clang -c -std=c++11 test.cpp

$ ../clang34/bin/clang -c -std=c++11 -emit-ast test.cpp

$ ../clang34/bin/clang -c -std=c++11 test.ast
/home/wluo/space/test.cpp:21:5: error: static_assert failed "FOO"
    static_assert(!__is_trivial(_ValueType), "FOO");
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~
/home/wluo/space/test.cpp:27:5: note: in instantiation of function template
specialization
      'func<struct Data<const int> *>' requested here
    func(table_start);
    ^
1 error generated.


=== Suggested fix ===
In Clang 3.2, QualType::isTrivialType() relies on
ClassDecl->hasTrivialDefaultConstructor(), but in Clang 3.4 it uses
!ClassDecl->hasNonTrivialDefaultConstructor().

It seems ClassDecl->hasNonTrivialDefaultConstructor() is returning false in
this case, which I don't think is correct. Data<const int> has a user-provided
default constructor, and according to C++ [class.ctor] one of the requirements
for the default trivial constructor is that it isn't user-provided.

The following change seems to fix the issue:

--- a/llvm-3.4/tools/clang/lib/AST/Type.cpp
+++ b/llvm-3.4/tools/clang/lib/AST/Type.cpp
@@ -1064,9 +1064,15 @@ bool QualType::isTrivialType(ASTContext &Context) const
{
       //   A trivial class is a class that has a default constructor,
       //   has no non-trivial default constructors, and is trivially
       //   copyable.
+#if 1
+      // See BZ xxxxx comment x for why we don't want to use
+      // hasNonTrivialDefaultConstructor().
+      return ClassDecl->isTrivial();
+#else
       return ClassDecl->hasDefaultConstructor() &&
              !ClassDecl->hasNonTrivialDefaultConstructor() &&
              ClassDecl->isTriviallyCopyable();
+#endif
     }

     return true;

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