[RFC] c++: Diagnose [basic.scope.block]/2 violations even for block externs [PR52953]
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Commit Message
Hi!
C++17 had in [basic.block.scope]/2
"A parameter name shall not be redeclared in the outermost block of the function
definition nor in the outermost block of any handler associated with a
function-try-block."
and in [basic.block.scope]/4 similar rule for selection/iteration
statements. My reading of that is that it applied even for block local
externs in all those spots, while they declare something at namespace scope,
the redeclaration happens in that outermost block etc. and introduces names
into that.
Those wordings seemed to have been moved somewhere else in C++20, but what's
worse, they were moved back and completely rewritten in
P1787R6: Declarations and where to find them
which has been applied as a DR (but admittedly, we don't claim yet to
implement that).
The current wording at https://eel.is/c++draft/basic.scope#block-2
and https://eel.is/c++draft/basic.scope#scope-2.10 seem to imply at least
to me that it doesn't apply to extern block local decls because their
target scope is the namespace scope and [basic.scope.block]/2 says
"and whose target scope is the block scope"...
Now, it is unclear if that is actually the intent or not.
There seems to be quite large implementation divergence on this as well.
Unpatched g++ e.g. on the redeclaration-5.C testcase diagnoses just
lines 55,58,67,70 (i.e. where the previous declaration is in for's
condition).
clang++ trunk diagnoses just lines 8 and 27, i.e. redeclaration in the
function body vs. parameter both in normal fn and lambda (but not e.g.
function-try-block and others, including ctors, but it diagnoses those
for non-extern decls).
ICC 19 diagnoses lines 8,32,38,41,45,52,55,58,61,64,67,70,76.
And MSCV trunk diagnoses 8,27,32,38,41,45,48,52,55,58,67,70,76,87,100,137
although the last 4 are just warnings.
g++ with the patch diagnoses
8,15,27,32,38,41,45,48,52,55,58,61,64,67,70,76,87,100,121,137
as the dg-error directives test.
So, I'm not really sure what to do. Intuitively the patch seems right
because even block externs redeclare stuff and change meaning of the
identifiers and void foo () { int i; extern int i (int); } is rejected
by all compilers.
2023-08-31 Jakub Jelinek <jakub@redhat.com>
PR c++/52953
* name-lookup.cc (check_local_shadow): Defer punting on
DECL_EXTERNAL (decl) from the start of function to right before
the -Wshadow* checks.
* g++.dg/diagnostic/redeclaration-4.C: New test.
* g++.dg/diagnostic/redeclaration-5.C: New test.
Jakub
Comments
On 8/31/23 04:08, Jakub Jelinek wrote:
> Hi!
>
> C++17 had in [basic.block.scope]/2
> "A parameter name shall not be redeclared in the outermost block of the function
> definition nor in the outermost block of any handler associated with a
> function-try-block."
> and in [basic.block.scope]/4 similar rule for selection/iteration
> statements. My reading of that is that it applied even for block local
> externs in all those spots, while they declare something at namespace scope,
> the redeclaration happens in that outermost block etc. and introduces names
> into that.
> Those wordings seemed to have been moved somewhere else in C++20, but what's
> worse, they were moved back and completely rewritten in
> P1787R6: Declarations and where to find them
> which has been applied as a DR (but admittedly, we don't claim yet to
> implement that).
> The current wording at https://eel.is/c++draft/basic.scope#block-2
> and https://eel.is/c++draft/basic.scope#scope-2.10 seem to imply at least
> to me that it doesn't apply to extern block local decls because their
> target scope is the namespace scope and [basic.scope.block]/2 says
> "and whose target scope is the block scope"...
> Now, it is unclear if that is actually the intent or not.
Yes, I suspect that should be
If a declaration that is not a name-independent declaration and
<del>whose target scope is</del><ins>that binds a name in</ins> the
block scope S of a
which seems to also be needed to prohibit the already-diagnosed
void f(int i) { union { int i; }; }
void g(int i) { enum { i }; }
I've suggested this to Core.
> There seems to be quite large implementation divergence on this as well.
>
> Unpatched g++ e.g. on the redeclaration-5.C testcase diagnoses just
> lines 55,58,67,70 (i.e. where the previous declaration is in for's
> condition).
>
> clang++ trunk diagnoses just lines 8 and 27, i.e. redeclaration in the
> function body vs. parameter both in normal fn and lambda (but not e.g.
