[clang-tidy] fix wrong float to float conversion check when floating point type is not standard type

compare type kind is the wrong way to compare floating point type compatibility.
more generic compatibility check is needed.

Author: HerrCai0907

clang-tools-extra/clang-tidy/bugprone/NarrowingConversionsCheck.cpp

@@ -513,7 +513,9 @@ void NarrowingConversionsCheck::handleFloatingCast(const ASTContext &Context,
       return;
     }
     const BuiltinType *FromType = getBuiltinType(Rhs);
-    if (ToType->getKind() < FromType->getKind())
+    if (!llvm::APFloatBase::isRepresentableBy(
+            Context.getFloatTypeSemantics(FromType->desugar()),
+            Context.getFloatTypeSemantics(ToType->desugar())))
       diagNarrowType(SourceLoc, Lhs, Rhs);
   }
 }

clang-tools-extra/docs/ReleaseNotes.rst

@@ -206,6 +206,11 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/forwarding-reference-overload>` check by fixing
   a crash when determining if an ``enable_if[_t]`` was found.
 
+- Improve :doc:`bugprone-narrowing-conversions
+  <clang-tidy/checks/bugprone/narrowing-conversions>` to avoid incorrect check
+  results when floating point type is not ``float``, ``double`` and
+  ``long double``.
+
 - Improved :doc:`bugprone-optional-value-conversion
   <clang-tidy/checks/bugprone/optional-value-conversion>` to support detecting
   conversion directly by ``std::make_unique`` and ``std::make_shared``.

clang-tools-extra/test/clang-tidy/checkers/bugprone/narrowing-conversions-narrowingfloatingpoint-option.cpp

@@ -36,6 +36,15 @@ void narrow_double_to_float_not_ok(double d) {
   f = narrow_double_to_float_return();
 }
 
+float narrow_float16_to_float_return(_Float16 f) {
+  return f;
+}
+
+_Float16 narrow_float_to_float16_return(float f) {
+  return f;
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: narrowing conversion from 'float' to '_Float16' [bugprone-narrowing-conversions]
+}
+
 void narrow_fp_constants() {
   float f;
   f = 0.5; // [dcl.init.list] 7.2 : in-range fp constant to narrower float is not a narrowing.