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

Patrick Palka ppalka@redhat.com
Thu Nov 18 22:48:08 GMT 2021


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.

> 
> > +	}
> > +
> >         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
> > +}
> > 
> 
> 



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