https://bugs.llvm.org/show_bug.cgi?id=47414
Bug ID: 47414
Summary: Premature substitution into constraints
Product: clang
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: C++2a
Assignee: unassignedclangb...@nondot.org
Reporter: ca...@carter.net
CC: blitzrak...@gmail.com, erik.pilking...@gmail.com,
llvm-bugs@lists.llvm.org, richard-l...@metafoo.co.uk
Compiling this program:
template <class, class> constexpr bool is_same_v = false;
template <class T> constexpr bool is_same_v<T, T> = true;
template <class T> concept C = is_same_v<T, void>;
template <class T> requires C<T> using alias = T;
template <class T> struct S {
void f() requires C<T> && is_same_v<alias<T>, void> {}
};
int main() { S<int>{}; }
with "clang++ -std=c++2a" diagnoses (https://godbolt.org/z/qhhec8):
<source>:9:41: error: constraints not satisfied for alias template 'alias'
[with T = int]
void f() requires C<T> && is_same_v<alias<T>, void> {}
^~~~~~~~
<source>:12:14: note: in instantiation of template class 'S<int>' requested
here
int main() { S<int>{}; }
^
<source>:6:29: note: because 'int' does not satisfy 'C'
template <class T> requires C<T> using alias = T;
^
<source>:4:32: note: because 'is_same_v<int, void>' evaluated to false
template <class T> concept C = is_same_v<T, void>;
^
Despite that `C<T>` is unsatisfied, the compiler is substituting into the
conjoined `is_same_v<alias<T>, void>` atomic constraint.
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