[clang-tidy] Fix cppcoreguidelines-narrowing-conversions false positive

Fix https://llvm.org/PR49498.
The check notices 2 sides of a conditional operator have types with a different constness and so tries to examine the implicit cast.
As one side is infinity, the float narrowing detection sees when its casted to a double(which it already was) it thinks the result is out of range.

I've fixed this by just disregarding expressions where the builtin type(without quals) match as no conversion would take place.

However this has opened a can of worms. Currenty `float a = std::numeric_limits<double>::infinity();` is marked as narrowing.
Whats more suspicious is `double a = std::numeric_limits<float>::infinity();` is also marked as narrowing.
It could be argued `double inf -> float inf` is narrowing, but `float inf -> double inf` definitely isnt.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D98416

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

@@ -510,6 +510,8 @@ void NarrowingConversionsCheck::handleBinaryOperator(const ASTContext &Context,
   const BuiltinType *RhsType = getBuiltinType(Rhs);
   if (RhsType == nullptr || LhsType == nullptr)
     return;
+  if (LhsType == RhsType)
+    return;
   if (RhsType->getKind() == BuiltinType::Bool && LhsType->isSignedInteger())
     return handleBooleanToSignedIntegral(Context, SourceLoc, Lhs, Rhs);
   if (RhsType->isInteger() && LhsType->getKind() == BuiltinType::Bool)
@@ -549,6 +551,8 @@ void NarrowingConversionsCheck::handleImplicitCast(
   const Expr &Rhs = *Cast.getSubExpr();
   if (Lhs.isInstantiationDependent() || Rhs.isInstantiationDependent())
     return;
+  if (getBuiltinType(Lhs) == getBuiltinType(Rhs))
+    return;
   if (handleConditionalOperator(Context, Lhs, Rhs))
     return;
   SourceLocation SourceLoc = Lhs.getExprLoc();

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

@@ -54,4 +54,11 @@ void narrow_fp_constants() {
   f = __builtin_nan("0"); // double NaN is not narrowing.
 }
 
+double false_positive_const_qualified_cast(bool t) {
+  double b = 1.0;
+  constexpr double a = __builtin_huge_val();
+  // PR49498 The constness difference of 'a' and 'b' results in an implicit cast.
+  return t ? b : a;
+}
+
 } // namespace floats