https://bugs.llvm.org/show_bug.cgi?id=49891
Bug ID: 49891
Summary: No hint to use -std=c++20 when using 'concept' or
'requires' with older -std modes
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
template<typename T>
concept snackable = requires (sizeof(T) != 1)
template<typename T>
struct S { };
template<typename T> requires snackable<T>
struct S<T> { };
Compiling this C++20 code without -std=c++20 produces a load of parser errors:
c.C:2:3: error: unknown type name 'concept'
concept snackable = requires (sizeof(T) != 1)
^
c.C:2:23: error: use of undeclared identifier 'requires'
concept snackable = requires (sizeof(T) != 1)
^
c.C:7:22: error: unknown type name 'requires'
template<typename T> requires snackable<T>
^
c.C:7:31: error: variable template partial specialization does not specialize
any template argument; to define the primary template, remove the template
argument list
template<typename T> requires snackable<T>
^ ~~~
c.C:7:43: error: expected ';' at end of declaration
template<typename T> requires snackable<T>
^
;
c.C:8:12: error: use of undeclared identifier 'T'
struct S<T> { };
^
6 errors generated.
There's no hint that you need to enable C++20.
GCC does better:
c.C:2:3: error: 'concept' does not name a type
2 | concept snackable = requires (sizeof(T) != 1)
| ^~~~~~~
c.C:2:3: note: 'concept' only available with '-std=c++20' or '-fconcepts'
c.C:7:22: error: 'requires' does not name a type
7 | template<typename T> requires snackable<T>
| ^~~~~~~~
c.C:7:22: note: 'requires' only available with '-std=c++20' or '-fconcepts'
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