Clang: only use [[fallthrough]] in C++ files

It appears that [[fallthrough]] is valid in C++ but not in C. And in some Clang versions (e.g. Clang 11 on macOS), Clang is pedantic about this and considers it an error to use [[fallthrough]] in a .c file such as src/util/format.c. This changes makes gcc_fallthrough a no-op under Clang in C files.

Clang: only use [[fallthrough]] in C++ files
dca05193 · Jacob Vosmaer · Max Kellermann · b9a7f304 · dca05193 · dca05193
Commit dca05193 authored Dec 30, 2019 by Jacob Vosmaer Committed by Max Kellermann Dec 31, 2019
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NEWS NEWS +1 -0

Compiler.h src/util/Compiler.h +1 -1

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--- a/NEWS
+++ b/NEWS
 ver 0.21.19 (not yet released)
+* fix build failure with clang 10
 ver 0.21.18 (2019/12/24)
 * protocol

--- a/src/util/Compiler.h
+++ b/src/util/Compiler.h
@@ -145,7 +145,7 @@
 #if GCC_CHECK_VERSION(7,0)
 #define gcc_fallthrough __attribute__((fallthrough))
-#elif CLANG_CHECK_VERSION(10,0)
+#elif CLANG_CHECK_VERSION(10,0) && defined(__cplusplus)
 #define gcc_fallthrough [[fallthrough]]
 #else
 #define gcc_fallthrough