No AST representation created for explicit instantiation declarations of function templates

[Quuxplusone]

Bugzilla Link	PR20666
Status	NEW
Importance	P normal
Reported by	Eli Bendersky (eliben@gmail.com)
Reported on	2014-08-14 18:24:01 -0700
Last modified on	2019-01-14 02:54:22 -0800
Version	trunk
Hardware	PC Linux
CC	dgregor@apple.com, eliben@gmail.com, jbytheway+llvm@gmail.com, llvm-bugs@lists.llvm.org, nlewycky@google.com, richard-llvm@metafoo.co.uk, steveire@gmail.com
Fixed by commit(s)
Attachments
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Blocked by
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Explicit (extern) template declarations have no representation in the AST:

  template <typename T>
  int __ostream_insert() {
    return 0;
  }

  extern template int __ostream_insert<char>();

  template<> int __ostream_insert<long>() {
    return 1;
  }

Produces:

    TranslationUnitDecl 0x3763300 <<invalid sloc>> <invalid sloc>
    |-TypedefDecl 0x3763840 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128'
    |-TypedefDecl 0x37638a0 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128'
    |-TypedefDecl 0x3763c60 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list '__va_list_tag [1]'
    |-FunctionTemplateDecl 0x3763e50 </tmp/jp.cpp:1:3, line:4:3> line:2:7 __ostream_insert
    | |-TemplateTypeParmDecl 0x3763cb0 <line:1:13, col:22> col:22 typename T
    | |-FunctionDecl 0x3763db0 <line:2:3, line:4:3> line:2:7 __ostream_insert 'int (void)'
    | | `-CompoundStmt 0x3763ed8 <col:26, line:4:3>
    | |   `-ReturnStmt 0x3763eb8 <line:3:5, col:12>
    | |     `-IntegerLiteral 0x3763e98 <col:12> 'int' 0
    | |-FunctionDecl 0x37a4f60 <line:2:3, line:4:3> line:2:7 __ostream_insert 'int (void)'
    | | `-TemplateArgument type 'char'
    | `-Function 0x37a50a0 '__ostream_insert' 'int (void)'
    `-FunctionDecl 0x37a50a0 prev 0x37a51c0 <line:8:3, line:10:3> line:8:18 __ostream_insert 'int (void)'
      |-TemplateArgument type 'long'
      `-CompoundStmt 0x37a5380 <col:43, line:10:3>
        `-ReturnStmt 0x37a5360 <line:9:5, col:12>
          `-IntegerLiteral 0x37a5340 <col:12> 'int' 1

It appears as a child of the FunctionTemplateDecl:

    | |-FunctionDecl 0x37a4f60 <line:2:3, line:4:3> line:2:7 __ostream_insert 'int (void)'
    | | `-TemplateArgument type 'char'

But the source location is incorrect (points to the parent template, not the
extern instantiation.

[Quuxplusone]

Isn't FD->getTemplateSpecializationKind() == TSK_ExplicitSpecializationDeclaration?

[Quuxplusone]

Specifically, the problem we encounter is that its source location is incorrect. It's <line:2:3, line:4:3>, same as the template decl.

[Quuxplusone]

More symptoms of this:

template<typename T> void f() {}
extern template void f<int>();
extern template void f<int>();

... has no declarations on either line 2 or line 3, and only one declaration of
'void f<int>()' in the AST. And:

template<typename T> void f() {}
typedef int Foo;
extern template void f<Foo>();

... has no references to Foo in the AST (defeating rename refactorings and the
like).

[Quuxplusone]

I ran into an aspect of this issue while trying to improve include-what-you-
use's handling of arguments to explicit template declarations and similar
things.

For example, in the following code:

 using my_int = int;
 constexpr int h(int n) { return n*2; }

 template<class>
 void template_function() {}

 template void template_function<my_int[h(7)]>();

the relevant part of the AST is just:

 `-FunctionTemplateDecl 0x561a3ac05e20 <line:4:1, line:5:27> col:6 template_function
   |-TemplateTypeParmDecl 0x561a3ac05c78 <line:4:10> col:10 class depth 0 index 0
   |-FunctionDecl 0x561a3ac05d80 <line:5:1, col:27> col:6 template_function 'void ()'
   | `-CompoundStmt 0x561a3ac05e70 <col:26, col:27>
   `-FunctionDecl 0x561a3ac06138 <col:1, col:27> col:6 template_function 'void ()'
     |-TemplateArgument type 'int [14]'
     `-CompoundStmt 0x561a3ac05e70 <col:26, col:27>

To compute uses correctly we need to see that the argument mentioned my_int,
rather than int, and that h was called.  I'd expect DeclRefExpr entries
somewhere in the AST for both my_int and h, but as far as I can see there is
nothing.  We just get the type int[14].