Produce Pack Expansion Types for Polyvariadic Functions

Also begin resolving (T...) as a pack expansion.

Author: CodaFi

lib/Sema/TypeCheckDecl.cpp

@@ -2110,7 +2110,14 @@ static Type validateParameterType(ParamDecl *decl) {
   }
 
   if (decl->isVariadic()) {
-    Ty = VariadicSequenceType::get(Ty);
+    // Handle the monovariadic/polyvariadic interface type split.
+    if (Ty->hasTypeSequence()) {
+      // Polyvariadic types (T...) for <T...> resolve to pack expansions.
+      Ty = PackExpansionType::get(Ty);
+    } else {
+      // Monovariadic types (T...) for <T> resolve to [T].
+      Ty = VariadicSequenceType::get(Ty);
+    }
     if (!ctx.getArrayDecl()) {
       ctx.Diags.diagnose(decl->getTypeRepr()->getLoc(),
                          diag::sugar_type_not_found, 0);

lib/Sema/TypeCheckType.cpp

@@ -3559,12 +3559,31 @@ NeverNullType TypeResolver::resolveTupleType(TupleTypeRepr *repr,
     elementOptions = elementOptions.withoutContext(true);
   }
 
-  // Variadic tuples are not permitted.
   bool complained = false;
   if (repr->hasEllipsis()) {
-    diagnose(repr->getEllipsisLoc(), diag::tuple_ellipsis);
-    repr->removeEllipsis();
-    complained = true;
+    if (repr->getNumElements() == 1 && !repr->hasElementNames()) {
+      // This is probably a pack expansion. Try to resolve the pattern type.
+      auto patternTy = resolveType(repr->getElementType(0), elementOptions);
+      if (patternTy->hasError())
+        complained = true;
+
+      // If there's no reference to a variadic generic parameter, complain
+      // - the pack won't actually expand to anything meaningful.
+      if (!patternTy->hasTypeSequence())
+        diagnose(repr->getLoc(), diag::expansion_not_variadic, patternTy)
+          .highlight(repr->getParens());
+
+      return PackExpansionType::get(patternTy);
+    } else {
+      // Variadic tuples are not permitted.
+      //
+      // FIXME: We could probably make this work.
+      // (T, U, V...) is a reasonable pack expansion to support with a kind of
+      // "guaranteed bound" of at least two elements.
+      diagnose(repr->getEllipsisLoc(), diag::tuple_ellipsis);
+      repr->removeEllipsis();
+      complained = true;
+    }
   }
 
   bool hadError = false;