https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86956
Bug ID: 86956
Summary: Use of an alias template unexpectedly breaks
compilation
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: v.reshetnikov at gmail dot com
Target Milestone: ---
/********************** BEGIN SOURCE **********************/
template<class>
struct Outer {
template<bool>
struct Inner;
template<bool x>
using Alias = Inner<x>;
};
template<>
template<bool v>
struct Outer<void>::Inner {
static constexpr bool value = v;
};
static_assert(Outer<void>::Inner<true>::value); // OK
static_assert(Outer<void>::Alias<true>::value); // error
/*********************** END SOURCE ***********************/
The first static_assert compiles OK, but the second results in unexpected
errors:
/********************** BEGIN OUTPUT **********************/
<source>: In instantiation of 'struct Outer<void>::Inner<true>':
<source>:17:39: required from here
<source>:12:27: error: type/value mismatch at argument 1 in template parameter
list for 'template<bool <anonymous> > struct Outer<void>::Inner<<enumerator> >'
12 | struct Outer<void>::Inner {
| ^
<source>:12:27: note: expected a constant of type 'bool', got 'void'
<source>:12:27: error: type/value mismatch at argument 1 in template parameter
list for 'Outer<void>::Inner<<enumerator> >::Inner'
<source>:12:27: note: expected a constant of type 'bool', got 'void'
<source>:13:27: error: type/value mismatch at argument 1 in template parameter
list for 'template<bool <anonymous> > struct Outer<void>::Inner<<enumerator> >'
13 | static constexpr bool value = v;
| ^~~~~
<source>:13:27: note: expected a constant of type 'bool', got 'void'
<source>:13:27: error: type/value mismatch at argument 1 in template parameter
list for 'value'
<source>:13:27: note: expected a constant of type 'bool', got 'void'
<source>:17:41: error: 'value' is not a member of 'Outer<void>::Alias<true>'
{aka 'Outer<void>::Inner<true>'}
17 | static_assert(Outer<void>::Alias<true>::value); // error
| ^~~~~
Compiler returned: 1
/*********************** END OUTPUT ***********************/
The part "expected a constant of type 'bool', got 'void'" looks inexplicable;
"Inner<<enumerator> >" is also difficult to decipher. The issue is discovered
with build 9.0.0 20180813 (experimental), but appears to exist in earlier
versions as well.
For comparison, Clang compiles this code successfully.
