C++20+: constant evaluation erroneous failure involving allocation and range for

[ilazaric]

Issue Summary

Statement for (auto _ : foo()); seems to break in constexpr if the object foo() returns involves a sufficiently large allocation.

struct vec {
  int* data;

  constexpr vec() : data(new int[NUM]) {
    for (int i = 0; i < NUM; ++i) data[i] = 0;
  }

  constexpr auto begin() { return data; }
  constexpr auto end() { return data + NUM; }

  constexpr ~vec() { delete[] data; }
};

constexpr int bar() {
  for (auto _ : vec());
  return 1;
}

static_assert(bar());

This snippet compiles with --c++20 -DNUM=1354, and fails to compile with --c++20 -DNUM=1355.
Failure:

"<source>", line 19: error: expression must have a constant value
  static_assert(bar());
                ^
"<source>", line 19: note: some dynamic allocations (total number = 1) were not deallocated
  static_assert(bar());
                ^
"<source>", line 4: note: allocation occurred here
    constexpr vec() : data(new int[NUM]) {
                           ^

1 error detected in the compilation of "<source>".
Compiler returned: 2

On godbolt: https://godbolt.org/z/zKP5shPnn

Same example except the range for is for (auto&& _ : vec()); breaks with a significantly smaller NUM=678.
On godbolt: https://godbolt.org/z/x5Mb9qa5v

Reproducing Source Code

struct vec {
  int* data;

  constexpr vec() : data(new int[NUM]) {
    for (int i = 0; i < NUM; ++i) data[i] = 0;
  }

  constexpr auto begin() { return data; }
  constexpr auto end() { return data + NUM; }

  constexpr ~vec() { delete[] data; }
};

constexpr int bar() {
  for (auto _ : vec());
  return 1;
}

static_assert(bar());

Command-line Options

cpfe --c++20 -DNUM=1355 repro.cpp

Type of Issue

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

Additional Details

No response

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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