[Clang][WIP][RFC] Bypass TAD during overload resolution if a perfect match exists

This implements the same overload resolution behavior as GCC,
as described in https://wg21.link/p3606 (section 1-2, not 3)

If during overload resolution, there is a non-template candidate
that would be always be picked - because each of the argument
is a perfect match (ie the source and target types are the same),
we do not perform deduction for any template candidate
that might exists.

The goal is to be able to merge llvm#122423 without being too disruptive.

This change means that the selection of the best viable candidate and
template argument deduction become interleaved.

To avoid rewriting half of Clang we store in `OverloadCandidateSet`
enough information to be able to deduce template candidates from
`OverloadCandidateSet::BestViableFunction`. Which means
the lifetime of any object used by template argument must outlive
a call to `Add*Template*Candidate`.

This two phase resolution is not performed for some initialization
as there are cases where template candidate are better match
in these cases per the standard. It's also bypassed for code completion.

The change has a nice impact on compile times
https://llvm-compile-time-tracker.com/compare.php?from=719b029c16eeb1035da522fd641dfcc4cee6be74&to=bf7041045c9408490c395230047c5461de72fc39&stat=instructions%3Au

Fixes llvm#62096
Fixes llvm#74581

Author: cor3ntin

clang/include/clang/Sema/Overload.h

@@ -38,6 +38,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <utility>
+#include <variant>
 
 namespace clang {
 
@@ -743,6 +744,12 @@ class Sema;
       Standard.setAllToTypes(T);
     }
 
+    bool isPerfect(const ASTContext &C) const {
+      return (isStandard() && Standard.isIdentityConversion() &&
+              C.hasSameType(Standard.getFromType(), Standard.getToType(2))) ||
+             getKind() == StaticObjectArgumentConversion;
+    }
+
     // True iff this is a conversion sequence from an initializer list to an
     // array or std::initializer.
     bool hasInitializerListContainerType() const {
@@ -979,6 +986,18 @@ class Sema;
       return false;
     }
 
+    bool isPerfectMatch(const ASTContext &Ctx) const {
+      if (!Viable)
+        return false;
+      for (auto &C : Conversions) {
+        if (!C.isInitialized())
+          return false;
+        if (!C.isPerfect(Ctx))
+          return false;
+      }
+      return true;
+    }
+
     bool TryToFixBadConversion(unsigned Idx, Sema &S) {
       bool CanFix = Fix.tryToFixConversion(
                       Conversions[Idx].Bad.FromExpr,
@@ -1015,6 +1034,61 @@ class Sema;
           RewriteKind(CRK_None) {}
   };
 
+  struct NonDeducedConversionTemplateOverloadCandidate {
+    FunctionTemplateDecl *FunctionTemplate;
+    DeclAccessPair FoundDecl;
+    CXXRecordDecl *ActingContext;
+    Expr *From;
+    QualType ToType;
+
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AllowObjCConversionOnExplicit : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AllowExplicit : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AllowResultConversion : 1;
+  };
+
+  struct NonDeducedMethodTemplateOverloadCandidate {
+    FunctionTemplateDecl *FunctionTemplate;
+    DeclAccessPair FoundDecl;
+    ArrayRef<Expr *> Args;
+    CXXRecordDecl *ActingContext;
+    Expr::Classification ObjectClassification;
+    QualType ObjectType;
+
+    OverloadCandidateParamOrder PO;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned SuppressUserConversions : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned PartialOverloading : 1;
+  };
+
+  struct NonDeducedFunctionTemplateOverloadCandidate {
+    FunctionTemplateDecl *FunctionTemplate;
+    DeclAccessPair FoundDecl;
+    ArrayRef<Expr *> Args;
+
+    CallExpr::ADLCallKind IsADLCandidate;
+    OverloadCandidateParamOrder PO;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned SuppressUserConversions : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned PartialOverloading : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AllowExplicit : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned AggregateCandidateDeduction : 1;
+  };
+
+  using NonDeducedTemplateOverloadCandidate =
+      std::variant<NonDeducedConversionTemplateOverloadCandidate,
+                   NonDeducedMethodTemplateOverloadCandidate,
+                   NonDeducedFunctionTemplateOverloadCandidate>;
+
+  static_assert(
+      std::is_trivially_destructible_v<NonDeducedTemplateOverloadCandidate>);
+
   /// OverloadCandidateSet - A set of overload candidates, used in C++
   /// overload resolution (C++ 13.3).
   class OverloadCandidateSet {
@@ -1043,6 +1117,8 @@ class Sema;
       /// C++ [over.match.call.general]
       /// Resolve a call through the address of an overload set.
       CSK_AddressOfOverloadSet,
+
+      CSK_CodeCompletion,
     };
 
