c++: Fix up floating point conversion rank comparison for _Float32 and float if float/double are same size [PR115511]

On AVR and SH with some options sizeof (float) == sizeof (double) and
the 2 types have the same set of values.
http://eel.is/c++draft/conv.rank#2.2 for this says that double still
has bigger rank than float and http://eel.is/c++draft/conv.rank#2.2
says that extended type with the same set of values as more than one
standard floating point type shall have the same rank as double.
I've implemented the latter rule as
   if (cnt > 1 && mv2 == long_double_type_node)
     return -2;
with the _Float64/double/long double case having same mode case (various
targets with -mlong-double-64) in mind.
But never thought there are actually targets where float and double
are the same, that needs handling too, if cnt > 1 (that is the extended
type mv1 has same set of values as 2 or 3 of float/double/long double)
and mv2 is float, we need to return 2, because mv1 in that case should
have same rank as double and double has bigger rank than float.

2024-06-17  Jakub Jelinek  <jakub@redhat.com>

	PR target/111343
	PR c++/115511
	* typeck.cc (cp_compare_floating_point_conversion_ranks): If an
	extended floating point type mv1 has same set of values as more
	than one standard floating point type and mv2 is float, return 2.

	* g++.dg/cpp23/ext-floating18.C: New test.

1 files changed, 26 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp23/ext-floating18.C b/gcc/testsuite/g++.dg/cpp23/ext-floating18.C
new file mode 100644
index 0000000..ece2546
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/ext-floating18.C
@@ -0,0 +1,26 @@
+// P1467R9 - Extended floating-point types and standard names.
+// { dg-do compile { target c++23 } }
+// { dg-options "" }
+// { dg-add-options float32 }
+
+constexpr int foo (float) { return 1; }
+constexpr int foo (double) { return 2; }
+constexpr int foo (long double) { return 3; }
+
+#ifdef __STDCPP_FLOAT32_T__
+#if __FLT_MAX_EXP__ == __FLT32_MAX_EXP__ \
+    && __FLT_MAX_DIG__ == __FLT32_MAX_DIG__
+#if __FLT_MAX_EXP__ == __DBL_MAX_EXP__ \
+    && __FLT_MAX_DIG__ == __DBL_MAX_DIG__
+static_assert (foo (1.0f32) == 2);
+#else
+static_assert (foo (1.0f32) == 1);
+#endif
+#endif
+#endif
+#ifdef __STDCPP_FLOAT64_T__
+#if __DBL_MAX_EXP__ == __FLT64_MAX_EXP__ \
+    && __DBL_MAX_DIG__ == __FLT64_MAX_DIG__
+static_assert (foo (1.0f64) == 2);
+#endif
+#endif