[clang] simplify constant template parameter type deduction for partial ordering

This simplifies things a little bit. This is mostly NFCish, except
the reference type deduction workaround now applies in partial ordering
as well, but we don't have any test cases and any reason to suspect
this is significant.

Author: mizvekov

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -483,7 +483,7 @@ DeduceNonTypeTemplateArgument(Sema &S, TemplateParameterList *TemplateParams,
     return TemplateDeductionResult::Inconsistent;
   }
   Deduced[NTTP.getIndex()] = Result;
-  if (!S.getLangOpts().CPlusPlus17)
+  if (!S.getLangOpts().CPlusPlus17 && !PartialOrdering)
     return TemplateDeductionResult::Success;
 
   if (NTTP.isExpandedParameterPack())
@@ -2653,23 +2653,11 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
       switch (A.getKind()) {
       case TemplateArgument::Expression: {
         const Expr *E = A.getAsExpr();
-        // When checking NTTP, if either the parameter or the argument is
-        // dependent, as there would be otherwise nothing to deduce, we force
-        // the argument to the parameter type using this dependent implicit
-        // cast, in order to maintain invariants. Now we can deduce the
-        // resulting type from the original type, and deduce the original type
-        // against the parameter we are checking.
-        if (const auto *ICE = dyn_cast<ImplicitCastExpr>(E);
-            ICE && ICE->getCastKind() == clang::CK_Dependent) {
+        // The type of the value is the type of the expression before any
+        // implicit conversions.
+        if (const auto *ICE = dyn_cast<ImplicitCastExpr>(E)) {
           E = ICE->getSubExpr();
-          if (auto Result = DeduceTemplateArgumentsByTypeMatch(
-                  S, TemplateParams, ICE->getType(), E->getType(), Info,
-                  Deduced, TDF_SkipNonDependent,
-                  PartialOrdering ? PartialOrderingKind::NonCall
-                                  : PartialOrderingKind::None,
-                  /*DeducedFromArrayBound=*/false, HasDeducedAnyParam);
-              Result != TemplateDeductionResult::Success)
-            return Result;
+          assert(!isa<ImplicitCastExpr>(E));
         }
         return DeduceNonTypeTemplateArgument(
             S, TemplateParams, NTTP, DeducedTemplateArgument(A), E->getType(),