Sema: Handle PackElement locator in repairFailures()

Author: slavapestov

lib/Sema/CSSimplify.cpp

@@ -5877,6 +5877,19 @@ bool ConstraintSystem::repairFailures(
     break;
   }
 
+  case ConstraintLocator::PackElement: {
+    path.pop_back();
+
+    if (!path.empty() && path.back().is<LocatorPathElt::PackType>())
+      path.pop_back();
+
+    if (!path.empty() && path.back().is<LocatorPathElt::PackType>())
+      path.pop_back();
+
+    return repairFailures(lhs, rhs, matchKind, conversionsOrFixes,
+                          getConstraintLocator(anchor, path));
+  }
+
   case ConstraintLocator::SequenceElementType: {
     // This is going to be diagnosed as `missing conformance`,
     // so no need to create duplicate fixes.

lib/Sema/ConstraintSystem.cpp

@@ -6708,7 +6708,14 @@ SourceRange constraints::getSourceRange(ASTNode anchor) {
 
 static Optional<Requirement> getRequirement(ConstraintSystem &cs,
                                             ConstraintLocator *reqLocator) {
-  auto reqLoc = reqLocator->getLastElementAs<LocatorPathElt::AnyRequirement>();
+  ArrayRef<LocatorPathElt> path = reqLocator->getPath();
+
+  // If we have something like ... -> type req # -> pack element #, we're
+  // solving a requirement of the form T : P where T is a type parameter pack
+  if (path.back().is<LocatorPathElt::PackElement>())
+    path = path.drop_back();
+
+  auto reqLoc = path.back().getAs<LocatorPathElt::AnyRequirement>();
   if (!reqLoc)
     return None;
 

test/Constraints/variadic_generic_constraints.swift

@@ -26,4 +26,12 @@ func takesC<T...: C>(_: T...) {}  // expected-note {{where 'T' = 'NotSubclassOfC
 takesC()  // ok
 takesC(SubclassOfC(), SubclassOfC(), SubclassOfC())  // ok
 
-takesC(SubclassOfC(), NotSubclassOfC(), SubclassOfC())  // expected-error {{global function 'takesC' requires that 'NotSubclassOfC' inherit from 'C'}}
+takesC(SubclassOfC(), NotSubclassOfC(), SubclassOfC())  // expected-error {{global function 'takesC' requires that 'NotSubclassOfC' inherit from 'C'}}
+
+func takesParallelSequences<T..., U...>(t: T..., u: U...) where T: Sequence, U: Sequence, T.Element == U.Element {}
+// expected-note@-1 {{where 'T.Element' = 'String', 'U.Element' = 'Int'}}
+
+takesParallelSequences()  // ok
+takesParallelSequences(t: Array<Int>(), u: Set<Int>())  // ok
+takesParallelSequences(t: Array<String>(), Set<Int>(), u: Set<String>(), Array<Int>())  // ok
+takesParallelSequences(t: Array<String>(), Set<Int>(), u: Array<Int>(), Set<String>())  // expected-error {{global function 'takesParallelSequences(t:u:)' requires the types 'String' and 'Int' be equivalent}}