     /// Information about operator rewrites to consider when adding operator
@@ -1116,6 +1192,7 @@ class Sema;
   private:
     SmallVector<OverloadCandidate, 16> Candidates;
     llvm::SmallPtrSet<uintptr_t, 16> Functions;
+    SmallVector<NonDeducedTemplateOverloadCandidate, 8> NonDeducedCandidates;
 
     // Allocator for ConversionSequenceLists. We store the first few of these
     // inline to avoid allocation for small sets.
@@ -1126,7 +1203,7 @@ class Sema;
     OperatorRewriteInfo RewriteInfo;
 
     constexpr static unsigned NumInlineBytes =
-        24 * sizeof(ImplicitConversionSequence);
+        32 * sizeof(ImplicitConversionSequence);
     unsigned NumInlineBytesUsed = 0;
     alignas(void *) char InlineSpace[NumInlineBytes];
 
@@ -1144,8 +1221,8 @@ class Sema;
       // It's simpler if this doesn't need to consider alignment.
       static_assert(alignof(T) == alignof(void *),
                     "Only works for pointer-aligned types.");
-      static_assert(std::is_trivial<T>::value ||
-                        std::is_same<ImplicitConversionSequence, T>::value,
+      static_assert(std::is_trivially_destructible_v<T> ||
+                        (std::is_same_v<ImplicitConversionSequence, T>),
                     "Add destruction logic to OverloadCandidateSet::clear().");
 
       unsigned NBytes = sizeof(T) * N;
@@ -1199,8 +1276,12 @@ class Sema;
     iterator begin() { return Candidates.begin(); }
     iterator end() { return Candidates.end(); }
 
-    size_t size() const { return Candidates.size(); }
-    bool empty() const { return Candidates.empty(); }
+    size_t size() const {
+      return Candidates.size() + NonDeducedCandidates.size();
+    }
+    bool empty() const {
+      return Candidates.empty() && NonDeducedCandidates.empty();
+    }
 
     /// Allocate storage for conversion sequences for NumConversions
     /// conversions.
@@ -1216,6 +1297,19 @@ class Sema;
       return ConversionSequenceList(Conversions, NumConversions);
     }
 
+    llvm::MutableArrayRef<Expr *> getPersistentArgsArray(unsigned N) {
+      Expr **Exprs = slabAllocate<Expr *>(N);
+      return llvm::MutableArrayRef<Expr *>(Exprs, N);
+    }
+
+    template <typename... T>
+    llvm::MutableArrayRef<Expr *> getPersistentArgsArray(T *...Exprs) {
+      llvm::MutableArrayRef<Expr *> Arr =
+          getPersistentArgsArray(sizeof...(Exprs));
+      llvm::copy(std::initializer_list<Expr *>{Exprs...}, Arr.data());
+      return Arr;
+    }
+
     /// Add a new candidate with NumConversions conversion sequence slots
     /// to the overload set.
     OverloadCandidate &addCandidate(unsigned NumConversions = 0,
@@ -1231,10 +1325,36 @@ class Sema;
       return C;
     }
 
+    void AddNonDeducedTemplateCandidate(
+        FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+        ArrayRef<Expr *> Args, bool SuppressUserConversions,
+        bool PartialOverloading, bool AllowExplicit,
+        CallExpr::ADLCallKind IsADLCandidate, OverloadCandidateParamOrder PO,
+        bool AggregateCandidateDeduction);
+
+    void AddNonDeducedMethodTemplateCandidate(
+        FunctionTemplateDecl *MethodTmpl, DeclAccessPair FoundDecl,
+        CXXRecordDecl *ActingContext, QualType ObjectType,
+        Expr::Classification ObjectClassification, ArrayRef<Expr *> Args,
+        bool SuppressUserConversions, bool PartialOverloading,
+        OverloadCandidateParamOrder PO);
+
+    void AddNonDeducedConversionTemplateCandidate(
+        FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+        CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
+        bool AllowObjCConversionOnExplicit, bool AllowExplicit,
+        bool AllowResultConversion);
+
+    void InjectNonDeducedTemplateCandidates(Sema &S);
+
     /// Find the best viable function on this overload set, if it exists.
     OverloadingResult BestViableFunction(Sema &S, SourceLocation Loc,
                                          OverloadCandidateSet::iterator& Best);
 
