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
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.
--- gcc/cp/name-lookup.cc.jj2023-08-30 20:07:18.584830291 +0200
+++ gcc/cp/name-lookup.cc 2023-08-30 20:01:05.796967755 +0200
@@ -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;
--- gcc/testsuite/g++.dg/diagnostic/redeclaration-4.C.jj2023-08-30
20:01:37.013537549 +0200
+++ gcc/testsuite/g++.dg/diagnostic/redeclaration-4.C 2023-08-30
20:12:03.763900190 +0200
@@ -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)
