On Tue, 14 Nov 2023, Jason Merrill wrote:
> On 11/14/23 11:10, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> >
> > -- >8 --
> >
> > For decltype((x)) within a lambda where x is not captured, we dubiously
> > require that the lambda has a capture default, unlike for decltype(x).
> > This patch fixes this inconsistency; I couldn't find a justification for
> > it in the standard.
>
> The relevant passage seems to be
>
> https://eel.is/c++draft/expr.prim#id.unqual-3
>
> "If naming the entity from outside of an unevaluated operand within S would
> refer to an entity captured by copy in some intervening lambda-expression,
> then let E be the innermost such lambda-expression.
>
> If there is such a lambda-expression and if P is in E's function parameter
> scope but not its parameter-declaration-clause, then the type of the
> expression is the type of a class member access expression ([expr.ref]) naming
> the non-static data member that would be declared for such a capture in the
> object parameter ([dcl.fct]) of the function call operator of E."
>
> In this case I guess there is no such lambda-expression because naming x won't
> refer to a capture by copy if the lambda doesn't capture anything, so we
> ignore the lambda.
>
> Maybe refer to that in a comment? OK with that change.
>
> I'm surprised that it refers specifically to capture by copy, but I guess a
> capture by reference should have the same decltype as the captured variable?
Ah, seems like it. So maybe we should get rid of the redundant
by-reference capture-default handling, to more closely mirror the
standard?
Also now that r14-6026-g73e2bdbf9bed48 made capture_decltype return
NULL_TREE to mean the capture is dependent, it seems we should just
inline capture_decltype into finish_decltype_type rather than
introducing another special return value to mean "fall back to ordinary
handling".
How does the following look? Bootstrapped and regtested on
x86_64-pc-linux-gnu.
-- >8 --
PR c++/83167
gcc/cp/ChangeLog:
* semantics.cc (capture_decltype): Inline into its only caller ...
(finish_decltype_type): ... here. Update nearby comment to refer
to recent standard. Restrict uncaptured variable handling to just
lambdas with a by-copy capture-default.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/lambda/lambda-decltype4.C: New test.
---
gcc/cp/semantics.cc | 107 +++++++-----------
.../g++.dg/cpp0x/lambda/lambda-decltype4.C | 15 +++
2 files changed, 55 insertions(+), 67 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index fbbc18336a0..fb4c3992e34 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -53,7 +53,6 @@ along with GCC; see the file COPYING3. If not see
static tree maybe_convert_cond (tree);
static tree finalize_nrv_r (tree *, int *, void *);
-static tree capture_decltype (tree);
/* Used for OpenMP non-static data member privatization. */
@@ -11856,21 +11855,48 @@ finish_decltype_type (tree expr, bool
id_expression_or_member_access_p,
}
else
{
- /* Within a lambda-expression:
-
- Every occurrence of decltype((x)) where x is a possibly
- parenthesized id-expression that names an entity of
- automatic storage duration is treated as if x were
- transformed into an access to a corresponding data member
- of the closure type that would have been declared if x
- were a use of the denoted entity. */
if (outer_automatic_var_p (STRIP_REFERENCE_REF (expr))
&& current_function_decl
&& LAMBDA_FUNCTION_P (current_function_decl))
{
- type = capture_decltype (STRIP_REFERENCE_REF (expr));
- if (!type)
- goto dependent;
+ /* [expr.prim.id.unqual]/3: If naming the entity from outside of an
+ unevaluated operand within S would refer to an entity captured by
+ copy in some intervening lambda-expression, then let E be the
+ innermost such lambda-expression.
