[PATCH] c++: nested lambda capturing a capture proxy [PR94376]

[Jason Merrill]

On 11/18/21 17:48, Patrick Palka wrote:
> On Thu, 18 Nov 2021, Jason Merrill wrote:
> 
>> On 11/2/21 14:06, Patrick Palka wrote:
>>> Here when determining the type of the FIELD_DECL for the by-value
>>> capture of 'i' in the inner lambda, we incorrectly give it the
>>> type const int instead of int since the effective initializer is
>>> the proxy for the outer capture, and this proxy is const qualified
>>> since the outer lambda is non-mutable.
>>>
>>> This patch fixes this by handling capturing of capture proxies specially
>>> in lambda_capture_field_type, namely we instead consider the type of
>>> their FIELD_DECL which unlike the proxy has the true cv-quals of the
>>> captured entity.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>>> trunk?
>>>
>>> 	PR c++/94376
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* lambda.c (lambda_capture_field_type): Simplify by handling the
>>> 	is_this case first and specially.  When capturing by-value a
>>> 	capture proxy, consider the type of the corresponding field
>>> 	instead.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp0x/lambda/lambda-nested9.C: New test.
>>> ---
>>>    gcc/cp/lambda.c                               | 19 +++++++--
>>>    .../g++.dg/cpp0x/lambda/lambda-nested9.C      | 41 +++++++++++++++++++
>>>    2 files changed, 56 insertions(+), 4 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
>>>
>>> diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
>>> index 2e9d38bbe83..f16c121dc78 100644
>>> --- a/gcc/cp/lambda.c
>>> +++ b/gcc/cp/lambda.c
>>> @@ -196,7 +196,9 @@ lambda_capture_field_type (tree expr, bool
>>> explicit_init_p,
>>>      tree type;
>>>      bool is_this = is_this_parameter (tree_strip_nop_conversions (expr));
>>>    -  if (!is_this && explicit_init_p)
>>> +  if (is_this)
>>> +    type = TREE_TYPE (expr);
>>> +  else if (explicit_init_p)
>>>        {
>>>          tree auto_node = make_auto ();
>>>          @@ -210,7 +212,7 @@ lambda_capture_field_type (tree expr, bool
>>> explicit_init_p,
>>>          else
>>>    	type = do_auto_deduction (type, expr, auto_node);
>>>        }
>>> -  else if (!is_this && type_dependent_expression_p (expr))
>>> +  else if (type_dependent_expression_p (expr))
>>>        {
>>>          type = cxx_make_type (DECLTYPE_TYPE);
>>>          DECLTYPE_TYPE_EXPR (type) = expr;
>>> @@ -220,10 +222,19 @@ lambda_capture_field_type (tree expr, bool
>>> explicit_init_p,
>>>        }
>>>      else
>>>        {
>>> +      if (!by_reference_p && is_capture_proxy (expr))
>>> +	{
>>> +	  /* When capturing by-value another capture proxy from an enclosing
>>> +	     lambda, consider the type of the corresponding field instead,
>>> +	     as the proxy may be const-qualifed if the enclosing lambda is
>>> +	     non-mutable (PR94376).  */
>>> +	  gcc_assert (TREE_CODE (DECL_VALUE_EXPR (expr)) == COMPONENT_REF);
>>> +	  expr = TREE_OPERAND (DECL_VALUE_EXPR (expr), 1);
>>
>> Is there a reason not to use DECL_CAPTURED_VARIABLE?
> 
> IIRC that'd work for normal capture proxies, but DECL_CAPTURED_VARIABLE
> is empty for explicitly-initialized captures and their capture proxies
> have the same issue.

Then the patch is OK as is.

>>
>>> +	}
>>> +
>>>          type = non_reference (unlowered_expr_type (expr));
>>>    -      if (!is_this
>>> -	  && (by_reference_p || TREE_CODE (type) == FUNCTION_TYPE))
>>> +      if (by_reference_p || TREE_CODE (type) == FUNCTION_TYPE)
>>>    	type = build_reference_type (type);
>>>        }
>>>    diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
>>> b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
>>> new file mode 100644
>>> index 00000000000..ff7da3b0ce1
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-nested9.C
>>> @@ -0,0 +1,41 @@
>>> +// PR c++/94376
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +int main() {
>>> +  // We used to incorrectly reject the first two cases.
>>> +  int i = 0;
>>> +  [=] () {
>>> +    [=] () mutable {
>>> +      ++i;
>>> +    };
>>> +  };
>>> +
>>> +#if __cpp_init_captures
>>> +  [j=0] () {
>>> +    [=] () mutable {
>>> +      ++j;
>>> +    };
>>> +  };
>>> +#endif
>>> +
>>> +  [=] () {
>>> +    [&] () mutable {
>>> +      ++i; // { dg-error "read-only" }
>>> +    };
>>> +  };
>>> +
>>> +  const int j = 0;
>>> +  [=] () {
>>> +    [=] () mutable {
>>> +      ++j; // { dg-error "read-only" }
>>> +    };
>>> +  };
>>> +
>>> +#if __cpp_init_captures
>>> +  [j=0] () {
>>> +    [&] () mutable {
>>> +      ++j; // { dg-error "read-only" }
>>> +    };
>>> +  };
>>> +#endif
>>> +}
>>>
>>
>>
>