> function-try-block and others, including ctors, but it diagnoses those
> for non-extern decls).
>
> ICC 19 diagnoses lines 8,32,38,41,45,52,55,58,61,64,67,70,76.
>
> And MSCV trunk diagnoses 8,27,32,38,41,45,48,52,55,58,67,70,76,87,100,137
> although the last 4 are just warnings.
>
> g++ with the patch diagnoses
> 8,15,27,32,38,41,45,48,52,55,58,61,64,67,70,76,87,100,121,137
> as the dg-error directives test.
>
> So, I'm not really sure what to do. Intuitively the patch seems right
> because even block externs redeclare stuff and change meaning of the
> identifiers and void foo () { int i; extern int i (int); } is rejected
> by all compilers.
I think this direction makes sense, though we might pedwarn on these
rather than error to reduce possible breakage.
> 2023-08-31 Jakub Jelinek <jakub@redhat.com>
>
> PR c++/52953
> * name-lookup.cc (check_local_shadow): Defer punting on
> DECL_EXTERNAL (decl) from the start of function to right before
> the -Wshadow* checks.
Don't we want to consider externs for the -Wshadow* checks as well?
Jason
@@ -3096,10 +3096,6 @@ check_local_shadow (tree decl)
if (TREE_CODE (decl) == PARM_DECL && !DECL_CONTEXT (decl))
return;
- /* External decls are something else. */
- if (DECL_EXTERNAL (decl))
- return;
-
tree old = NULL_TREE;
cp_binding_level *old_scope = NULL;
if (cxx_binding *binding = outer_binding (DECL_NAME (decl), NULL, true))
@@ -3219,6 +3215,9 @@ check_local_shadow (tree decl)
return;
}
+ if (DECL_EXTERNAL (decl))
+ return;
+
/* If '-Wshadow=compatible-local' is specified without other
-Wshadow= flags, we will warn only when the type of the
shadowing variable (DECL) can be converted to that of the
@@ -3274,6 +3273,9 @@ check_local_shadow (tree decl)
return;
}
+ if (DECL_EXTERNAL (decl))
+ return;
+
if (!warn_shadow)
return;
@@ -0,0 +1,167 @@
+// PR c++/52953
+// { dg-do compile }
+// { dg-options "-pedantic-errors -Wno-switch-unreachable" }
+
+void
+foo (int x) // { dg-message "'int x' previously declared here" }
+{
+ extern int x; // { dg-error "declaration of 'int x' shadows a parameter" }
+}
+
+void
+bar (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+ extern int x; // { dg-error "declaration of 'int x' shadows a parameter" }
+}
+catch (...)
+{
+}
+
+volatile int v;
+
+void
+baz ()
+{
+#if __cplusplus >= 201103L
+ auto f = [] (int x) { extern int x; };// { dg-error "declaration of 'int x' shadows a parameter" "" { target c++11 } }
+ // { dg-message "'int x' previously declared here" "" { target c++11 } .-1 }
+#endif
+ if (int x = 1) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ }
+ if (int x = 0) // { dg-message "'int x' previously declared here" }
+ ;
+ else
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ }
+ if (int x = 1) // { dg-message "'int x' previously declared here" }
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ if (int x = 0) // { dg-message "'int x' previously declared here" }
+ ;
+ else
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ switch (int x = 1) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ default:;
+ }
+ switch (int x = 1) // { dg-message "'int x' previously declared here" }
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ while (int x = v)
+ {
+ extern int x; // { dg-error "'int x' conflicts with a previous declaration" }
+ }
+ while (int x = v)
+ extern int x; // { dg-error "'int x' conflicts with a previous declaration" }
+ for (int x = v; x; ++x) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ }
+ for (int x = v; x; ++x) // { dg-message "'int x' previously declared here" }
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ for (; int x = v; )
+ {
+ extern int x; // { dg-error "'int x' conflicts with a previous declaration" }
+ }
+ for (; int x = v; )
+ extern int x; // { dg-error "'int x' conflicts with a previous declaration" }
+ try
+ {
+ }
+ catch (int x) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ }
+}
+
+void
+corge (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+}
+catch (...)
+{
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+}
+
+void
+fred (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+}
+catch (int)
+{
+}
+catch (long)
+{
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+}
+
+void
+garply (int x)
+{
+ try
+ {
+ extern int x;
+ }
+ catch (...)
