[clang-tidy] readability-redundant-member-init fails to account for explicit user-provided default constructors

[connorsmacd-hh]

The readability-redundant-member-init check issues warnings on initializers even when the relevant default constructor is marked explicit and is user-provided.

Example code (godbolt here) with clang-tidy args -checks=-*,readability-redundant-member-init:

struct A
{
    explicit A() {}
};

struct B
{
    A a1 {}; // Compiles, but tidy detects a redundant member init
    A a2; // No warning from tidy, but does not compile
};

auto b = B{};

In the above example, clang-tidy flags the a1 line with:

<source>:8:10: warning: initializer for member 'a1' is redundant [readability-redundant-member-init]
    8 |     A a1 {}; // Compiles, but tidy detects a redundant member init

When I do what tidy wants in the a2 line, this fails to compile with:

<source>:12:12: error: chosen constructor is explicit in copy-initialization
   12 | auto b = B{};
      |            ^
<source>:3:14: note: explicit constructor declared here
    3 |     explicit A() {}
      |              ^
<source>:9:7: note: in implicit initialization of field 'a2' with omitted initializer
    9 |     A a2; // No warning from tidy, but does not compile
      |       ^
1 error generated.
Compiler returned: 1

Interestingly, clang-tidy no longer warns if the constructor is instead explicitly defaulted, i.e., explicit A() = default;.

For context, I encountered this issue when working with boost::circular_buffer's default constructor (docs here):

explicit circular_buffer(const allocator_type & alloc = allocator_type()) noexcept;

It seems to me that readability-redundant-member-init should ignore instances where the initializer is invoking an explicit default constructor. Is that correct or am I doing something wrong?

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Connor S. MacDonald (connorsmacd-hh)

The `readability-redundant-member-init` check issues warnings on initializers even when the relevant default constructor is **marked `explicit`** and is **user-provided**.

Example code (godbolt here) with clang-tidy args -checks=-*,readability-redundant-member-init:

struct A
{
    explicit A() {}
};

struct B
{
    A a1 {}; // Compiles, but tidy detects a redundant member init
    A a2; // No warning from tidy, but does not compile
};

auto b = B{};

In the above example, clang-tidy flags the a1 line with:

<source>:8:10: warning: initializer for member 'a1' is redundant [readability-redundant-member-init]
    8 |     A a1 {}; // Compiles, but tidy detects a redundant member init

When I do what tidy wants in the a2 line, this fails to compile with:

<source>:12:12: error: chosen constructor is explicit in copy-initialization
   12 | auto b = B{};
      |            ^
<source>:3:14: note: explicit constructor declared here
    3 |     explicit A() {}
      |              ^
<source>:9:7: note: in implicit initialization of field 'a2' with omitted initializer
    9 |     A a2; // No warning from tidy, but does not compile
      |       ^
1 error generated.
Compiler returned: 1

Interestingly, clang-tidy no longer warns if the constructor is instead explicitly defaulted, i.e., explicit A() = default;.

For context, I encountered this issue when working with boost::circular_buffer's default constructor (docs here):

explicit circular_buffer(const allocator_type & alloc = allocator_type()) noexcept;

It seems to me that readability-redundant-member-init should ignore instances where the initializer is invoking an explicit default constructor. Is that correct or am I doing something wrong?