diff --git a/cmake/Assertion.cmake b/cmake/Assertion.cmake
index 78d9478..827ec51 100644
--- a/cmake/Assertion.cmake
+++ b/cmake/Assertion.cmake
@@ -66,105 +66,102 @@ endmacro()
 function(assert)
   cmake_parse_arguments(PARSE_ARGV 0 ARG "" "" "")
   if(${ARG_UNPARSED_ARGUMENTS})
-    # Do nothing if the assertion succeeds.
-  else()
-    if(ARGC EQUAL 2)
-      if(NOT ARGV0 STREQUAL NOT)
-        if(ARGV0 STREQUAL DEFINED)
-          fail("expected variable" ARGV1 "to be defined")
-        elseif(ARGV0 STREQUAL EXISTS)
-          fail("expected path" ARGV1 "to exist")
-        elseif(ARGV0 STREQUAL IS_DIRECTORY)
-          fail("expected path" ARGV1 "to be a directory")
-        else()
-          set(ARGS "${ARGV0} ${ARGV1}")
-          fail("expected" ARGS "to resolve to true")
-        endif()
-      else()
-        set(ARGS "${ARGV0} ${ARGV1}")
-        fail("expected" ARGS "to resolve to true")
+    # Assertion passed, exit immediately.
+    return()
+  endif()
+
+  # Assertion failed, determine the fatal error message based on the given
+  # condition.
+  if(ARGC EQUAL 2)
+    if(NOT ARGV0 STREQUAL NOT)
+      if(ARGV0 STREQUAL DEFINED)
+        fail("expected variable" ARGV1 "to be defined")
+        return()
+      elseif(ARGV0 STREQUAL EXISTS)
+        fail("expected path" ARGV1 "to exist")
+        return()
+      elseif(ARGV0 STREQUAL IS_DIRECTORY)
+        fail("expected path" ARGV1 "to be a directory")
+        return()
+      endif()
+    endif()
+  elseif(ARGC EQUAL 3)
+    if(ARGV0 STREQUAL NOT)
+      if(ARGV1 STREQUAL DEFINED)
+        fail("expected variable" ARGV2 "not to be defined")
+        return()
+      elseif(ARGV1 STREQUAL EXISTS)
+        fail("expected path" ARGV2 "not to exist")
+        return()
+      elseif(ARGV1 STREQUAL IS_DIRECTORY)
+        fail("expected path" ARGV2 "not to be a directory")
+        return()
       endif()
-    elseif(ARGC EQUAL 3)
-      if(ARGV0 STREQUAL NOT)
-        if(ARGV1 STREQUAL DEFINED)
-          fail("expected variable" ARGV2 "not to be defined")
-        elseif(ARGV1 STREQUAL EXISTS)
-          fail("expected path" ARGV2 "not to exist")
-        elseif(ARGV1 STREQUAL IS_DIRECTORY)
-          fail("expected path" ARGV2 "not to be a directory")
+    else()
+      if(ARGV1 STREQUAL MATCHES)
+        if(DEFINED "${ARGV0}")
+          fail("expected string" "${ARGV0}" "of variable" ARGV0
+            "to match" ARGV2)
         else()
-          set(ARGS "${ARGV0} ${ARGV1} ${ARGV2}")
-          fail("expected" ARGS "to resolve to true")
+          fail("expected string" ARGV0 "to match" ARGV2)
         endif()
-      else()
-        if(ARGV1 STREQUAL MATCHES)
-          if(DEFINED "${ARGV0}")
+        return()
+      elseif(ARGV1 STREQUAL STREQUAL)
+        if(DEFINED "${ARGV0}")
+          if(DEFINED "${ARGV2}")
             fail("expected string" "${ARGV0}" "of variable" ARGV0
-              "to match" ARGV2)
+              "to be equal to string" "${ARGV2}" "of variable" ARGV2)
           else()
-            fail("expected string" ARGV0 "to match" ARGV2)
+            fail("expected string" "${ARGV0}" "of variable" ARGV0
+              "to be equal to" ARGV2)
           endif()
-        elseif(ARGV1 STREQUAL STREQUAL)
-          if(DEFINED "${ARGV0}")
-            if(DEFINED "${ARGV2}")
-              fail("expected string" "${ARGV0}" "of variable" ARGV0
-                "to be equal to string" "${ARGV2}" "of variable" ARGV2)
-            else()
-              fail("expected string" "${ARGV0}" "of variable" ARGV0
-                "to be equal to" ARGV2)
-            endif()
+        else()
+          if(DEFINED "${ARGV2}")
+            fail("expected string" ARGV0
+              "to be equal to string" "${ARGV2}" "of variable" ARGV2)
           else()
-            if(DEFINED "${ARGV2}")
-              fail("expected string" ARGV0
-                "to be equal to string" "${ARGV2}" "of variable" ARGV2)
-            else()
-              fail("expected string" ARGV0 "to be equal to" ARGV2)
-            endif()
+            fail("expected string" ARGV0 "to be equal to" ARGV2)
           endif()
-        else()
-          set(ARGS "${ARGV0} ${ARGV1} ${ARGV2}")
-          fail("expected" ARGS "to resolve to true")
         endif()
+        return()
       endif()
-    elseif(ARGC EQUAL 4)
-      if(ARGV0 STREQUAL NOT)
-        if(ARGV2 STREQUAL MATCHES)
-          if(DEFINED "${ARGV1}")
+    endif()
+  elseif(ARGC EQUAL 4)
+    if(ARGV0 STREQUAL NOT)
+      if(ARGV2 STREQUAL MATCHES)
+        if(DEFINED "${ARGV1}")
+          fail("expected string" "${ARGV1}" "of variable" ARGV1
+            "not to match" ARGV3)
+        else()
+          fail("expected string" ARGV1 "not to match" ARGV3)
+        endif()
+        return()
+      elseif(ARGV2 STREQUAL STREQUAL)
+        if(DEFINED "${ARGV1}")
+          if(DEFINED "${ARGV3}")
             fail("expected string" "${ARGV1}" "of variable" ARGV1
-              "not to match" ARGV3)
+              "not to be equal to string" "${ARGV3}" "of variable" ARGV3)
           else()
-            fail("expected string" ARGV1 "not to match" ARGV3)
+            fail("expected string" "${ARGV1}" "of variable" ARGV1
+              "not to be equal to" ARGV3)
           endif()
-        elseif(ARGV2 STREQUAL STREQUAL)
-          if(DEFINED "${ARGV1}")
-            if(DEFINED "${ARGV3}")
-              fail("expected string" "${ARGV1}" "of variable" ARGV1
-                "not to be equal to string" "${ARGV3}" "of variable" ARGV3)
-            else()
-              fail("expected string" "${ARGV1}" "of variable" ARGV1
-                "not to be equal to" ARGV3)
-            endif()
+        else()
+          if(DEFINED "${ARGV3}")
+            fail("expected string" ARGV1
+              "not to be equal to string" "${ARGV3}" "of variable" ARGV3)
           else()
-            if(DEFINED "${ARGV3}")
-              fail("expected string" ARGV1
-                "not to be equal to string" "${ARGV3}" "of variable" ARGV3)
-            else()
-              fail("expected string" ARGV1 "not to be equal to" ARGV3)
-            endif()
+            fail("expected string" ARGV1 "not to be equal to" ARGV3)
           endif()
-        else()
-          set(ARGS "${ARGV0} ${ARGV1} ${ARGV2} ${ARGV3}")
-          fail("expected" ARGS "to resolve to true")
         endif()
-      else()
-        set(ARGS "${ARGV0} ${ARGV1} ${ARGV2} ${ARGV3}")
-        fail("expected" ARGS "to resolve to true")
+        return()
       endif()
-    else()
-      string(REPLACE ";" " " ARGS "${ARG_UNPARSED_ARGUMENTS}")
-      fail("expected" ARGS "to resolve to true")
     endif()
   endif()
+
+  # Fallback to this message if it could not determine the fatal error message
+  # from the given condition.
+  string(REPLACE ";" " " ARGS "${ARG_UNPARSED_ARGUMENTS}")
+  fail("expected" ARGS "to resolve to true")
 endfunction()
 
