in src/java/org/jetbrains/plugins/clojure/psi/resolve/completion/ClojureClassNameCompletionContributor.java [46:56]
public static boolean shouldRunClassName(CompletionParameters parameters, PrefixMatcher prefixMatcher,
boolean checkInvocationCount) {
final PsiElement position = parameters.getPosition();
if (checkInvocationCount && parameters.getInvocationCount() < 2) return false;
final PsiElement parent = position.getParent();
if (!(parent instanceof ClSymbol)) return false;
if (((ClSymbol) parent).getQualifierSymbol() != null) return false;
if (checkInvocationCount && parameters.getInvocationCount() >= 2) return true;
final String prefix = prefixMatcher.getPrefix();
return prefix.length() > 0 && prefix.substring(0, 1).toUpperCase().equals(prefix.substring(0, 1));
}