Reflection: assignment to a default-initialized local reflection fails with "attempt to access run-time storage"

[stevensudit-corvid]

Issue Summary

With the experimental reflection support enabled, a consteval function cannot assign to a local variable of reflection type that was declared without an initializer. Constant evaluation stops at the assignment with "attempt to access run-time storage". I understand reflection is still experimental; this looks like a small gap in it.

gcc accepts the code with -freflection. I ran into it in a consteval helper that declares a std::meta::info and then assigns it in each branch of an if.

Reproducing Source Code

using info = decltype(^^int);
consteval info f() { info r; r = ^^int; return r; }
static_assert(f() == ^^int);

Command-line Options

cpfe --c++26 repro.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

The diagnostic, at 158320e (version 7.0), in the linux-gcc-reflection-release configuration:

"repro.cpp", line 3: error: expression must have a constant value
  static_assert(f() == ^^int);
                ^
"repro.cpp", line 2: note: attempt to access run-time storage
  consteval info f() { info r; r = ^^int; return r; }
                                 ^

With gcc 16's <meta> in the GNU mode, a local std::meta::info behaves the same way.

Any one of these changes makes it compile:

• The variable has an initializer: info r{}; or info r = info{};.
• The variable is only read: consteval info f() { info r; return r; } compiles, and the result compares equal to info{}.
• The type is another scalar: the same function with int, a pointer or decltype(nullptr) compiles.
• The reflection is a member of a local class object, or is created with new info.

It still fails when the variable is assigned and never read, when it is in a nested block, and for an element of info r[1];.

Reading the source, three places seem to combine into this:

• def_initializer gives a default-initialized variable of reflection type init_kind = initk_zero (decl_inits.c:7353, "Default-initialization for reflection types is zero-initialization").
• At block entry, the interpreter allocates the locals that have no initializer only when init_kind == initk_none (interpret.c:8452), so this variable gets no storage. In a debug build, the init_kind at that line is initk_zero for info r; and initk_none for int r;.
• A read of the unallocated variable has a case that produces the null reflection (interpret.c:32254, "Reflection variables are default initialized to the NULL reflection"). The eok_assign case has no counterpart, finds a run-time address as its destination, and reports it (interpret.c:30229).

Reproducing (Standard) Configuration(s)

linux-gcc-reflection-release

Reproducing (Non-standard) Configuration

No response

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• I have checked open issues and existing discussions before filing this report.
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[wchilders-nvidia]

Confirmed as #215.