[clang] constexpr template method cannot be used in a constant expression

[Pawday]

Online example

struct ConstexprTemplate
{
    static constexpr int sizeof_char()
    {
        return sizeof_template<char>();
    }

    static constexpr int sizeof_short()
    {
        return sizeof_template<short>();
    }

    static constexpr int sizeof_int()
    {
        return sizeof_template<int>();
    }

    static constexpr int sizeof_long()
    {
        return sizeof_template<long>();
    }

    template <typename I>
    static constexpr I sizeof_template()
    {
        return sizeof(I);
    }
};

constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
constexpr int sizeof_long = ConstexprTemplate::sizeof_long();

clang version 18.1.8 ([email protected]:llvm/llvm-project.git 3b5b5c1)

Full diagnostic message:

Example.cc:30:15: error: constexpr variable 'sizeof_char' must be initialized by a constant expression
   30 | constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
      |               ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:5:16: note: undefined function 'sizeof_template<char>' cannot be used in a constant expression
    5 |         return sizeof_template<char>();
      |                ^
Example.cc:30:29: note: in call to 'sizeof_char()'
   30 | constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:31:15: error: constexpr variable 'sizeof_short' must be initialized by a constant expression
   31 | constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
      |               ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:10:16: note: undefined function 'sizeof_template<short>' cannot be used in a constant expression
   10 |         return sizeof_template<short>();
      |                ^
Example.cc:31:30: note: in call to 'sizeof_short()'
   31 | constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:32:15: error: constexpr variable 'sizeof_int' must be initialized by a constant expression
   32 | constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
      |               ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:15:16: note: undefined function 'sizeof_template<int>' cannot be used in a constant expression
   15 |         return sizeof_template<int>();
      |                ^
Example.cc:32:28: note: in call to 'sizeof_int()'
   32 | constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:33:15: error: constexpr variable 'sizeof_long' must be initialized by a constant expression
   33 | constexpr int sizeof_long = ConstexprTemplate::sizeof_long();
      |               ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:20:16: note: undefined function 'sizeof_template<long>' cannot be used in a constant expression
   20 |         return sizeof_template<long>();
      |                ^
Example.cc:33:29: note: in call to 'sizeof_long()'
   33 | constexpr int sizeof_long = ConstexprTemplate::sizeof_long();
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
4 errors generated.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Pavel Korotkevich (Pawday)

# [Online example](https://godbolt.org/z/qzaaGfe5h) ```c++ struct ConstexprTemplate { static constexpr int sizeof_char() { return sizeof_template<char>(); }

static constexpr int sizeof_short()
{
    return sizeof_template<short>();
}

static constexpr int sizeof_int()
{
    return sizeof_template<int>();
}

static constexpr int sizeof_long()
{
    return sizeof_template<long>();
}

template <typename I>
static constexpr I sizeof_template()
{
    return sizeof(I);
}

};

constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
constexpr int sizeof_long = ConstexprTemplate::sizeof_long();

clang version 18.1.8 (git@<!-- -->github.com:llvm/llvm-project.git 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)

Full diagnostic message:
```txt
Example.cc:30:15: error: constexpr variable 'sizeof_char' must be initialized by a constant expression
   30 | constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
      |               ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:5:16: note: undefined function 'sizeof_template&lt;char&gt;' cannot be used in a constant expression
    5 |         return sizeof_template&lt;char&gt;();
      |                ^
Example.cc:30:29: note: in call to 'sizeof_char()'
   30 | constexpr int sizeof_char = ConstexprTemplate::sizeof_char();
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:31:15: error: constexpr variable 'sizeof_short' must be initialized by a constant expression
   31 | constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
      |               ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:10:16: note: undefined function 'sizeof_template&lt;short&gt;' cannot be used in a constant expression
   10 |         return sizeof_template&lt;short&gt;();
      |                ^
Example.cc:31:30: note: in call to 'sizeof_short()'
   31 | constexpr int sizeof_short = ConstexprTemplate::sizeof_short();
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:32:15: error: constexpr variable 'sizeof_int' must be initialized by a constant expression
   32 | constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
      |               ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:15:16: note: undefined function 'sizeof_template&lt;int&gt;' cannot be used in a constant expression
   15 |         return sizeof_template&lt;int&gt;();
      |                ^
Example.cc:32:28: note: in call to 'sizeof_int()'
   32 | constexpr int sizeof_int = ConstexprTemplate::sizeof_int();
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
Example.cc:33:15: error: constexpr variable 'sizeof_long' must be initialized by a constant expression
   33 | constexpr int sizeof_long = ConstexprTemplate::sizeof_long();
      |               ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:20:16: note: undefined function 'sizeof_template&lt;long&gt;' cannot be used in a constant expression
   20 |         return sizeof_template&lt;long&gt;();
      |                ^
Example.cc:33:29: note: in call to 'sizeof_long()'
   33 | constexpr int sizeof_long = ConstexprTemplate::sizeof_long();
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example.cc:24:24: note: declared here
   24 |     static constexpr I sizeof_template()
      |                        ^
4 errors generated.

[tbaederr]

I think this is another case of constant evaluation not causing template instantiation. CC @cor3ntin

[shafik]

@tbaederr @cor3ntin I think we are talking about #73232 so folks don't have to go hunting to understand the details.