[Sema] Diagnose wrong type coercion involving metatypes

Author: LucianoPAlmeida

lib/Sema/CSDiagnostics.cpp

@@ -1905,6 +1905,12 @@ bool ContextualFailure::diagnoseAsError() {
     diagnostic = diag::cannot_convert_condition_value;
     break;
   }
+      
+  case ConstraintLocator::InstanceType: {
+    if (diagnoseCoercionToUnrelatedType())
+      return true;
+    break;
+  }
 
   case ConstraintLocator::TernaryBranch: {
     auto *ifExpr = cast<IfExpr>(getRawAnchor());
@@ -2235,11 +2241,12 @@ bool ContextualFailure::diagnoseCoercionToUnrelatedType() const {
   auto *anchor = getAnchor();
 
   if (auto *coerceExpr = dyn_cast<CoerceExpr>(anchor)) {
-    auto fromType = getFromType();
+    auto fromType = getType(coerceExpr->getSubExpr());
     auto toType = getType(coerceExpr->getCastTypeLoc());
+    
     auto diagnostic =
         getDiagnosticFor(CTP_CoerceOperand,
-                         /*forProtocol=*/toType->isAnyExistentialType());
+                         /*forProtocol=*/toType->isExistentialType());
 
     auto diag =
         emitDiagnostic(anchor->getLoc(), *diagnostic, fromType, toType);
@@ -4772,6 +4779,13 @@ bool MissingContextualConformanceFailure::diagnoseAsError() {
       break;
     }
 
+    case ConstraintLocator::InstanceType: {
+      // If the missing conformance involves a coercion of metatypes
+      if (diagnoseCoercionToUnrelatedType())
+        return true;
+      break;
+    }
+
     default:
       break;
     }

lib/Sema/CSSimplify.cpp

@@ -2203,6 +2203,15 @@ ConstraintSystem::matchExistentialTypes(Type type1, Type type2,
           if (!(anchor && isa<AssignExpr>(anchor)))
             return getTypeMatchFailure(locator);
         }
+        
+        auto *anchor = locator.getAnchor();
+        if (isa<CoerceExpr>(anchor)) {
+          auto *fix = ContextualMismatch::create(
+              *this, type1, type2, getConstraintLocator(locator));
+          if (recordFix(fix))
+            return getTypeMatchFailure(locator);
+          break;
+        }
 
         auto *fix = MissingConformance::forContextual(
             *this, type1, proto, getConstraintLocator(locator));
@@ -4059,9 +4068,20 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         subKind = ConstraintKind::Bind;
       }
 
-      return matchTypes(
-          instanceType1, instanceType2, subKind, subflags,
-          locator.withPathElement(ConstraintLocator::InstanceType));
+      auto result =
+          matchTypes(instanceType1, instanceType2, subKind, subflags,
+                     locator.withPathElement(ConstraintLocator::InstanceType));
+      if (shouldAttemptFixes() && result.isFailure()) {
+        auto *anchor = locator.getAnchor();
+        if (anchor && isa<CoerceExpr>(anchor)) {
+          auto *fix =
+              ContextualMismatch::create(*this, instanceType1, instanceType2,
+                                         getConstraintLocator(locator));
+          conversionsOrFixes.push_back(fix);
+          break;
+        }
+      }
+      return result;
     }
 
     case TypeKind::Function: {