Throw the correct exception type

Author: tannergooding

src/coreclr/jit/importercalls.cpp

@@ -3247,7 +3247,7 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
 
                         default:
                         {
-                            return impUnsupportedNamedIntrinsic(CORINFO_HELP_THROW_PLATFORM_NOT_SUPPORTED, method, sig,
+                            return impUnsupportedNamedIntrinsic(CORINFO_HELP_THROW_TYPE_NOT_SUPPORTED, method, sig,
                                                                 mustExpand);
                         }
                     }
@@ -5206,9 +5206,9 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
 #ifdef TARGET_WASM
         NYI_WASM("Unhandled must expand intrinsic");
 #else
-        assert(!"Unhandled must expand intrinsic, throwing PlatformNotSupportedException");
+        assert(!"Unhandled must expand intrinsic, throwing NotImplementedException");
 #endif
-        return impUnsupportedNamedIntrinsic(CORINFO_HELP_THROW_PLATFORM_NOT_SUPPORTED, method, sig, mustExpand);
+        return impUnsupportedNamedIntrinsic(CORINFO_HELP_THROW_NOT_IMPLEMENTED, method, sig, mustExpand);
     }
 
     // Optionally report if this intrinsic is special
@@ -12028,6 +12028,19 @@ GenTree* Compiler::impUnsupportedNamedIntrinsic(unsigned              helper,
 
     if (mustExpand || opts.OptimizationEnabled())
     {
+        if ((helper == CORINFO_HELP_THROW_TYPE_NOT_SUPPORTED) && IsReadyToRun())
+        {
+            if (!mustExpand)
+            {
+                return nullptr;
+            }
+
+            // If we're ready to run and must expand, we need to throw a PlatformNotSupportedException since
+            // there is no R2R helper for the regular NotSupportedException with the relevant
+            // type message.
+            helper = CORINFO_HELP_THROW_PLATFORM_NOT_SUPPORTED;
+        }
+
         impSpillSideEffects(true, CHECK_SPILL_ALL DEBUGARG("impUnsupportedNamedIntrinsic"));
 
         for (unsigned i = 0; i < sig->numArgs; i++)