java-symbol-solver-core/src/main/java/com/github/javaparser/symbolsolver/resolution/typeinference/TypeHelper.java
public static boolean isProperType(ResolvedType type) {
        if (type instanceof InferenceVariable) {
            return false;
        }
        if (type instanceof ResolvedReferenceType) {
            ResolvedReferenceType referenceType = (ResolvedReferenceType) type;
            return referenceType.typeParametersValues().stream().allMatch(it -> isProperType(it));
        }
        if (type instanceof ResolvedWildcard) {
            ResolvedWildcard wildcard = (ResolvedWildcard)type;
            if (wildcard.isBounded()) {
                return isProperType(wildcard.getBoundedType());
            } else {
                return true;
            }
        }
        if (type.isPrimitive()) {
            return true;
        }
        if (type.isTypeVariable()) {
            // FIXME I am not sure...
            return false;
        }
        if (type.isArray()) {
            return isProperType(type.asArrayType().getComponentType());
        }
        throw new UnsupportedOperationException(type.toString());
    }
public static boolean isProperType(ResolvedType type) {
        if (type instanceof InferenceVariable) {
            return false;
        }
        if (type instanceof ResolvedReferenceType) {
            ResolvedReferenceType referenceType = (ResolvedReferenceType) type;
            return referenceType.typeParametersValues().stream().allMatch(it -> isProperType(it));
        }
        if (type instanceof ResolvedWildcard) {
            ResolvedWildcard wildcard = (ResolvedWildcard)type;
            if (wildcard.isBounded()) {
                return isProperType(wildcard.getBoundedType());
            } else {
                return true;
            }
        }
        if (type.isPrimitive()) {
            return true;
        }
        if (type.isTypeVariable()) {
            // FIXME I am not sure...
            return false;
        }
        if (type.isArray()) {
            return isProperType(type.asArrayType().getComponentType());
        }
        throw new UnsupportedOperationException(type.toString());
    }
The term proper type excludes such "types" that mention inference variables.