Handle const-qualified named arguments

A named argument that arrives const-qualified was not recognized by
is_named_arg, silently dropping its name. This happens both when a
named argument is passed through an intermediate function returning
const T& (https://github.com/fmtlib/fmt/issues/4866) and in fmt's own
compiled format path, which passes arguments as const T&.

Make is_named_arg and is_static_named_arg see through top-level const so
the name is preserved instead of dropped.

.github/workflows/linux.yml

@@ -83,7 +83,9 @@ jobs:
         sudo apt update
         sudo apt install libatomic1 libc6-dev libgomp1 libitm1 libmpc3
         # https://launchpad.net/ubuntu/xenial/amd64/g++-4.9/4.9.3-13ubuntu2
-        wget --no-verbose \
+        # launchpad periodically returns 503s, so retry to avoid flaky CI.
+        wget --no-verbose --tries=5 --waitretry=10 \
+          --retry-connrefused --retry-on-http-error=503 \
           http://launchpadlibrarian.net/230069137/libmpfr4_3.1.3-2_amd64.deb \
           http://launchpadlibrarian.net/253728424/libasan1_4.9.3-13ubuntu2_amd64.deb \
           http://launchpadlibrarian.net/445346135/libubsan0_5.4.0-6ubuntu1~16.04.12_amd64.deb \
@@ -114,7 +116,9 @@ jobs:
         sudo apt update
         sudo apt install libtinfo5
         # https://code.launchpad.net/ubuntu/xenial/amd64/clang-3.6/1:3.6.2-3ubuntu2
-        wget --no-verbose \
+        # launchpad periodically returns 503s, so retry to avoid flaky CI.
+        wget --no-verbose --tries=5 --waitretry=10 \
+          --retry-connrefused --retry-on-http-error=503 \
           http://launchpadlibrarian.net/230019046/libffi6_3.2.1-4_amd64.deb \
           http://launchpadlibrarian.net/445346109/libasan2_5.4.0-6ubuntu1~16.04.12_amd64.deb \
           http://launchpadlibrarian.net/445346135/libubsan0_5.4.0-6ubuntu1~16.04.12_amd64.deb \

include/fmt/base.h

@@ -1059,6 +1059,10 @@ template <typename T> struct is_static_named_arg : std::false_type {};
 template <typename T, typename Char>
 struct is_named_arg<named_arg<T, Char>> : std::true_type {};
 
+template <typename T> struct is_named_arg<const T> : is_named_arg<T> {};
+template <typename T>
+struct is_static_named_arg<const T> : is_static_named_arg<T> {};
+
 template <typename T, typename Char = char> struct named_arg : view {
   const Char* name;
   const T& value;

test/compile-test.cc

@@ -163,6 +163,10 @@ TEST(compile_test, named) {
             fmt::format(FMT_COMPILE("{a0} {a1}"), "a0"_a = 41, "a1"_a = 43));
   EXPECT_EQ("41 43",
             fmt::format(FMT_COMPILE("{a1} {a0}"), "a0"_a = 43, "a1"_a = 41));
+
+  // A statically-named argument with a format spec compiles to spec_field,
+  // which passes the argument to make_format_args as const (#4866).
+  EXPECT_EQ("4.2", fmt::format(FMT_COMPILE("{arg:3.1f}"), "arg"_a = 4.2));
 #  endif
 }