[c++20] Implement P0846R0: allow (ADL-only) calls to template-ids whose

template name is not visible to unqualified lookup.

In order to support this without a severe degradation in our ability to
diagnose typos in template names, this change significantly restructures
the way we handle template-id-shaped syntax for which lookup of the
template name finds nothing.

Instead of eagerly diagnosing an undeclared template name, we now form a
placeholder template-name representing a name that is known to not find
any templates. When the parser sees such a name, it attempts to
disambiguate whether we have a less-than comparison or a template-id.
Any diagnostics or typo-correction for the name are delayed until its
point of use.

The upshot should be a small improvement of our diagostic quality
overall: we now take more syntactic context into account when trying to
resolve an undeclared identifier on the left hand side of a '<'. In
fact, this works well enough that the backwards-compatible portion (for
an undeclared identifier rather than a lookup that finds functions but
no function templates) is enabled in all language modes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@360308 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 61 insertions, 36 deletions

diff --git a/lib/Parse/ParseDeclCXX.cpp b/lib/Parse/ParseDeclCXX.cpp
index c4fe23c60c..e884884163 100644
--- a/lib/Parse/ParseDeclCXX.cpp
+++ b/lib/Parse/ParseDeclCXX.cpp
@@ -597,10 +597,11 @@ bool Parser::ParseUsingDeclarator(DeclaratorContext Context,
 
   // Parse nested-name-specifier.
   IdentifierInfo *LastII = nullptr;
-  ParseOptionalCXXScopeSpecifier(D.SS, nullptr, /*EnteringContext=*/false,
-                                 /*MayBePseudoDtor=*/nullptr,
-                                 /*IsTypename=*/false,
-                                 /*LastII=*/&LastII);
+  if (ParseOptionalCXXScopeSpecifier(D.SS, nullptr, /*EnteringContext=*/false,
+                                     /*MayBePseudoDtor=*/nullptr,
+                                     /*IsTypename=*/false,
+                                     /*LastII=*/&LastII))
+    return true;
   if (D.SS.isInvalid())
     return true;
 
@@ -1111,7 +1112,8 @@ TypeResult Parser::ParseBaseTypeSpecifier(SourceLocation &BaseLoc,
 
   // Parse optional nested-name-specifier
   CXXScopeSpec SS;
-  ParseOptionalCXXScopeSpecifier(SS, nullptr, /*EnteringContext=*/false);
+  if (ParseOptionalCXXScopeSpecifier(SS, nullptr, /*EnteringContext=*/false))
+    return true;
 
   BaseLoc = Tok.getLocation();
 
@@ -1135,7 +1137,8 @@ TypeResult Parser::ParseBaseTypeSpecifier(SourceLocation &BaseLoc,
   if (Tok.is(tok::annot_template_id)) {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     if (TemplateId->Kind == TNK_Type_template ||
-        TemplateId->Kind == TNK_Dependent_template_name) {
+        TemplateId->Kind == TNK_Dependent_template_name ||
+        TemplateId->Kind == TNK_Undeclared_template) {
       AnnotateTemplateIdTokenAsType(/*IsClassName*/true);
 
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
@@ -1554,6 +1557,36 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
 
   TemplateParameterLists *TemplateParams = TemplateInfo.TemplateParams;
 
