I noticed that concepts-lambda14.C had two useless requires-expressions:
static_assert(requires { C<T>; });
always succeeds, because C<T> is always a valid expression for any type,
regardless of whether C is satisfied for a particular type. Presumably the
user means
static_assert(requires { requires C<T>; });
to make the C<T> a nested-requirement. Of course,
static_assert(C<T>);
is much simpler and means the same thing; this is more relevant in the
middle of a longer requires-expression, such as the bug this warning found
in cmcstl2:
template<class I>
META_CONCEPT input_iterator =
input_or_output_iterator<I> &&
readable<I> &&
requires(I& i, const I& ci) {
typename iterator_category_t<I>;
derived_from<iterator_category_t<I>, input_iterator_tag>;
i++;
};
where 'requires' is missing before 'derived_from'.
Tested x86_64-pc-linux-gnu, applying to trunk.
gcc/ChangeLog:
* doc/invoke.texi: Document -Wmissing-requires.
gcc/c-family/ChangeLog:
* c.opt: Add -Wmissing-requires.
gcc/cp/ChangeLog:
* parser.c (cp_parser_simple_requirement): Warn about missing
requires.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-lambda14.C: Add expected warnings.
---
gcc/doc/invoke.texi | 22 +++++++++++++++++++
gcc/c-family/c.opt | 4 ++++
gcc/cp/parser.c | 19 ++++++++++++++++
.../g++.dg/cpp2a/concepts-lambda14.C | 4 ++--
4 files changed, 47 insertions(+), 2 deletions(-)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index f7bb193b51d..8969bac664d 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8752,6 +8752,28 @@ s x = @{ @};
This warning is included in @option{-Wextra}. To get other @option{-Wextra}
warnings without this one, use @option{-Wextra
-Wno-missing-field-initializers}.
+@item -Wno-missing-requires
+@opindex Wmissing-requires
+@opindex Wno-missing-requires
+
+By default, the compiler warns about a concept-id appearing as a C++20
simple-requirement:
+
+@smallexample
+bool satisfied = requires @{ C<T> @};
+@end smallexample
+
+Here @samp{satisfied} will be true if @samp{C<T>} is a valid
+expression, which it is for all T. Presumably the user meant to write
+
+@smallexample
+bool satisfied = requires @{ requires C<T> @};
+@end smallexample
+
+so @samp{satisfied} is only true if concept @samp{C} is satisfied for
+type @samp{T}.
+
+This warning can be disabled with @option{-Wno-missing-requires}.
+
@item -Wno-multichar
@opindex Wno-multichar
@opindex Wmultichar
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 91929706aff..c5fe90003f2 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -839,6 +839,10 @@ Wmissing-field-initializers
C ObjC C++ ObjC++ Var(warn_missing_field_initializers) Warning
EnabledBy(Wextra)
Warn about missing fields in struct initializers.
+Wmissing-requires
+C++ ObjC++ Var(warn_missing_requires) Init(1) Warning
+Warn about likely missing requires keyword.
+
Wmultistatement-macros
C ObjC C++ ObjC++ Var(warn_multistatement_macros) Warning LangEnabledBy(C ObjC
C++ ObjC++,Wall)
Warn about unsafe macros expanding to multiple statements used as a body of a
clause such as if, else, while, switch, or for.
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index a959c71dfa3..797e70ba5bb 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -29911,6 +29911,25 @@ cp_parser_simple_requirement (cp_parser *parser)
if (expr.get_location() == UNKNOWN_LOCATION)
expr.set_location (start);
+ for (tree t = expr; ; )
+ {
+ if (TREE_CODE (t) == TRUTH_ANDIF_EXPR
+ || TREE_CODE (t) == TRUTH_ORIF_EXPR)
+ {
+ t = TREE_OPERAND (t, 0);
+ continue;
+ }
+ if (concept_check_p (t))
+ {
+ gcc_rich_location richloc (get_start (start));
+ richloc.add_fixit_insert_before (start, "requires ");
+ warning_at (&richloc, OPT_Wmissing_requires, "testing "
+ "if a concept-id is a valid expression; add "
+ "%<requires%> to check satisfaction");
+ }
+ break;
+ }
+
return finish_simple_requirement (expr.get_location (), expr);
}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda14.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda14.C
index bdc893da857..02b6b6a8438 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda14.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda14.C
@@ -11,9 +11,9 @@ void foo() noexcept(!__is_same(T, void)) { }
template<typename U>
auto f() {
- return []<typename T>(T, bool a = requires { C<T>; }){
+ return []<typename T>(T, bool a = requires { C<T>; }){ // { dg-warning
Wmissing-requires }
static_assert(requires { requires C<U> && (C<T> || C<T>); }); // {
dg-error "assert" }
- static_assert(requires { C<T>; });
+ static_assert(requires { C<T>; }); // { dg-warning Wmissing-requires }
static_assert(requires { { foo<T>() } noexcept -> C; });
static_assert(!requires { typename T::blah; }); // { dg-error "assert" }
return 0;
base-commit: 7465c2ed6f1a1dfb2bec4b18bcad5fe3210b3f4e
--
2.27.0
