diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 20e9bfdcf..06460a68f 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -25,7 +25,7 @@ endif()
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 
 # Initialize CXXFLAGS.
-set(CMAKE_CXX_FLAGS                "-Wall -Wunused-parameter -std=c++11 ${LEAN_EXTRA_CXX_FLAGS}")
+set(CMAKE_CXX_FLAGS                "-Wall -Wextra -std=c++11 ${LEAN_EXTRA_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS_DEBUG          "-g -DLEAN_DEBUG -DLEAN_TRACE")
 set(CMAKE_CXX_FLAGS_MINSIZEREL     "-Os -DNDEBUG")
 set(CMAKE_CXX_FLAGS_RELEASE        "-O3 -DNDEBUG")
diff --git a/src/util/hash.cpp b/src/util/hash.cpp
index e7f27882c..ffaea7460 100644
--- a/src/util/hash.cpp
+++ b/src/util/hash.cpp
@@ -33,7 +33,10 @@ unsigned hash_str(unsigned length, char const * str, unsigned init_value) {
         a += reinterpret_cast<unsigned const *>(str)[0];
         b += reinterpret_cast<unsigned const *>(str)[1];
         c += reinterpret_cast<unsigned const *>(str)[2];
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wextra"
         mix(a, b, c);
+#pragma GCC diagnostic pop
         str += 12; len -= 12;
     }
 
@@ -55,7 +58,10 @@ unsigned hash_str(unsigned length, char const * str, unsigned init_value) {
     case 1 :  a+=str[0];
         /* case 0: nothing left to add */
     }
-    mix(a, b, c);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wextra"
+        mix(a, b, c);
+#pragma GCC diagnostic pop
     /*-------------------------------------------- report the result */
     return c;
 }
diff --git a/src/util/sexpr/format.h b/src/util/sexpr/format.h
index ea50ab980..df67bc296 100644
--- a/src/util/sexpr/format.h
+++ b/src/util/sexpr/format.h
@@ -88,7 +88,7 @@ private:
         lean_assert(sexpr_kind(s) == format_kind::TEXT);
         return cdr(s);
     }
-    static inline size_t const sexpr_text_length(sexpr const & s) {
+    static inline size_t sexpr_text_length(sexpr const & s) {
         lean_assert(sexpr_kind(s) == format_kind::TEXT);
         std::stringstream ss;
         sexpr const & content = cdr(s);