[Diagnostics] Diagnose an attempt to assign a value to an overloaded name

Example:

```swift
struct X {
}

func X(_: Int) -> Int {
  return 42
}

X = 42
```

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -69,6 +69,9 @@ ERROR(no_overloads_match_exactly_in_call_no_labels,none,
 ERROR(no_overloads_match_exactly_in_call_special,none,
       "no exact matches in call to %0",
       (DescriptiveDeclKind))
+ERROR(no_overloads_match_exactly_in_assignment,none,
+      "no exact matches in assignment to %0",
+      (DeclBaseName))
 
 ERROR(ambiguous_subscript,none,
       "ambiguous subscript with base type %0 and index type %1",
@@ -3401,6 +3404,10 @@ ERROR(assignment_dynamic_property_has_immutable_base,none,
 ERROR(assignment_bang_has_immutable_subcomponent,none,
       "cannot assign through '!': %0", (StringRef))
 
+NOTE(candidate_is_not_assignable,none,
+     "candidate is not assignable: %0 %1",
+     (DescriptiveDeclKind, DeclName))
+
 NOTE(change_to_mutating,none,
      "mark %select{method|accessor}0 'mutating' to make 'self' mutable",
      (bool))

lib/Sema/CSDiagnostics.cpp

@@ -1440,6 +1440,17 @@ bool RValueTreatedAsLValueFailure::diagnoseAsError() {
   return failure.diagnose();
 }
 
+bool RValueTreatedAsLValueFailure::diagnoseAsNote() {
+  auto overload = getChoiceFor(getLocator());
+  if (!(overload && overload->choice.isDecl()))
+    return false;
+
+  auto *decl = overload->choice.getDecl();
+  emitDiagnostic(decl, diag::candidate_is_not_assignable,
+                 decl->getDescriptiveKind(), decl->getFullName());
+  return true;
+}
+
 static Decl *findSimpleReferencedDecl(const Expr *E) {
   if (auto *LE = dyn_cast<LoadExpr>(E))
     E = LE->getSubExpr();

lib/Sema/CSDiagnostics.h

@@ -638,6 +638,7 @@ class RValueTreatedAsLValueFailure final : public FailureDiagnostic {
       : FailureDiagnostic(cs, locator) {}
 
   bool diagnoseAsError() override;
+  bool diagnoseAsNote() override;
 };
 
 class TrailingClosureAmbiguityFailure final : public FailureDiagnostic {

lib/Sema/CSSimplify.cpp

@@ -2815,6 +2815,14 @@ bool ConstraintSystem::repairFailures(
       return true;
     }
 
+    if (auto *ODRE = dyn_cast<OverloadedDeclRefExpr>(anchor)) {
+      if (lhs->is<LValueType>()) {
+        conversionsOrFixes.push_back(
+            TreatRValueAsLValue::create(*this, getConstraintLocator(locator)));
+        return true;
+      }
+    }
+
     if (auto *AE = dyn_cast<AssignExpr>(anchor)) {
       if (repairByInsertingExplicitCall(lhs, rhs))
         return true;

lib/Sema/ConstraintSystem.cpp

@@ -2701,6 +2701,10 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
       DE.diagnose(commonAnchor->getLoc(),
                   diag::could_not_find_subscript_member_did_you_mean,
                   getType(UDE->getBase()));
+    } else if (fix->getKind() == FixKind::TreatRValueAsLValue) {
+      DE.diagnose(commonAnchor->getLoc(),
+                  diag::no_overloads_match_exactly_in_assignment,
+                  decl->getBaseName());
     } else {
       auto name = decl->getFullName();
       // Three choices here:

test/NameBinding/import-resolution-overload.swift

@@ -46,7 +46,7 @@ scopedFunction = 42
 // FIXME: Should be an error -- a type name and a function cannot overload.
 var _ : Int = TypeNameWins(42)
 
-TypeNameWins = 42 // expected-error {{ambiguous reference to member 'TypeNameWins'}}
+TypeNameWins = 42 // expected-error {{no exact matches in assignment to 'TypeNameWins'}}
 var _ : TypeNameWins // no-warning
 
 // rdar://problem/21739333