Issue 958 in include-what-you-use: iwyu removes necessary header files when forward declaring template function

[notifi...@include-what-you-use.org]

New issue 958 by iwansmith: iwyu removes necessary header files when forward declaring template function
https://github.com/include-what-you-use/include-what-you-use/issues/958

Consider this example:

file1.h
```
struct X{};
```

file2.h
```
template<typename T>
void templFunc();
```

file2.cxx
```
#include "file2.h"
#include "file1.h"

template<typename T>
void templFunc() {}

template<>
void templFunc<X>();

```

And the output of iwyu:
```
[iwans@home]$ include-what-you-use file2.cxx

file2.cxx should add these lines:
struct X;

file2.cxx should remove these lines:
- #include "file1.h" // lines 1-1

The full include-list for file2.cxx:
struct X;
---

iwyu has incorrectly removed file1.h and forward declared the struct X

[notifi...@include-what-you-use.org]

Comment #1 on issue 958 by kimgr: iwyu removes necessary header files when forward declaring template function
https://github.com/include-what-you-use/include-what-you-use/issues/958

After accepting IWYU's changes, I get:
```
$ include-what-you-use tests/958/file2.cxx

(tests/958/file2.h has correct #includes/fwd-decls)

(tests/958/file2.cxx has correct #includes/fwd-decls)

$ clang++-13 -fsyntax-only -c tests/958/file2.cxx
$
```

So it seems like that reduced example still compiles. Is there another reason you prefer the include for `X`, or does the repro need to be larger?

[notifi...@include-what-you-use.org]

Comment #1 on issue 958 by iwansmith: iwyu removes necessary header files when forward declaring template function
https://github.com/include-what-you-use/include-what-you-use/issues/958

My bad, the example doesn't work. Here's a more complete example I've verified fails to compile:

file1.h
```
struct X{
int x = 42;
};
```

file2.h
```
template<typename T>
int templFunc(T);

```

file2.cxx
```
#include "file2.h"
#include "file1.h"

template<typename T>

int templFunc(T val) { return val.x;}

template
int templFunc<X>(X val);
```

[iwans@home ~/Projects/C++]$ include-what-you-use file2.cxx

(file2.h has correct #includes/fwd-decls)

file2.cxx should add these lines:

file2.cxx should remove these lines:

- #include "file1.h" // lines 2-2

The full include-list for file2.cxx:

#include "file2.h"
---

After removing #include "file1.h"

```
[iwans@home ~/Projects/C++]$ g++ -c -fPIC file2.cxx
file2.cxx:7:15: error: 'X' was not declared in this scope
int templFunc<X>(X val);
^
```

[notifi...@include-what-you-use.org]

Comment #2 on issue 958 by iwansmith: iwyu removes necessary header files when forward declaring template function

https://github.com/include-what-you-use/include-what-you-use/issues/958

My bad, the example doesn't work. Here's a more complete example I've verified fails to compile:

file1.h
```
struct X{
int x = 42;
};
```

file2.h
```
template<typename T>
int templFunc(T);
```

file2.cxx
```
#include "file2.h"
#include "file1.h"

template<typename T>
int templFunc(T val) { return val.x;}

template
int templFunc<X>(X val);
```

Running iwyu

[notifi...@include-what-you-use.org]

[notifi...@include-what-you-use.org]

Comment #3 on issue 958 by kimgr: iwyu removes necessary header files when forward declaring template function
https://github.com/include-what-you-use/include-what-you-use/issues/958

I wasn't aware that was valid code, I expected:
```
template<> // note the <> brackets

int templFunc<X>(X val);
```

for a function template specialization, but it appears
```

template
int templFunc<X>(X val);
```

is a valid explicit instantiation: https://en.cppreference.com/w/cpp/language/function_template#Explicit_instantiation

That instantiation is kind of hidden away in the AST, and I suspect we don't take action on its template arguments in IWYU. I'm going to have to experiment more with that, but this does look like an omission bug.