diff --git a/config/companion_libs/isl.in b/config/companion_libs/isl.in
index eefbf34d..f1af3a00 100644
--- a/config/companion_libs/isl.in
+++ b/config/companion_libs/isl.in
@@ -4,12 +4,31 @@
 # GCC 4.9 supports ISL 0.10 to 0.15
 # GCC 5   supports ISL 0.12 to 0.16
 # GCC 6   supports ISL 0.14 to 0.16
+# Starting with GCC 5, GCC's configure no longer checks the ISL version explicitly,
+# despite what the configure's message says ("checking for isl 0.15 or 0.16").
+# Instead, it verifies that certain interfaces are available, so it accepts 0.17.1
+# or 0.18 under that check. Include them as experimental just in case anyone needs
+# the bugfixes in these releases.
 choice
     bool
     prompt "ISL version"
 # Don't remove next line
 # CT_INSERT_VERSION_BELOW
 
+config ISL_V_0_18
+    bool
+    prompt "0.18"
+    depends on EXPERIMENTAL # Not officially recommended by GCC
+    depends on CC_GCC_5_or_later
+    select ISL_V_0_16_or_later
+
+config ISL_V_0_17_1
+    bool
+    prompt "0.17.1"
+    depends on EXPERIMENTAL # Not officially recommended by GCC
+    depends on CC_GCC_5_or_later
+    select ISL_V_0_16_or_later
+
 config ISL_V_0_16_1
     bool
     prompt "0.16.1"
@@ -58,6 +77,8 @@ config ISL_VERSION
     string
 # Don't remove next line
 # CT_INSERT_VERSION_STRING_BELOW
+    default "0.18" if ISL_V_0_18
+    default "0.17.1" if ISL_V_0_17_1
     default "0.16.1" if ISL_V_0_16_1
     default "0.15" if ISL_V_0_15
     default "0.14" if ISL_V_0_14