Add sequence folding for the assignment operator.

Author: DougGregor

Sources/SwiftOperatorPrecedence/OperatorPrecedence+Folding.swift

@@ -60,10 +60,15 @@ extension OperatorPrecedence {
     }
 
     // The ternary operator has a fixed precedence group name.
-    if let ternaryExpr = expr.as(UnresolvedTernaryExprSyntax.self) {
+    if expr.is(UnresolvedTernaryExprSyntax.self) {
       return "TernaryPrecedence"
     }
 
+    // An assignment operator has fixed precedence.
+    if expr.is(AssignmentExprSyntax.self) {
+      return "AssignmentPrecedence"
+    }
+
     // FIXME: Handle all of the language-defined precedence relationships.
     return nil
   }
@@ -81,8 +86,7 @@ extension OperatorPrecedence {
       return ExprSyntax(
         InfixOperatorExprSyntax(
           leftOperand: lhs,
-          binaryOperatorExpr.unexpectedBeforeOperatorToken,
-          operatorOperand: op,
+          operatorOperand: ExprSyntax(binaryOperatorExpr),
           rightOperand: rhs)
       )
     }
@@ -102,6 +106,16 @@ extension OperatorPrecedence {
       )
     }
 
+    // An assignment operator x = y.
+    if let assignExpr = op.as(AssignmentExprSyntax.self) {
+      return ExprSyntax(
+        InfixOperatorExprSyntax(
+          leftOperand: lhs,
+          operatorOperand: ExprSyntax(assignExpr),
+          rightOperand: rhs)
+      )
+    }
+
     // FIXME: Fallback that we should never need
     fatalError("Unknown binary operator")
   }

Tests/SwiftOperatorPrecedenceTest/OperatorPrecedence.swift

@@ -118,6 +118,12 @@ public class OperatorPrecedenceTests: XCTestCase {
     XCTAssertFalse(foldedAll.containsExprSequence)
   }
 
+  func testAssignExprs() throws {
+    let opPrecedence = OperatorPrecedence.standardOperators
+    try opPrecedence.assertExpectedFold("a = b + c", "(a = (b + c))")
+    try opPrecedence.assertExpectedFold("a = b = c", "(a = (b = c))")
+  }
+
   func testParsedLogicalExprs() throws {
     let logicalOperatorSources =
     """