Matching variadic template arguments using an AST Matcher

Question

I have the following sample code.

template<class T, class... Args>
T add(T first, Args... rest) {
    return first + add(rest...);
}

int add(int a, int b) {
    return a + b;
}

When I run it through clang-check -ast-dump, I get the following tree.

FunctionDecl 0x4df0b08 <tmp.cpp:2:1, line:4:1> line:2:3 add 'T (T, Args...)'
|-ParmVarDecl 0x4df0928 <col:7, col:9> col:9 referenced first 'T'
|-ParmVarDecl 0x4df09f8 <col:16, col:24> col:24 referenced rest 'Args...' pack
`-CompoundStmt 0x4df0dd0 <col:30, line:4:1>
  `-ReturnStmt 0x4df0dc0 <line:3:5, col:31>
    `-BinaryOperator 0x4df0da0 <col:12, col:31> '<dependent type>' '+'
      |-DeclRefExpr 0x4df0cd0 <col:12> 'T' lvalue ParmVar 0x4df0928 'first' 'T'
      `-CallExpr 0x4df0d78 <col:20, col:31> '<dependent type>'
        |-UnresolvedLookupExpr 0x4df0cf0 <col:20> '<overloaded function type>' lvalue (ADL) = 'add' 0x4df0ba8
        `-PackExpansionExpr 0x4df0d58 <col:24, col:28> '<dependent type>' lvalue
          `-DeclRefExpr 0x4df0d38 <col:24> 'Args' lvalue ParmVar 0x4df09f8 'rest' 'Args...'

FunctionDecl 0x4df0f60 <tmp.cpp:6:1, line:8:1> line:6:5 add 'int (int, int)'
|-ParmVarDecl 0x4df0e00 <col:9, col:13> col:13 used a 'int'
|-ParmVarDecl 0x4df0e80 <col:16, col:20> col:20 used b 'int'
`-CompoundStmt 0x4df10b0 <col:23, line:8:1>
  `-ReturnStmt 0x4df10a0 <line:7:5, col:16>
    `-BinaryOperator 0x4df1080 <col:12, col:16> 'int' '+'
      |-DeclRefExpr 0x4df1010 <col:12> 'int' lvalue ParmVar 0x4df0e00 'a' 'int'
      `-DeclRefExpr 0x4df1030 <col:16> 'int' lvalue ParmVar 0x4df0e80 'b' 'int'

I would like to write a matcher for the first case, where one of the arguments of the function is variadic. From the AST Matcher Reference, I have found that there is a isVariadic matcher, though as the documentation says,

Example matches f, but not g or h. The function i will not match, even when
compiled in C mode.
  void f(...);
  void g(int);
  template <typename... Ts> void h(Ts...);
  void i();

Is there any way to match the variadic parameter function declaration, and further bind the variadic parameter declaration to some node? I would like to do something like functionDecl(has(parmVarDecl(hasType(packExpansionType().bind("parameter_type"))))), but it seems that this not possible since there is no packExpansionType matcher. Am I missing something here?

Answer 1

For future reference, I found a solution to this. It is actually possible to define custom matchers within a check.

namespace {
    const AstTypeMatcher<PackExpansionType> packExpansionType;
    const internal::VariadicDynCastAllOfMatcher<Stmt, PackExpansionExpr> packExpansionExpr;
}

// You can also use the macros from ASTMatchersMacros.h

...