[llvm-bugs] [Bug 51078] New: consteval function inside of class template cannot use template type parameters in some contexts

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51078

            Bug ID: 51078
           Summary: consteval function inside of class template cannot use
                    template type parameters in some contexts
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: evanacox.dev at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Problem is reproducible in Clang 12.0.0 and Clang trunk.

Clang seems to not be able to deal with consteval functions that use type
parameters,
but only in specific contexts.  

Code:

    template <typename T> struct Foo {
      static consteval unsigned long foo() {
        return sizeof(T);
      }

      static consteval auto bar() {
        return sizeof(T);
      }

      static consteval T baz() {
        return T();
      }

      void f() {
        constexpr auto result = foo();
      }

      void g() {
        // different error from `foo()`?
        constexpr auto result = bar();
      }

      void h() {
        constexpr auto result = baz();
      }

      // all of these work fine 
      int array1[foo()];
      int array2[bar()];
      int array3[baz()];
    };

Clang Trunk: https://godbolt.org/z/hx97WTjG3

Local Compile:

    $ clang++ -v
    clang version 12.0.0 (Fedora 12.0.0-2.fc34)
    Target: x86_64-unknown-linux-gnu
    Thread model: posix
    InstalledDir: /usr/bin
    Found candidate GCC installation:
/usr/bin/../lib/gcc/x86_64-redhat-linux/11
    Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/11
    Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/11
    Candidate multilib: .;@m64
    Candidate multilib: 32;@m32
    Selected multilib: .;@m64

    $ clang++ -std=c++20 ./bug.cc
    ./bug.cc:15:29: error: call to consteval function 'Foo::foo' is not a
constant expression
        constexpr auto result = foo();
                            ^
    ./bug.cc:20:29: error: cannot take address of consteval function 'bar'
outside of an immediate invocation
        constexpr auto result = bar();
                            ^
    ./bug.cc:6:25: note: declared here
      static consteval auto bar() {
                        ^
    ./bug.cc:24:29: error: cannot take address of consteval function 'baz'
outside of an immediate invocation
        constexpr auto result = baz();
                            ^
    ./bug.cc:10:22: note: declared here
      static consteval T baz() {
                     ^
    3 errors generated.

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