[ConstraintSystem] Use new condition element in constraint generation/diagnostics

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -1918,6 +1918,16 @@ bool ContextualFailure::diagnoseAsError() {
     break;
   }
 
+  case ConstraintLocator::Condition: {
+    // Tailored diagnostics for optional or assignment use
+    // in condition expression.
+    if (diagnoseConversionToBool())
+      return true;
+
+    diagnostic = diag::cannot_convert_condition_value;
+    break;
+  }
+
   case ConstraintLocator::ContextualType: {
     if (diagnoseConversionToBool())
       return true;

lib/Sema/CSGen.cpp

@@ -2578,10 +2578,10 @@ namespace {
       if (!boolDecl)
         return Type();
 
-      CS.addConstraint(ConstraintKind::Conversion,
-                       CS.getType(expr->getCondExpr()),
-                       boolDecl->getDeclaredType(),
-                       CS.getConstraintLocator(expr->getCondExpr()));
+      CS.addConstraint(
+          ConstraintKind::Conversion, CS.getType(expr->getCondExpr()),
+          boolDecl->getDeclaredType(),
+          CS.getConstraintLocator(expr, LocatorPathElt::Condition()));
 
       // The branches must be convertible to a common type.
       return CS.addJoinConstraint(CS.getConstraintLocator(expr),

lib/Sema/CSSimplify.cpp

@@ -3186,6 +3186,12 @@ bool ConstraintSystem::repairFailures(
     break;
   }
 
+  case ConstraintLocator::Condition: {
+    conversionsOrFixes.push_back(IgnoreContextualType::create(
+        *this, lhs, rhs, getConstraintLocator(locator)));
+    break;
+  }
+
   default:
     break;
   }