c++: Link extended FP conversion pedwarns to -Wnarrowing [PR111842]

Several users have been confused by the status of these warnings,
which can be misunderstood as "this might not be what you want",
rather than diagnostics required by the C++ standard. Add the text "ISO
C++ does not allow" to make this clear.

Also link them to -Wnarrowing so that they can be disabled or promoted
to errors independently of other pedwarns.

	PR c++/111842
	PR c++/112498

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal): Use OPT_Wnarrowing for
	pedwarns about illformed conversions involving extended
	floating-point types. Clarify that ISO C++ requires these
	diagnostics.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/ext-floating16.C: New test.
	* g++.dg/cpp23/ext-floating17.C: New test.

2 files changed, 82 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp23/ext-floating16.C b/gcc/testsuite/g++.dg/cpp23/ext-floating16.C
new file mode 100644
index 0000000..d6a562d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/ext-floating16.C
@@ -0,0 +1,40 @@
+// P1467R9 - Extended floating-point types and standard names.
+// { dg-do compile { target c++23 } }
+// { dg-options "-pedantic-errors -Wno-narrowing" }
+// { dg-add-options float16 }
+// { dg-add-options float32 }
+// { dg-add-options float64 }
+// { dg-add-options float128 }
+
+#include "ext-floating.h"
+
+#ifdef __STRICT_ANSI__
+#undef __SIZEOF_FLOAT128__
+#endif
+
+using namespace std;
+
+#ifdef __STDCPP_FLOAT16_T__
+#ifdef __STDCPP_FLOAT32_T__
+float16_t f16c = 1.0F32;		// { dg-bogus "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float32' with greater conversion rank" "" { target { float16 && float32 } } }
+#endif
+#ifdef __STDCPP_FLOAT64_T__
+float16_t f16e = 1.0F64;		// { dg-bogus "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float64' with greater conversion rank" "" { target { float16 && float64 } } }
+#endif
+#ifdef __STDCPP_FLOAT128_T__
+float16_t f16g = 1.0F128;		// { dg-bogus "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float128' with greater conversion rank" "" { target { float16 && float128 } } }
+#endif
+#endif
+#ifdef __STDCPP_FLOAT32_T__
+#ifdef __STDCPP_FLOAT64_T__
+float32_t f32e = 1.0F64;		// { dg-bogus "converting to 'std::float32_t' \\\{aka '_Float32'\\\} from '_Float64' with greater conversion rank" "" { target { float32 && float64 } } }
+#endif
+#ifdef __STDCPP_FLOAT128_T__
+float32_t f32g = 1.0F128;		// { dg-bogus "converting to 'std::float32_t' \\\{aka '_Float32'\\\} from '_Float128' with greater conversion rank" "" { target { float32 && float128 } } }
+#endif
+#endif
+#ifdef __STDCPP_FLOAT64_T__
+#ifdef __STDCPP_FLOAT128_T__
+float64_t f64g = 1.0F128;		// { dg-bogus "converting to 'std::float64_t' \\\{aka '_Float64'\\\} from '_Float128' with greater conversion rank" "" { target { float64 && float128 } } }
+#endif
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp23/ext-floating17.C b/gcc/testsuite/g++.dg/cpp23/ext-floating17.C
new file mode 100644
index 0000000..796e045
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/ext-floating17.C
@@ -0,0 +1,42 @@
+// P1467R9 - Extended floating-point types and standard names.
+// { dg-do compile { target c++23 } }
+// { dg-options "-Werror=narrowing" }
+// { dg-add-options float16 }
+// { dg-add-options float32 }
+// { dg-add-options float64 }
+// { dg-add-options float128 }
+// { dg-prune-output "some warnings being treated as errors" }
+
+#include "ext-floating.h"
+
+#ifdef __STRICT_ANSI__
+#undef __SIZEOF_FLOAT128__
+#endif
+
+using namespace std;
+
+#ifdef __STDCPP_FLOAT16_T__
+#ifdef __STDCPP_FLOAT32_T__
+float16_t f16c = 1.0F32;		// { dg-error "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float32' with greater conversion rank" "" { target { float16 && float32 } } }
+#endif
+#ifdef __STDCPP_FLOAT64_T__
+float16_t f16e = 1.0F64;		// { dg-error "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float64' with greater conversion rank" "" { target { float16 && float64 } } }
+#endif
+#ifdef __STDCPP_FLOAT128_T__
+float16_t f16g = 1.0F128;		// { dg-error "converting to 'std::float16_t' \\\{aka '_Float16'\\\} from '_Float128' with greater conversion rank" "" { target { float16 && float128 } } }
+#endif
+#endif
+#ifdef __STDCPP_FLOAT32_T__
+#ifdef __STDCPP_FLOAT64_T__
+float32_t f32e = 1.0F64;		// { dg-error "converting to 'std::float32_t' \\\{aka '_Float32'\\\} from '_Float64' with greater conversion rank" "" { target { float32 && float64 } } }
+#endif
+#ifdef __STDCPP_FLOAT128_T__
+float32_t f32g = 1.0F128;		// { dg-error "converting to 'std::float32_t' \\\{aka '_Float32'\\\} from '_Float128' with greater conversion rank" "" { target { float32 && float128 } } }
+#endif
+#endif
+#ifdef __STDCPP_FLOAT64_T__
+#ifdef __STDCPP_FLOAT128_T__
+float64_t f64g = 1.0F128;		// { dg-error "converting to 'std::float64_t' \\\{aka '_Float64'\\\} from '_Float128' with greater conversion rank" "" { target { float64 && float128 } } }
+#endif
+#endif
+