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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - False-positive -Wnon-c-typedef-for-linkage, pre-C++20"
href="https://bugs.llvm.org/show_bug.cgi?id=50751">50751</a>
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<th>Summary</th>
<td>False-positive -Wnon-c-typedef-for-linkage, pre-C++20
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>maltsevm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Clang emits a warning when an unnamed class used in a typedef contains
constructs that are not compatible with pure C:
$ cat test.cpp
typedef struct {
void foo();
} X;
$ clang -std=c++17 -c test.cpp
<source>:1:15: warning: anonymous non-C-compatible type given name for linkage
purposes by typedef declaration; add a tag name here
[-Wnon-c-typedef-for-linkage]
typedef struct {
^
X
<source>:2:5: note: type is not C-compatible due to this member declaration
void foo();
^~~~~~~~~~
<source>:3:3: note: type is given name 'X' for linkage purposes by this typedef
declaration
} X;
^
1 warning generated.
Compiler returned: 0
When compiling with -pedantic-errors this warning is converted into an error.
The warning is based on a change in the C++ standard introduced by the C++20
proposal P1766. As the C++20 standard suggests ([diff.cpp17.dcl.dcl]/1), the
code is valid in C++17 and earlier versions:
"Affected subclause: 9.2.3
Change: Unnamed classes with a typedef name for linkage purposes can contain
only C-compatible constructs.
Rationale: Necessary for implementability.
Effect on original feature: Valid C++ 2017 code may be ill-formed in this
International Standard.
typedef struct {
void f() {}
} S;"
This should not be a "pedantic-error" prior to C++20.</pre>
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