"no matching function for call" caused by "candidate template ignored" when templated function is defined in header file

[gdanov]

Observed behavior

Templated function calls are marked wrongly. Here is excerpt and example:

// utils.hpp
typedef std::function<String()> retfun;

template <typename T> retfun mk_tag(String name, T value) { 
   return [name, value]() { return name + "=" + String(value); };
}

here is the using file

// client.cpp

void setup(){
  mk_tag("abc", "def");
}

screenshot from VSCode configured to use ccls

sorry for the extra context, if not obvious — this is arduino/platformio project.

Variant of the same code that works: removing the std::function return type:

// ccls does not complain when using this
template <typename T> String mk_tag(String name, T value) { 
   return name;
}

so, std::function somehow contributes to the problem. experimented with returning a vector<String>, same problem. Looks like any non-trivial return type triggers the problem.

Expected behavior

The code compiles, so I'd expect no complaints. Also, to the best of my knowledge this is valid code.

System information

• ccls version

Homebrew ccls version 0.20210330-11-g790daca4
clang version 13.0.1

this is head, but with latest non-head behavior is the same

• clang version:

Apple clang version 13.0.0 (clang-1300.0.29.30)
Target: x86_64-apple-darwin21.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

• OS: OSX
• Editor: emacs, vscode
• Language client (and version): emacs-lsp latest elpa, latest vscode + ccls plug-in (v0.1.29)