Problem: travis-ci is using outdated clang(-format)

Solution: explicitly select clang-5.0

.travis.yml

@@ -84,8 +84,14 @@ matrix:
   - env: BUILD_TYPE=android CURVE=tweetnacl
     os: linux
     dist: trusty
-  - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1
+  - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1 CLANG_FORMAT=/usr/local/clang-5.0.0/bin/clang-format
     os: linux
+    addons:
+    apt:
+      sources:
+        - llvm-toolchain-trusty-5.0
+      packages:
+        - clang-5.0
 
 sudo: required
 

builds/cmake/Modules/ClangFormat.cmake

@@ -30,14 +30,12 @@ add_custom_target(
         clang-format-check
         COMMAND chmod +x clang-format-check.sh
         COMMAND ./clang-format-check.sh
-        #COMMAND ${CLANG_FORMAT} -style=file -output-replacements-xml ${ALL_SOURCE_FILES} >clang-format-replacements.xml
-        #COMMAND grep \"<replacement \" clang-format-replacements.xml >/dev/null && exit 1
-        COMMENT "Checking correct formatting according to .clang-format file"
+        COMMENT "Checking correct formatting according to .clang-format file using ${CLANG_FORMAT}"
 )
 
 add_custom_target(
         clang-format-diff
         COMMAND ${CLANG_FORMAT} -style=file -i ${ALL_SOURCE_FILES}
         COMMAND git diff ${ALL_SOURCE_FILES}
-        COMMENT "Formatting with clang-format and showing differences with latest commit"
+        COMMENT "Formatting with clang-format (using ${CLANG_FORMAT}) and showing differences with latest commit"
 )

builds/cmake/ci_build.sh

@@ -18,6 +18,10 @@ CMAKE_OPTS+=("-DCMAKE_PREFIX_PATH:PATH=${BUILD_PREFIX}")
 CMAKE_OPTS+=("-DCMAKE_LIBRARY_PATH:PATH=${BUILD_PREFIX}/lib")
 CMAKE_OPTS+=("-DCMAKE_INCLUDE_PATH:PATH=${BUILD_PREFIX}/include")
 
+if [ "$CLANG_FORMAT" != "" ] ; then
+    CMAKE_OPTS+=("-DCLANG_FORMAT=${CLANG_FORMAT}")
+fi
+
 if [ -z $CURVE ]; then
     CMAKE_OPTS+=("-DENABLE_CURVE=OFF")
 elif [ $CURVE == "libsodium" ]; then