BUG: Fortify string casts against floating point warnings

This removes the check for floating point warnings, which is enough
in practice.  (In principle ufuncs or structured dtypes can chain
casts in a way that causes us to check anyway.)

It also checks for isfinite in the scalar repr code so the warnings
shouldn't be set to begin with.

Closes gh-22843

2 files changed, 10 insertions, 4 deletions

diff --git a/numpy/core/src/multiarray/convert_datatype.c b/numpy/core/src/multiarray/convert_datatype.c
index 3973fc795..14341c103 100644
--- a/numpy/core/src/multiarray/convert_datatype.c
+++ b/numpy/core/src/multiarray/convert_datatype.c
@@ -2880,7 +2880,7 @@ add_other_to_and_from_string_cast(
         .name = "legacy_cast_to_string",
         .nin = 1,
         .nout = 1,
-        .flags = NPY_METH_REQUIRES_PYAPI,
+        .flags = NPY_METH_REQUIRES_PYAPI | NPY_METH_NO_FLOATINGPOINT_ERRORS,
         .dtypes = dtypes,
         .slots = slots,
     };
diff --git a/numpy/core/src/multiarray/scalartypes.c.src b/numpy/core/src/multiarray/scalartypes.c.src
index bb5d36ba1..57336589e 100644
--- a/numpy/core/src/multiarray/scalartypes.c.src
+++ b/numpy/core/src/multiarray/scalartypes.c.src
@@ -896,15 +896,21 @@ static PyObject *
 @name@type_@kind@_either(npy_@name@ val, TrimMode trim_pos, TrimMode trim_sci,
                          npy_bool sign)
 {
-    npy_@name@ absval;
 
     if (npy_legacy_print_mode <= 113) {
         return legacy_@name@_format@kind@(val);
     }
 
-    absval = val < 0 ? -val : val;
+    int use_positional;
+    if (npy_isnan(val) || val == 0) {
+         use_positional = 1;
+    }
+    else {
+        npy_@name@ absval = val < 0 ? -val : val;
+        use_positional = absval < 1.e16L && absval >= 1.e-4L;
+    }
 
-    if (absval == 0 || (absval < 1.e16L && absval >= 1.e-4L) ) {
+    if (use_positional) {
         return format_@name@(val, 0, -1, sign, trim_pos, -1, -1, -1);
     }
     return format_@name@(val, 1, -1, sign, trim_sci, -1, -1, -1);