+ {
+ extern int x;
+ }
+}
+
+struct S
+{
+ S (int x) // { dg-message "'int x' previously declared here" }
+ try : s (x)
+ {
+ extern int x; // { dg-error "declaration of 'int x' shadows a parameter" }
+ }
+ catch (...)
+ {
+ }
+ int s;
+};
+
+struct T
+{
+ T (int x) // { dg-message "'int x' previously declared here" }
+ try : t (x)
+ {
+ }
+ catch (...)
+ {
+ extern int x; // { dg-error "redeclaration of 'int x'" }
+ }
+ int t;
+};
+
+struct U
+{
+ U (int x) : u (x)
+ {
+ try
+ {
+ extern int x;
+ }
+ catch (...)
+ {
+ extern int x;
+ }
+ }
+ int u;
+};
+
+struct V
+{
+ V (int x) : v (x)
+ {
+ {
+ extern int x;
+ }
+ }
+ int v;
+};
@@ -0,0 +1,167 @@
+// PR c++/52953
+// { dg-do compile }
+// { dg-options "-pedantic-errors -Wno-switch-unreachable" }
+
+void
+foo (int x) // { dg-message "'int x' previously declared here" }
+{
+ extern int x (int); // { dg-error "declaration of 'int x\\\(int\\\)' shadows a parameter" }
+}
+
+void
+bar (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+ extern int x (int); // { dg-error "declaration of 'int x\\\(int\\\)' shadows a parameter" }
+}
+catch (...)
+{
+}
+
+volatile int v;
+
+void
+baz ()
+{
+#if __cplusplus >= 201103L
+ auto f = [] (int x) { extern int x (int); };// { dg-error "declaration of 'int x\\\(int\\\)' shadows a parameter" "" { target c++11 } }
+ // { dg-message "'int x' previously declared here" "" { target c++11 } .-1 }
+#endif
+ if (int x = 1) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ }
+ if (int x = 0) // { dg-message "'int x' previously declared here" }
+ ;
+ else
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ }
+ if (int x = 1) // { dg-message "'int x' previously declared here" }
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ if (int x = 0) // { dg-message "'int x' previously declared here" }
+ ;
+ else
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ switch (int x = 1) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ default:;
+ }
+ switch (int x = 1) // { dg-message "'int x' previously declared here" }
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ while (int x = v)
+ {
+ extern int x (int); // { dg-error "'int x\\\(int\\\)' redeclared as different kind of entity" }
+ }
+ while (int x = v)
+ extern int x (int); // { dg-error "'int x\\\(int\\\)' redeclared as different kind of entity" }
+ for (int x = v; x; ++x) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ }
+ for (int x = v; x; ++x) // { dg-message "'int x' previously declared here" }
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ for (; int x = v; )
+ {
+ extern int x (int); // { dg-error "'int x\\\(int\\\)' redeclared as different kind of entity" }
+ }
+ for (; int x = v; )
+ extern int x (int); // { dg-error "'int x\\\(int\\\)' redeclared as different kind of entity" }
+ try
+ {
+ }
+ catch (int x) // { dg-message "'int x' previously declared here" }
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ }
+}
+
+void
+corge (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+}
+catch (...)
+{
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+}
+
+void
+fred (int x) // { dg-message "'int x' previously declared here" }
+try
+{
+}
+catch (int)
+{
+}
+catch (long)
+{
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+}
+
+void
+garply (int x)
+{
+ try
+ {
+ extern int x (int);
+ }
+ catch (...)
+ {
+ extern int x (int);
+ }
+}
+
+struct S
+{
+ S (int x) // { dg-message "'int x' previously declared here" }
+ try : s (x)
+ {
+ extern int x (int); // { dg-error "declaration of 'int x\\\(int\\\)' shadows a parameter" }
+ }
+ catch (...)
+ {
+ }
+ int s;
+};
+
+struct T
+{
+ T (int x) // { dg-message "'int x' previously declared here" }
+ try : t (x)
+ {
+ }
+ catch (...)
+ {
+ extern int x (int); // { dg-error "redeclaration of 'int x\\\(int\\\)'" }
+ }
+ int t;
+};
+
+struct U
+{
+ U (int x) : u (x)
+ {
+ try
+ {
+ extern int x (int);
+ }
+ catch (...)
+ {
+ extern int x (int);
+ }
+ }
+ int u;
+};
+
+struct V
+{
+ V (int x) : v (x)
+ {
+ {
+ extern int x (int);
+ }
+ }
+ int v;
+};