[clang][AST] NULL node for function template body with -target x86_64-pc-win32 pre c++20

[5chmidti]

This bug was found during #70559.

Repro case:

template <typename T>
void f() {}

Clang creates valid ASTs for this snippet with:

• -target x86_64-pc-win32 -std=c++20
• -std=c++17 (linux)
• -std=c++20 (linux)

But not with:

• -target x86_64-pc-win32 -std=c++17
• -target x86_64-pc-win32 -std=c++14
• -target x86_64-pc-win32 -std=c++11

https://godbolt.org/z/9v98vnbsx

TranslationUnitDecl
`-FunctionTemplateDecl <line:1:1, line:2:11> col:6 f
  |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
  `-FunctionDecl <line:2:1, col:11> col:6 f 'void ()'
    `-CompoundStmt <col:10, col:11>

vs

TranslationUnitDecl
`-FunctionTemplateDecl <line:1:1, line:2:8> col:6 f
  |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
  `-FunctionDecl <line:2:1, col:8> col:6 f 'void ()'
    `-<<<NULL>>>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Julian Schmidt (5chmidti)

This bug was found during #70559.

Repro case:

template <typename T>
void f() {}

Clang creates valid ASTs for this snippet with:

• -target x86_64-pc-win32 -std=c++20
• -std=c++17 (linux)
• -std=c++20 (linux)

But not with:

• -target x86_64-pc-win32 -std=c++17
• -target x86_64-pc-win32 -std=c++14
• -target x86_64-pc-win32 -std=c++11

https://godbolt.org/z/9v98vnbsx

TranslationUnitDecl
`-FunctionTemplateDecl <line:1:1, line:2:11> col:6 f
  |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
  `-FunctionDecl <line:2:1, col:11> col:6 f 'void ()'
    `-CompoundStmt <col:10, col:11>

vs

TranslationUnitDecl
`-FunctionTemplateDecl <line:1:1, line:2:8> col:6 f
  |-TemplateTypeParmDecl <line:1:11, col:20> col:20 typename depth 0 index 0 T
  `-FunctionDecl <line:2:1, col:8> col:6 f 'void ()'
    `-<<<NULL>>>

[koplas]

Relevant info:

• How do you access the body of a template function in the AST?
• [C++20] [Modules] [Driver] Don't enable -fdelayed-template-parsing by default on windows with C++20 modules #69431

[5chmidti]

Thanks. I'm going to close this issue. As far as I can tell, this is expected behavior: the change for C++20 was due to MSVC changing the default for C++20, and the pre C++20 behavior is to use the delayed template parsing. Clang simply mirrors this behavior, from what I can tell. I'll figure something out for my CI failure.