No AST representation created for explicit instantiation declarations of function templates

[llvmbot]

Bugzilla Link	20666
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@jbytheway,@zygoloid,@steveire

Extended Description

Explicit (extern) template declarations have no representation in the AST:

template int __ostream_insert() { return 0; }

extern template int __ostream_insert();

template<> int __ostream_insert() { 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.

[jbytheway]

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 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].

[ec04fc15-fa35-46f2-80e1-5d271f2ef708]

More symptoms of this:

template void f() {} extern template void f(); extern template void f();

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

template void f() {} typedef int Foo; extern template void f();

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

[llvmbot]

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.

[llvmbot]

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