diff --git a/.travis.yml b/.travis.yml
index cd04032..175b072 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -58,4 +58,5 @@
       - libevent-dev
 
 script:
+  - deactivate # deactivate host compiler
   - ./travis-ci.sh
diff --git a/source/dub/compilers/buildsettings.d b/source/dub/compilers/buildsettings.d
index 16de290..3da87b1 100644
--- a/source/dub/compilers/buildsettings.d
+++ b/source/dub/compilers/buildsettings.d
@@ -108,8 +108,9 @@
 	void removeOptions(in BuildOption[] value...) { foreach (v; value) this.options &= ~v; }
 	void removeOptions(in BuildOptions value) { this.options &= ~value; }
 
+private:
 	// Adds vals to arr without adding duplicates.
-	private void add(ref string[] arr, in string[] vals, bool no_duplicates = true)
+	static void add(ref string[] arr, in string[] vals, bool no_duplicates = true)
 	{
 		if (!no_duplicates) {
 			arr ~= vals;
@@ -127,7 +128,16 @@
 		}
 	}
 
-	private void prepend(ref string[] arr, in string[] vals, bool no_duplicates = true)
+	unittest
+	{
+		auto ary = ["-dip1000", "-vgc"];
+		BuildSettings.add(ary, ["-dip1000", "-vgc"]);
+		assert(ary == ["-dip1000", "-vgc"]);
+		BuildSettings.add(ary, ["-dip1001", "-vgc"], false);
+		assert(ary == ["-dip1000", "-vgc", "-dip1001", "-vgc"]);
+	}
+
+	static void prepend(ref string[] arr, in string[] vals, bool no_duplicates = true)
 	{
 		if (!no_duplicates) {
 			arr = vals ~ arr;
@@ -145,8 +155,17 @@
 		}
 	}
 
+	unittest
+	{
+		auto ary = ["-dip1000", "-vgc"];
+		BuildSettings.prepend(ary, ["-dip1000", "-vgc"]);
+		assert(ary == ["-dip1000", "-vgc"]);
+		BuildSettings.prepend(ary, ["-dip1001", "-vgc"], false);
+		assert(ary == ["-dip1001", "-vgc", "-dip1000", "-vgc"]);
+	}
+
 	// add string import files (avoids file name duplicates in addition to path duplicates)
-	private void addSI(ref string[] arr, in string[] vals)
+	static void addSI(ref string[] arr, in string[] vals)
 	{
 		bool[string] existing;
 		foreach (v; arr) existing[Path(v).head.toString()] = true;
@@ -159,7 +178,16 @@
 		}
 	}
 
-	private void removePaths(ref string[] arr, in string[] vals)
+	unittest
+	{
+		auto ary = ["path/foo.txt"];
+		BuildSettings.addSI(ary, ["path2/foo2.txt"]);
+		assert(ary == ["path/foo.txt", "path2/foo2.txt"]);
+		BuildSettings.addSI(ary, ["path2/foo.txt"]); // no duplicate basenames
+		assert(ary == ["path/foo.txt", "path2/foo2.txt"]);
+	}
+
+	static void removePaths(ref string[] arr, in string[] vals)
 	{
 		bool matches(string s)
 		{
@@ -171,7 +199,18 @@
 		arr = arr.filter!(s => !matches(s))().array();
 	}
 
-	private void remove(ref string[] arr, in string[] vals)
+	unittest
+	{
+		auto ary = ["path1", "root/path1", "root/path2", "root2/path1"];
+		BuildSettings.removePaths(ary, ["path1"]);
+		assert(ary == ["root/path1", "root/path2", "root2/path1"]);
+		BuildSettings.removePaths(ary, ["*/path1"]);
+		assert(ary == ["root/path2"]);
+		BuildSettings.removePaths(ary, ["foo", "bar", "root/path2"]);
+		assert(ary == []);
+	}
+
+	static void remove(ref string[] arr, in string[] vals)
 	{
 		bool matches(string s)
 		{
@@ -182,6 +221,21 @@
 		}
 		arr = arr.filter!(s => !matches(s))().array();
 	}
+
+	unittest
+	{
+		import std.string : join;
+
+		auto ary = ["path1", "root/path1", "root/path2", "root2/path1"];
+		BuildSettings.remove(ary, ["path1"]);
+		assert(ary == ["root/path1", "root/path2", "root2/path1"]);
+		BuildSettings.remove(ary, ["root/path*"]);
+		assert(ary == ["root/path1", "root/path2", "root2/path1"]);
+		BuildSettings.removePaths(ary, ["foo", "root/path2", "bar", "root2/path1"]);
+		assert(ary == ["root/path1"]);
+		BuildSettings.remove(ary, ["root/path1", "foo"]);
+		assert(ary == []);
+	}
 }
 
 enum BuildSetting {
diff --git a/test/ddox.sh b/test/ddox.sh
index 44f836e..9675ed8 100755
--- a/test/ddox.sh
+++ b/test/ddox.sh
@@ -2,6 +2,12 @@
 
 . $(dirname "${BASH_SOURCE[0]}")/common.sh
 
+# gdc 4.8.5 not working with ddox due to missing
+# std.experimental.allocator.mallocator for libdparse
+if [ ${DC} = gdc ]; then
+    exit 0
+fi
+
 (cd $CURR_DIR/ddox/default && $DUB build -b ddox)
 grep -qF ddox_project $CURR_DIR/ddox/default/docs/index.html
 
diff --git a/travis-ci.sh b/travis-ci.sh
index 8cf47b3..9046bf3 100755
--- a/travis-ci.sh
+++ b/travis-ci.sh
@@ -2,6 +2,8 @@
 
 set -v -e -o pipefail
 
+source ~/dlang/*/activate # activate host compiler
+
 if [ -z "$FRONTEND" -o "$FRONTEND" \> 2.067.z ]; then
     vibe_ver=$(jq -r '.versions | .["vibe-d"]' < dub.selections.json)
     dub fetch vibe-d --version=$vibe_ver # get optional dependency
@@ -12,10 +14,21 @@
     # library-nonet fails to build with coverage (Issue 13742)
     dub test --compiler=${DC} -b unittest-cov
     ./build.sh -cov
+
+    # run tests with different compilers
+    DUB=`pwd`/bin/dub DC=${DC} test/run-unittest.sh
+    deactivate
+    git clean -dxf -- test
+    source $(~/dlang/install.sh ldc --activate)
+    DUB=`pwd`/bin/dub DC=${DC} test/run-unittest.sh
+    deactivate
+    git clean -dxf -- test
+    source $(~/dlang/install.sh gdc --activate)
+    DUB=`pwd`/bin/dub DC=${DC} test/run-unittest.sh
 else
     ./build.sh
+    DUB=`pwd`/bin/dub DC=${DC} test/run-unittest.sh
 fi
-DUB=`pwd`/bin/dub DC=${DC} test/run-unittest.sh
 
 if [ "$COVERAGE" = true ]; then
     wget https://codecov.io/bash -O codecov.sh