 # Asserts whether a command call throws a fatal error message.
@@ -262,34 +259,43 @@ function(assert_execute_process)
     set(ARG_COMMAND ${ARG_UNPARSED_ARGUMENTS})
   endif()
 
-  if(DEFINED ARG_OUTPUT)
-    string(JOIN "" EXPECTED_OUTPUT ${ARG_OUTPUT})
-  endif()
-
-  if(DEFINED ARG_ERROR)
-    string(JOIN "" EXPECTED_ERROR ${ARG_ERROR})
-  endif()
-
   execute_process(
     COMMAND ${ARG_COMMAND}
     RESULT_VARIABLE RES
     OUTPUT_VARIABLE OUT
     ERROR_VARIABLE ERR)
 
-  if(DEFINED ARG_ERROR AND RES EQUAL 0)
-    string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
-    fail("expected command" COMMAND "to fail")
-  elseif(NOT DEFINED ARG_ERROR AND NOT RES EQUAL 0)
-    string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
-    fail("expected command" COMMAND "not to fail with error" ERR)
-  elseif(DEFINED EXPECTED_OUTPUT AND NOT "${OUT}" MATCHES "${EXPECTED_OUTPUT}")
-    string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
-    fail("expected the output" OUT "of command" COMMAND
-      "to match" EXPECTED_OUTPUT)
-  elseif(DEFINED EXPECTED_ERROR AND NOT "${ERR}" MATCHES "${EXPECTED_ERROR}")
-    string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
-    fail("expected the error" ERR "of command" COMMAND
-      "to match" EXPECTED_ERROR)
+  if(DEFINED ARG_ERROR)
+    if(RES EQUAL 0)
+      string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
+      fail("expected command" COMMAND "to fail")
+      return()
+    endif()
+  else()
+    if(NOT RES EQUAL 0)
+      string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
+      fail("expected command" COMMAND "not to fail with error" ERR)
+      return()
+    endif()
+  endif()
+
+  if(DEFINED ARG_OUTPUT)
+    string(JOIN "" EXPECTED_OUTPUT ${ARG_OUTPUT})
+    if(NOT "${OUT}" MATCHES "${EXPECTED_OUTPUT}")
+      string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
+      fail("expected the output" OUT "of command" COMMAND
+        "to match" EXPECTED_OUTPUT)
+      return()
+    endif()
+  endif()
+
+  if(DEFINED ARG_ERROR)
+    string(JOIN "" EXPECTED_ERROR ${ARG_ERROR})
+    if(NOT "${ERR}" MATCHES "${EXPECTED_ERROR}")
+      string(REPLACE ";" " " COMMAND "${ARG_COMMAND}")
+      fail("expected the error" ERR "of command" COMMAND
+        "to match" EXPECTED_ERROR)
+    endif()
   endif()
 endfunction()