[cfe-dev] Possible ASTMatchers bug

[Aaron Ballman via cfe-dev]

On Sun, Jan 31, 2021 at 4:06 PM Hartogs Siegfried via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Hi!
>
> On the following code
>
> template<typename T>
> class C{};
> template<typename T>
> C<T> c;
> void func(){
>     c<float>;
> }
>
> with the matcher
>
> varDecl(hasType(classTemplateSpecializationDecl()))
>
> I get two matches which are exactly the same:
> Match #1:
>
> Binding for "root":
> VarTemplateSpecializationDecl 0x7fe879820a28 <test:16:1, col:6> col:6 used c 'C<float>':'C<float>' callinit
> |-TemplateArgument type 'float'
> | `-BuiltinType 0x7fe87901ee00 'float'
> `-CXXConstructExpr 0x7fe879852190 <col:6> 'C<float>':'C<float>' 'void () noexcept'
>
>
> Match #2:
>
> Binding for "root":
> VarTemplateSpecializationDecl 0x7fe879820a28 <test:16:1, col:6> col:6 used c 'C<float>':'C<float>' callinit
> |-TemplateArgument type 'float'
> | `-BuiltinType 0x7fe87901ee00 'float'
> `-CXXConstructExpr 0x7fe879852190 <col:6> 'C<float>':'C<float>' 'void () noexcept'
>
> Is this a bug? I'd expect there to be only one match.
> The goal is to find variables whose type refers to a class instantiation, is this a good approach?
> Behavior occurs in clang-query and the C++ API on Apple, on both clang 11.0.0 and 10.0.1; didn't yet have the possibility to test 11.0.1.

I think the AST matchers are properly reflecting the information in
the AST: https://godbolt.org/z/efYW3P

|-VarTemplateDecl <line:3:1, line:4:6> col:6 c
| |-TemplateTypeParmDecl <line:3:10, col:19> col:19 referenced
typename depth 0 index 0 T
| |-VarDecl <line:4:1, col:6> col:6 c 'C<T>'
| `-VarTemplateSpecializationDecl <col:1, col:6> col:6 used c
'C<float>':'C<float>' callinit
|   |-TemplateArgument type 'float'
|   | `-BuiltinType 'float'
|   `-CXXConstructExpr <col:6> 'C<float>':'C<float>' 'void () noexcept'
|-FunctionDecl <line:5:1, line:7:1> line:5:6 func 'void ()'
| `-CompoundStmt <col:12, line:7:1>
|   `-DeclRefExpr <line:6:5, col:12> 'C<float>':'C<float>' lvalue
VarTemplateSpecialization 0x55f3fcf22b50 'c' 'C<float>':'C<float>'
`-VarTemplateSpecializationDecl <line:4:1, col:6> col:6 used c
'C<float>':'C<float>' callinit
  |-TemplateArgument type 'float'
  | `-BuiltinType 'float'
  `-CXXConstructExpr <col:6> 'C<float>':'C<float>' 'void () noexcept'

So I don't think this is an AST matcher bug. However, I'm not certain
why the AST reflects the same information twice -- this has been the
behavior as far back as Clang 3.4: https://godbolt.org/z/xhao8d

~Aaron

>
> Thanks & best,
> Sigi
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