Adapt minimal boilerplate

I would agree with fmtlib/fmt#2086 on that.

linalg/bwc/lingen_call_companion.hpp

@@ -172,6 +172,10 @@ inline std::istream& operator>>(std::istream& is, lingen_call_companion::key & c
 
 namespace fmt {
     template <> struct /* fmt:: */ formatter<lingen_call_companion::key>: formatter<string_view> {
+    // only allow {} for formatting. No :, no :x, etc. It could be nice
+    // to allow them, though. Note that this should be constexpr with
+    // c++-14 or later
+    auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) { return ctx.begin(); }
     template <typename FormatContext>
 auto format(lingen_call_companion::key const & c, FormatContext& ctx) -> decltype(ctx.out()) {
             std::ostringstream os;

tests/sieve/test_fb_root.cpp

@@ -54,6 +54,10 @@ std::ostream& operator<<(std::ostream& os, fb_root_p1_t<T> const & R)
  */
 namespace fmt {
 template <typename T> struct /* fmt:: */ formatter<fb_root_p1_t<T>>: formatter<string_view> {
+    // only allow {} for formatting. No :, no :x, etc. It could be nice
+    // to allow them, though. Note that this should be constexpr with
+    // c++-14 or later
+    auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) { return ctx.begin(); }
     template <typename FormatContext>
         auto format(fb_root_p1_t<T> const & c, FormatContext& ctx) -> decltype(ctx.out()) {
             std::ostringstream os;
@@ -63,6 +67,10 @@ template <typename T> struct /* fmt:: */ formatter<fb_root_p1_t<T>>: formatter<s
 };
 
 template <> struct /* fmt:: */ formatter<qlattice_basis>: formatter<string_view> {
+    // only allow {} for formatting. No :, no :x, etc. It could be nice
+    // to allow them, though. Note that this should be constexpr with
+    // c++-14 or later
+    auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) { return ctx.begin(); }
     template <typename FormatContext>
         auto format(qlattice_basis const & c, FormatContext& ctx) -> decltype(ctx.out()) {
             std::ostringstream os;

utils/cxx_mpz.hpp

@@ -228,6 +228,10 @@ inline std::istream& operator>>(std::istream& is, cxx_mpq & x) { return is >> (m
 
 namespace fmt {
     template <> struct /* fmt:: */ formatter<cxx_mpz>: formatter<string_view> {
+    // only allow {} for formatting. No :, no :x, etc. It could be nice
+    // to allow them, though. Note that this should be constexpr with
+    // c++-14 or later
+    auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) { return ctx.begin(); }
     template <typename FormatContext>
 auto format(cxx_mpz const & c, FormatContext& ctx) -> decltype(ctx.out()) {
             std::ostringstream os;