+    OverloadingResult
+    BestViableFunctionImpl(Sema &S, SourceLocation Loc,
+                           OverloadCandidateSet::iterator &Best);
+
     SmallVector<OverloadCandidate *, 32> CompleteCandidates(
         Sema &S, OverloadCandidateDisplayKind OCD, ArrayRef<Expr *> Args,
         SourceLocation OpLoc = SourceLocation(),

clang/include/clang/Sema/Sema.h

@@ -60,6 +60,7 @@
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/ExternalSemaSource.h"
 #include "clang/Sema/IdentifierResolver.h"
+#include "clang/Sema/Overload.h"
 #include "clang/Sema/Ownership.h"
 #include "clang/Sema/ParsedAttr.h"
 #include "clang/Sema/Redeclaration.h"
@@ -10342,9 +10343,26 @@ class Sema final : public SemaBase {
       OverloadCandidateSet &CandidateSet, bool SuppressUserConversions = false,
       bool PartialOverloading = false, OverloadCandidateParamOrder PO = {});
 
+  void AddMethodTemplateCandidateImmediately(
+      OverloadCandidateSet &CandidateSet, FunctionTemplateDecl *MethodTmpl,
+      DeclAccessPair FoundDecl, CXXRecordDecl *ActingContext,
+      TemplateArgumentListInfo *ExplicitTemplateArgs, QualType ObjectType,
+      Expr::Classification ObjectClassification, ArrayRef<Expr *> Args,
+      bool SuppressUserConversions, bool PartialOverloading,
+      OverloadCandidateParamOrder PO);
+
   /// Add a C++ function template specialization as a candidate
   /// in the candidate set, using template argument deduction to produce
   /// an appropriate function template specialization.
+
+  void AddTemplateOverloadCandidateImmediately(
+      OverloadCandidateSet &CandidateSet,
+      FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+      TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
+      bool SuppressUserConversions, bool PartialOverloading, bool AllowExplicit,
+      ADLCallKind IsADLCandidate, OverloadCandidateParamOrder PO,
+      bool AggregateCandidateDeduction);
+
   void AddTemplateOverloadCandidate(
       FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
@@ -10389,6 +10407,13 @@ class Sema final : public SemaBase {
       OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
       bool AllowExplicit, bool AllowResultConversion = true);
 
+  void AddTemplateConversionCandidateImmediately(
+      OverloadCandidateSet &CandidateSet,
+      FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+      CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
+      bool AllowObjCConversionOnExplicit, bool AllowExplicit,
+      bool AllowResultConversion);
+
   /// AddSurrogateCandidate - Adds a "surrogate" candidate function that
   /// converts the given @c Object to a function pointer via the
   /// conversion function @c Conversion, and then attempts to call it

clang/lib/Sema/SemaCodeComplete.cpp

@@ -6364,7 +6364,8 @@ SemaCodeCompletion::ProduceCallSignatureHelp(Expr *Fn, ArrayRef<Expr *> Args,
   Expr *NakedFn = Fn->IgnoreParenCasts();
   // Build an overload candidate set based on the functions we find.
   SourceLocation Loc = Fn->getExprLoc();
-  OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
+  OverloadCandidateSet CandidateSet(Loc,
+                                    OverloadCandidateSet::CSK_CodeCompletion);
 
   if (auto ULE = dyn_cast<UnresolvedLookupExpr>(NakedFn)) {
     SemaRef.AddOverloadedCallCandidates(ULE, ArgsWithoutDependentTypes,
@@ -6567,7 +6568,8 @@ QualType SemaCodeCompletion::ProduceConstructorSignatureHelp(
   // FIXME: Provide support for variadic template constructors.
 
   if (CRD) {
-    OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
+    OverloadCandidateSet CandidateSet(Loc,
+                                      OverloadCandidateSet::CSK_CodeCompletion);
     for (NamedDecl *C : SemaRef.LookupConstructors(CRD)) {
       if (auto *FD = dyn_cast<FunctionDecl>(C)) {
         // FIXME: we can't yet provide correct signature help for initializer

clang/lib/Sema/SemaInit.cpp

@@ -10043,12 +10043,15 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     //   When [...] the constructor [...] is a candidate by
     //    - [over.match.copy] (in all cases)
     if (TD) {
-      SmallVector<Expr *, 8> TmpInits;
-      for (Expr *E : Inits)
+      MutableArrayRef<Expr *> TmpInits =
+          Candidates.getPersistentArgsArray(Inits.size());
+      for (auto [I, E] : llvm::enumerate(Inits)) {
         if (auto *DI = dyn_cast<DesignatedInitExpr>(E))
-          TmpInits.push_back(DI->getInit());
+          TmpInits[I] = DI->getInit();
         else
-          TmpInits.push_back(E);
+          TmpInits[I] = E;
+      }
+
       AddTemplateOverloadCandidate(
           TD, FoundDecl, /*ExplicitArgs=*/nullptr, TmpInits, Candidates,
           /*SuppressUserConversions=*/false,