+
+ If there is such a lambda-expression and if P is in E's function
+ parameter scope but not its parameter-declaration-clause, then the
+ type of the expression is the type of a class member access
+ expression naming the non-static data member that would be declared
+ for such a capture in the object parameter of the function call
+ operator of E." */
+ tree decl = STRIP_REFERENCE_REF (expr);
+ tree lam = CLASSTYPE_LAMBDA_EXPR (DECL_CONTEXT
(current_function_decl));
+ tree cap = lookup_name (DECL_NAME (decl), LOOK_where::BLOCK,
+ LOOK_want::HIDDEN_LAMBDA);
+
+ if (cap && is_capture_proxy (cap))
+ type = TREE_TYPE (cap);
+ else if (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam) == CPLD_COPY)
+ {
+ type = TREE_TYPE (decl);
+ if (TYPE_REF_P (type)
+ && TREE_CODE (TREE_TYPE (type)) != FUNCTION_TYPE)
+ type = TREE_TYPE (type);
+ }
+
+ if (type && !TYPE_REF_P (type))
+ {
+ tree obtype = TREE_TYPE (DECL_ARGUMENTS (current_function_decl));
+ if (WILDCARD_TYPE_P (non_reference (obtype)))
+ /* We don't know what the eventual obtype quals will be. */
+ goto dependent;
+ int quals = cp_type_quals (type);
+ if (INDIRECT_TYPE_P (obtype))
+ quals |= cp_type_quals (TREE_TYPE (obtype));
+ type = cp_build_qualified_type (type, quals);
+ type = build_reference_type (type);
+ }
}
else if (error_operand_p (expr))
type = error_mark_node;
@@ -11878,7 +11904,8 @@ finish_decltype_type (tree expr, bool
id_expression_or_member_access_p,
/* If the expression is just "this", we want the
cv-unqualified pointer for the "this" type. */
type = TYPE_MAIN_VARIANT (TREE_TYPE (expr));
- else
+
+ if (!type)
{
/* Otherwise, where T is the type of e, if e is an lvalue,
decltype(e) is defined as T&; if an xvalue, T&&; otherwise, T. */
@@ -12767,60 +12794,6 @@ apply_deduced_return_type (tree fco, tree return_type)
}
}
-/* DECL is a local variable or parameter from the surrounding scope of a
- lambda-expression. Returns the decltype for a use of the capture field
- for DECL even if it hasn't been captured yet. Or NULL_TREE if we can't give
- a correct answer at this point and we should build a DECLTYPE_TYPE. */
-
-static tree
-capture_decltype (tree decl)
-{
- tree lam = CLASSTYPE_LAMBDA_EXPR (DECL_CONTEXT (current_function_decl));
- tree cap = lookup_name (DECL_NAME (decl), LOOK_where::BLOCK,
- LOOK_want::HIDDEN_LAMBDA);
- tree type;
-
- if (cap && is_capture_proxy (cap))
- type = TREE_TYPE (cap);
- else
- switch (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam))
- {
- case CPLD_NONE:
- error ("%qD is not captured", decl);
- return error_mark_node;
-
- case CPLD_COPY:
- type = TREE_TYPE (decl);
- if (TYPE_REF_P (type)
- && TREE_CODE (TREE_TYPE (type)) != FUNCTION_TYPE)
- type = TREE_TYPE (type);
- break;
-
- case CPLD_REFERENCE:
- type = TREE_TYPE (decl);
- if (!TYPE_REF_P (type))
- type = build_reference_type (TREE_TYPE (decl));
- break;
-
- default:
- gcc_unreachable ();
- }
-
- if (!TYPE_REF_P (type))
- {
- tree obtype = TREE_TYPE (DECL_ARGUMENTS (current_function_decl));
- if (WILDCARD_TYPE_P (non_reference (obtype)))
- /* We don't know what the eventual obtype quals will be. */
- return NULL_TREE;
- int quals = cp_type_quals (type);
- if (INDIRECT_TYPE_P (obtype))
- quals |= cp_type_quals (TREE_TYPE (obtype));
- type = cp_build_qualified_type (type, quals);
- type = build_reference_type (type);
- }
- return type;
-}
-
/* Build a unary fold expression of EXPR over OP. If IS_RIGHT is true,
this is a right unary fold. Otherwise it is a left unary fold. */
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
new file mode 100644
index 00000000000..0062d7b8672
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
@@ -0,0 +1,15 @@
+// PR c++/83167
+// { dg-do compile { target c++11 } }
+
+int main() {
+ int x;
+ const int y = 42;
+
+ [] {
+ using ty1 = decltype((x));
+ using ty1 = int&;
+
+ using ty2 = decltype((y));
+ using ty2 = const int&;
+ };
+}
--
2.43.0.rc1
