`std::log1p(x)` with long double appears to use double precision

I think I can guess the source of the problem, but I want to be sure.

*check_log1p.cpp*

```cpp
#include
#include
#include

int main()
{
printf("numeric_limits::digits = %d\n",
std::numeric_limits::digits);
long double x = -5.0000000000000000000000005e-35L;
printf("%40.33Le\n", x);
long double yld = std::log1p(x);
printf("%40.33Le\n", yld);
long double yd = static_cast(std::log1p(static_cast(x)));
printf("%40.33Le\n", yd);
}
```
Compile and run:
```
% em++ check_log1p.cpp -sPRINTF_LONG_DOUBLE
% node a.out.js
numeric_limits::digits = 113
-5.000000000000000000000000500000000e-35
-4.999999999999999638373019459082555e-35
-4.999999999999999638373019459082555e-35
```
The value of `x` is small enough that `log1p(x)` should equal `x`. The last two lines being the same shows that the calculation of `std::log1p(x)` is being done with `double`, not `long double`.

My guess is that, even though you provide 113 bits precision in `long double`, your `std` math library does not support that precision. Is that correct?

The silent downcast to `double` is a nasty wart in the API. Is there any way to enable an error or warning when that happens?

**Version of emscripten/emsdk:**

```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.61 (67fa4c16496b157a7fc3377afd69ee0445e8a6e3)
clang version 19.0.0git (https:/github.com/llvm/llvm-project 7cfffe74eeb68fbb3fb9706ac7071f8caeeb6520)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /Users/warren/repos/git/forks/emsdk/upstream/bin
```

**Full link command and output with `-v` appended:**

Here's the compile command with `-v` added:

```
% em++ -v check_log1p.cpp -sPRINTF_LONG_DOUBLE
"/Users/warren/repos/git/forks/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -v check_log1p.cpp -c -o /var/folders/6f/wyccw81j5kj1c7v6p47zr9fr0000gn/T/emscripten_temp_3rlg3nz6/check_log1p_0.o
clang version 19.0.0git (https:/github.com/llvm/llvm-project 7cfffe74eeb68fbb3fb9706ac7071f8caeeb6520)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /Users/warren/repos/git/forks/emsdk/upstream/bin
(in-process)
"/Users/warren/repos/git/forks/emsdk/upstream/bin/clang-19" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name check_log1p.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/Users/warren/code-snippets/c++/test-emscripten-stuff -v -fcoverage-compilation-dir=/Users/warren/code-snippets/c++/test-emscripten-stuff -resource-dir /Users/warren/repos/git/forks/emsdk/upstream/lib/clang/19 -D EMSCRIPTEN -isysroot /Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /Users/warren/repos/git/forks/emsdk/upstream/lib/clang/19/include -internal-isystem /Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /var/folders/6f/wyccw81j5kj1c7v6p47zr9fr0000gn/T/emscripten_temp_3rlg3nz6/check_log1p_0.o -x c++ check_log1p.cpp
clang -cc1 version 19.0.0git based upon LLVM 19.0.0git default target x86_64-apple-darwin22.6.0
ignoring nonexistent directory "/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/compat
/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
/Users/warren/repos/git/forks/emsdk/upstream/lib/clang/19/include
/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
/Users/warren/repos/git/forks/emsdk/upstream/bin/clang --version
/Users/warren/repos/git/forks/emsdk/upstream/bin/wasm-ld -o a.out.wasm -L/Users/warren/repos/git/forks/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten /var/folders/6f/wyccw81j5kj1c7v6p47zr9fr0000gn/T/emscripten_temp_3rlg3nz6/check_log1p_0.o -lGL-getprocaddr -lal -lhtml5 -lprintf_long_double-debug -lstubs-debug -lnoexit -lc-debug -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /var/folders/6f/wyccw81j5kj1c7v6p47zr9fr0000gn/T/tmp3en6quuulibemscripten_js_symbols.so --strip-debug --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__get_temp_ret --export=__set_temp_ret --export=__wasm_call_ctors --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --stack-first --table-base=1
/Users/warren/repos/git/forks/emsdk/upstream/bin/llvm-objcopy a.out.wasm a.out.wasm --remove-section=.debug* --remove-section=producers
/Users/warren/repos/git/forks/emsdk/upstream/bin/wasm-emscripten-finalize --dyncalls-i64 --pass-arg=legalize-js-interface-exported-helpers a.out.wasm -o a.out.wasm --detect-features
/Users/warren/repos/git/forks/emsdk/node/18.20.3_64bit/bin/node /Users/warren/repos/git/forks/emsdk/upstream/emscripten/src/compiler.mjs /var/folders/6f/wyccw81j5kj1c7v6p47zr9fr0000gn/T/tmp4ac3r4z7.json
```