+  auto RecoverFromUndeclaredTemplateName = [&](IdentifierInfo *Name,
+                                               SourceLocation NameLoc,
+                                               SourceRange TemplateArgRange,
+                                               bool KnownUndeclared) {
+    Diag(NameLoc, diag::err_explicit_spec_non_template)
+        << (TemplateInfo.Kind == ParsedTemplateInfo::ExplicitInstantiation)
+        << TagTokKind << Name << TemplateArgRange << KnownUndeclared;
+
+    // Strip off the last template parameter list if it was empty, since
+    // we've removed its template argument list.
+    if (TemplateParams && TemplateInfo.LastParameterListWasEmpty) {
+      if (TemplateParams->size() > 1) {
+        TemplateParams->pop_back();
+      } else {
+        TemplateParams = nullptr;
+        const_cast<ParsedTemplateInfo &>(TemplateInfo).Kind =
+            ParsedTemplateInfo::NonTemplate;
+      }
+    } else if (TemplateInfo.Kind == ParsedTemplateInfo::ExplicitInstantiation) {
+      // Pretend this is just a forward declaration.
+      TemplateParams = nullptr;
+      const_cast<ParsedTemplateInfo &>(TemplateInfo).Kind =
+          ParsedTemplateInfo::NonTemplate;
+      const_cast<ParsedTemplateInfo &>(TemplateInfo).TemplateLoc =
+          SourceLocation();
+      const_cast<ParsedTemplateInfo &>(TemplateInfo).ExternLoc =
+          SourceLocation();
+    }
+  };
+
   // Parse the (optional) class name or simple-template-id.
   IdentifierInfo *Name = nullptr;
   SourceLocation NameLoc;
@@ -1574,38 +1607,26 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
         // try to give any location information for the list.
         LAngleLoc = RAngleLoc = SourceLocation();
       }
-
-      Diag(NameLoc, diag::err_explicit_spec_non_template)
-          << (TemplateInfo.Kind == ParsedTemplateInfo::ExplicitInstantiation)
-          << TagTokKind << Name << SourceRange(LAngleLoc, RAngleLoc);
-
-      // Strip off the last template parameter list if it was empty, since
-      // we've removed its template argument list.
-      if (TemplateParams && TemplateInfo.LastParameterListWasEmpty) {
-        if (TemplateParams->size() > 1) {
-          TemplateParams->pop_back();
-        } else {
-          TemplateParams = nullptr;
-          const_cast<ParsedTemplateInfo&>(TemplateInfo).Kind
-            = ParsedTemplateInfo::NonTemplate;
-        }
-      } else if (TemplateInfo.Kind
-                                == ParsedTemplateInfo::ExplicitInstantiation) {
-        // Pretend this is just a forward declaration.
-        TemplateParams = nullptr;
-        const_cast<ParsedTemplateInfo&>(TemplateInfo).Kind
-          = ParsedTemplateInfo::NonTemplate;
-        const_cast<ParsedTemplateInfo&>(TemplateInfo).TemplateLoc
-          = SourceLocation();
-        const_cast<ParsedTemplateInfo&>(TemplateInfo).ExternLoc
-          = SourceLocation();
-      }
+      RecoverFromUndeclaredTemplateName(
+          Name, NameLoc, SourceRange(LAngleLoc, RAngleLoc), false);
     }
   } else if (Tok.is(tok::annot_template_id)) {
     TemplateId = takeTemplateIdAnnotation(Tok);
     NameLoc = ConsumeAnnotationToken();
 
-    if (TemplateId->Kind != TNK_Type_template &&
+    if (TemplateId->Kind == TNK_Undeclared_template) {
+      // Try to resolve the template name to a type template.
+      Actions.ActOnUndeclaredTypeTemplateName(getCurScope(), TemplateId->Template,
+                                              TemplateId->Kind, NameLoc, Name);
+      if (TemplateId->Kind == TNK_Undeclared_template) {
+        RecoverFromUndeclaredTemplateName(
+            Name, NameLoc,
+            SourceRange(TemplateId->LAngleLoc, TemplateId->RAngleLoc), true);
+        TemplateId = nullptr;
+      }
+    }
+
+    if (TemplateId && TemplateId->Kind != TNK_Type_template &&
         TemplateId->Kind != TNK_Dependent_template_name) {
       // The template-name in the simple-template-id refers to
       // something other than a class template. Give an appropriate
@@ -1616,7 +1637,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
 
       // FIXME: Name may be null here.
       Diag(TemplateId->LAngleLoc, diag::err_template_spec_syntax_non_template)
-        << TemplateId->Name << static_cast<int>(TemplateId->Kind) << Range;
+          << TemplateId->Name << static_cast<int>(TemplateId->Kind) << Range;
 
       DS.SetTypeSpecError();
       SkipUntil(tok::semi, StopBeforeMatch);
@@ -3451,7 +3472,8 @@ void Parser::ParseConstructorInitializer(Decl *ConstructorDecl) {
 MemInitResult Parser::ParseMemInitializer(Decl *ConstructorDecl) {
   // parse '::'[opt] nested-name-specifier[opt]
   CXXScopeSpec SS;
-  ParseOptionalCXXScopeSpecifier(SS, nullptr, /*EnteringContext=*/false);
+  if (ParseOptionalCXXScopeSpecifier(SS, nullptr, /*EnteringContext=*/false))
+    return true;
 
   // : identifier
   IdentifierInfo *II = nullptr;
@@ -3477,11 +3499,14 @@ MemInitResult Parser::ParseMemInitializer(Decl *ConstructorDecl) {
                                            ? takeTemplateIdAnnotation(Tok)
                                            : nullptr;
     if (TemplateId && (TemplateId->Kind == TNK_Type_template ||
-                       TemplateId->Kind == TNK_Dependent_template_name)) {
+                       TemplateId->Kind == TNK_Dependent_template_name ||
+                       TemplateId->Kind == TNK_Undeclared_template)) {
       AnnotateTemplateIdTokenAsType(/*IsClassName*/true);
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
       TemplateTypeTy = getTypeAnnotation(Tok);
       ConsumeAnnotationToken();
+      if (!TemplateTypeTy)
+        return true;
     } else {
       Diag(Tok, diag::err_expected_member_or_base_name);
       return true;