Diagnose missing 'template' keywords in more cases.

We track when we see a name-shaped expression followed by a '<' token
and parse the '<' as a comparison. Then:

 * if we see a token sequence that cannot possibly be an expression but
   can be a template argument (in particular, a type-id) that follows
   either a ',' or the '<', diagnose that the '<' was supposed to start
   a template argument list, and
 * if we see '>()', diagnose that the '<' was supposed to start a
   template argument list.

This only changes the diagnostic for error cases, and in practice
appears to catch the most common cases where a missing 'template'
keyword leads to parse errors within a template.

Differential Revision: https://reviews.llvm.org/D48571

llvm-svn: 335687

1 files changed, 27 insertions, 6 deletions

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 8f316024cc6..68a5196f6b8 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -508,20 +508,41 @@ void Sema::diagnoseExprIntendedAsTemplateName(Scope *S, ExprResult TemplateName,
 
   DeclContext *LookupCtx = nullptr;
   NamedDecl *Found = nullptr;
+  bool MissingTemplateKeyword = false;
 
   // Figure out what name we looked up.
-  if (auto *ME = dyn_cast<MemberExpr>(TemplateName.get())) {
+  if (auto *DRE = dyn_cast<DeclRefExpr>(TemplateName.get())) {
+    NameInfo = DRE->getNameInfo();
+    SS.Adopt(DRE->getQualifierLoc());
+    LookupKind = LookupOrdinaryName;
+    Found = DRE->getFoundDecl();
+  } else if (auto *ME = dyn_cast<MemberExpr>(TemplateName.get())) {
     NameInfo = ME->getMemberNameInfo();
     SS.Adopt(ME->getQualifierLoc());
     LookupKind = LookupMemberName;
     LookupCtx = ME->getBase()->getType()->getAsCXXRecordDecl();
     Found = ME->getMemberDecl();
+  } else if (auto *DSDRE =
+                 dyn_cast<DependentScopeDeclRefExpr>(TemplateName.get())) {
+    NameInfo = DSDRE->getNameInfo();
+    SS.Adopt(DSDRE->getQualifierLoc());
+    MissingTemplateKeyword = true;
+  } else if (auto *DSME =
+                 dyn_cast<CXXDependentScopeMemberExpr>(TemplateName.get())) {
+    NameInfo = DSME->getMemberNameInfo();
+    SS.Adopt(DSME->getQualifierLoc());
+    MissingTemplateKeyword = true;
   } else {
-    auto *DRE = cast<DeclRefExpr>(TemplateName.get());
-    NameInfo = DRE->getNameInfo();
-    SS.Adopt(DRE->getQualifierLoc());
-    LookupKind = LookupOrdinaryName;
-    Found = DRE->getFoundDecl();
+    llvm_unreachable("unexpected kind of potential template name");
+  }
+
+  // If this is a dependent-scope lookup, diagnose that the 'template' keyword
+  // was missing.
+  if (MissingTemplateKeyword) {
+    Diag(NameInfo.getLocStart(), diag::err_template_kw_missing)
+        << "" << NameInfo.getName().getAsString()
+        << SourceRange(Less, Greater);
+    return;
   }
 
   // Try to correct the name by looking for templates and C++ named casts.