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

[mizvekov]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/160433.diff

1 Files Affected:

• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+5-17)

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 62e867c44ad14..b0d2554d819cb 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/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(),

[mizvekov]

@erichkeane sorry to ping so soon, but this patch is quite small and its starting to form a deep stack of patches.

[erichkeane]

I'm not positive exactly what is the effect of this, but I'm willing to ship it for now and see.