[ConstraintSystem] Find solutions for code with invalid '_' using

holes.

Author: hborla

lib/Sema/CSDiag.cpp

@@ -1447,29 +1447,6 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
     auto lhsType = CS.getType(lhsExpr)->getRValueType();
     auto rhsType = CS.getType(rhsExpr)->getRValueType();
 
-    // TODO(diagnostics): There are still cases not yet handled by new
-    // diagnostics framework e.g.
-    //
-    // var tuple = (1, 2, 3)
-    // switch tuple {
-    //   case (let (_, _, _)) + 1: break
-    // }
-    if (callExpr->isImplicit() && overloadName == "~=") {
-      auto flags = ParameterTypeFlags();
-      if (calleeInfo.candidates.size() == 1)
-        if (auto fnType = calleeInfo.candidates[0].getFunctionType())
-          flags = fnType->getParams()[0].getParameterFlags();
-
-      auto *locator = CS.getConstraintLocator(
-          callExpr,
-          {ConstraintLocator::ApplyArgument,
-           LocatorPathElt::ApplyArgToParam(0, 0, flags)},
-          /*summaryFlags=*/0);
-
-      ArgumentMismatchFailure failure(CS, lhsType, rhsType, locator);
-      return failure.diagnosePatternMatchingMismatch();
-    }
-
     if (isContextualConversionFailure(argTuple))
       return false;
 
@@ -1754,22 +1731,6 @@ void FailureDiagnosis::diagnoseAmbiguity(Expr *E) {
   if (auto *assignment = dyn_cast<AssignExpr>(E)) {
     if (isa<DiscardAssignmentExpr>(assignment->getDest())) {
       auto *srcExpr = assignment->getSrc();
-
-      bool diagnosedInvalidUseOfDiscardExpr = false;
-      srcExpr->forEachChildExpr([&](Expr *expr) -> Expr * {
-        if (auto *DAE = dyn_cast<DiscardAssignmentExpr>(expr)) {
-          diagnose(DAE->getLoc(), diag::discard_expr_outside_of_assignment)
-              .highlight(srcExpr->getSourceRange());
-          diagnosedInvalidUseOfDiscardExpr = true;
-          return nullptr;
-        }
-
-        return expr;
-      });
-
-      if (diagnosedInvalidUseOfDiscardExpr)
-        return;
-
       diagnoseAmbiguity(srcExpr);
       return;
     }
@@ -1783,15 +1744,6 @@ void FailureDiagnosis::diagnoseAmbiguity(Expr *E) {
     return;
   }
 
-  // A DiscardAssignmentExpr (spelled "_") needs contextual type information to
-  // infer its type. If we see one at top level, diagnose that it must be part
-  // of an assignment so we don't get a generic "expression is ambiguous" error.
-  if (isa<DiscardAssignmentExpr>(E)) {
-    diagnose(E->getLoc(), diag::discard_expr_outside_of_assignment)
-      .highlight(E->getSourceRange());
-    return;
-  }
-  
   // Diagnose ".foo" expressions that lack context specifically.
   if (auto UME =
         dyn_cast<UnresolvedMemberExpr>(E->getSemanticsProvidingExpr())) {

lib/Sema/CSGen.cpp

@@ -2827,7 +2827,8 @@ namespace {
 
     Type visitDiscardAssignmentExpr(DiscardAssignmentExpr *expr) {
       auto locator = CS.getConstraintLocator(expr);
-      auto typeVar = CS.createTypeVariable(locator, TVO_CanBindToNoEscape);
+      auto typeVar = CS.createTypeVariable(locator, TVO_CanBindToNoEscape |
+                                                    TVO_CanBindToHole);
       return LValueType::get(typeVar);
     }
 

test/Constraints/diagnostics.swift

@@ -1271,7 +1271,7 @@ func badTypes() {
 // rdar://34357545
 func unresolvedTypeExistential() -> Bool {
   return (Int.self==_{})
-  // expected-error@-1 {{expression type 'Bool' is ambiguous without more context}}
+  // expected-error@-1 {{'_' can only appear in a pattern or on the left side of an assignment}}
 }
 
 func rdar43525641(_ a: Int, _ b: Int = 0, c: Int = 0, _ d: Int) {}

test/Parse/matching_patterns.swift

@@ -288,8 +288,7 @@ case (_, var e, 3) +++ (1, 2, 3):
 // expected-error@-2{{'var' binding pattern cannot appear in an expression}}
   ()
 case (let (_, _, _)) + 1:
-// expected-error@-1 2 {{'var' binding pattern cannot appear in an expression}}
-// expected-error@-2 {{expression pattern of type 'Int' cannot match values of type '(Int, Int, Int)'}}
+// expected-error@-1 {{expression pattern of type 'Int' cannot match values of type '(Int, Int, Int)'}}
   ()
 }
 

test/expr/expressions.swift

@@ -708,7 +708,7 @@ func test() {
 func unusedExpressionResults() {
   // Unused l-value
   _ // expected-error{{'_' can only appear in a pattern or on the left side of an assignment}}
-
+  // expected-error@-1 {{expression resolves to an unused variable}}
 
   // <rdar://problem/20749592> Conditional Optional binding hides compiler error
   let optionalc:C? = nil