Improve compiler error recovery for null coalesce assignment operator (#80907)

Redo of #74218
Fixes #73894

Author: DoctorKrolic

src/Compilers/CSharp/Portable/Binder/Binder_Operators.cs

@@ -5777,12 +5777,10 @@ private BoundExpression BindNullCoalescingAssignmentOperator(AssignmentExpressio
             ReportSuppressionIfNeeded(leftOperand, diagnostics);
             BoundExpression rightOperand = BindValue(node.Right, diagnostics, BindValueKind.RValue);
 
-            // If either operand is bad, bail out preventing more cascading errors
+            // Prevent more cascading errors if there are any on either operand
             if (leftOperand.HasAnyErrors || rightOperand.HasAnyErrors)
             {
-                leftOperand = BindToTypeForErrorRecovery(leftOperand);
-                rightOperand = BindToTypeForErrorRecovery(rightOperand);
-                return new BoundNullCoalescingAssignmentOperator(node, leftOperand, rightOperand, CreateErrorType(), hasErrors: true);
+                diagnostics = BindingDiagnosticBag.Discarded;
             }
 
             // Given a ??= b, the type of a is A, the type of B is b, and if A is a nullable value type, the underlying

src/Compilers/CSharp/Test/IOperation/IOperation/IOperationTests_ICoalesceAssignmentOperation.cs

@@ -598,12 +598,12 @@ static void M(object o1, object o2, string s1, string s2)
                 Predecessors: [B0]
                 Statements (1)
                     IFlowCaptureOperation: 0 (OperationKind.FlowCapture, Type: null, IsInvalid, IsImplicit) (Syntax: 'o1')
-                      Value: 
+                      Value:
                         IParameterReferenceOperation: o1 (OperationKind.ParameterReference, Type: System.Object, IsInvalid) (Syntax: 'o1')
 
                 Jump if True (Regular) to Block[B3]
                     IIsNullOperation (OperationKind.IsNull, Type: System.Boolean, IsInvalid, IsImplicit) (Syntax: 'o1')
-                      Operand: 
+                      Operand:
                         IFlowCaptureReferenceOperation: 0 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1')
                     Leaving: {R3}
 
@@ -612,7 +612,7 @@ static void M(object o1, object o2, string s1, string s2)
                 Predecessors: [B1]
                 Statements (1)
                     IFlowCaptureOperation: 1 (OperationKind.FlowCapture, Type: null, IsInvalid, IsImplicit) (Syntax: 'o1')
-                      Value: 
+                      Value:
                         IFlowCaptureReferenceOperation: 0 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1')
 
                 Next (Regular) Block[B4]
@@ -623,15 +623,15 @@ static void M(object o1, object o2, string s1, string s2)
             Predecessors: [B1]
             Statements (1)
                 IFlowCaptureOperation: 1 (OperationKind.FlowCapture, Type: null, IsInvalid, IsImplicit) (Syntax: 'o2')
-                  Value: 
+                  Value:
                     IParameterReferenceOperation: o2 (OperationKind.ParameterReference, Type: System.Object, IsInvalid) (Syntax: 'o2')
 
             Next (Regular) Block[B4]
         Block[B4] - Block
             Predecessors: [B2] [B3]
             Statements (1)
                 IFlowCaptureOperation: 2 (OperationKind.FlowCapture, Type: null, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
-                  Value: 
+                  Value:
                     IInvalidOperation (OperationKind.Invalid, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
                       Children(1):
                           IFlowCaptureReferenceOperation: 1 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
@@ -647,12 +647,12 @@ static void M(object o1, object o2, string s1, string s2)
             Predecessors: [B4]
             Statements (1)
                 IFlowCaptureOperation: 3 (OperationKind.FlowCapture, Type: null, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
-                  Value: 
+                  Value:
                     IFlowCaptureReferenceOperation: 2 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
 
             Jump if True (Regular) to Block[B6]
                 IIsNullOperation (OperationKind.IsNull, Type: System.Boolean, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
-                  Operand: 
+                  Operand:
                     IFlowCaptureReferenceOperation: 3 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
                 Leaving: {R4}
                 Entering: {R5} {R6}
@@ -670,12 +670,12 @@ static void M(object o1, object o2, string s1, string s2)
                 Predecessors: [B5]
                 Statements (1)
                     IFlowCaptureOperation: 4 (OperationKind.FlowCapture, Type: null, IsImplicit) (Syntax: 's1')
-                      Value: 
+                      Value:
                         IParameterReferenceOperation: s1 (OperationKind.ParameterReference, Type: System.String) (Syntax: 's1')
 
                 Jump if True (Regular) to Block[B8]
                     IIsNullOperation (OperationKind.IsNull, Type: System.Boolean, IsImplicit) (Syntax: 's1')
-                      Operand: 
+                      Operand:
                         IFlowCaptureReferenceOperation: 4 (OperationKind.FlowCaptureReference, Type: System.String, IsImplicit) (Syntax: 's1')
                     Leaving: {R6}
 
@@ -684,7 +684,7 @@ static void M(object o1, object o2, string s1, string s2)
                 Predecessors: [B6]
                 Statements (1)
                     IFlowCaptureOperation: 5 (OperationKind.FlowCapture, Type: null, IsImplicit) (Syntax: 's1')
-                      Value: 
+                      Value:
                         IFlowCaptureReferenceOperation: 4 (OperationKind.FlowCaptureReference, Type: System.String, IsImplicit) (Syntax: 's1')
 
                 Next (Regular) Block[B9]
@@ -695,18 +695,22 @@ static void M(object o1, object o2, string s1, string s2)
             Predecessors: [B6]
             Statements (1)
                 IFlowCaptureOperation: 5 (OperationKind.FlowCapture, Type: null, IsImplicit) (Syntax: 's2')
-                  Value: 
+                  Value:
                     IParameterReferenceOperation: s2 (OperationKind.ParameterReference, Type: System.String) (Syntax: 's2')
 
             Next (Regular) Block[B9]
         Block[B9] - Block
             Predecessors: [B7] [B8]
             Statements (1)
-                ISimpleAssignmentOperation (OperationKind.SimpleAssignment, Type: ?, IsInvalid, IsImplicit) (Syntax: '(o1 ?? o2)  ...  (s1 ?? s2)')
-                  Left: 
+                ISimpleAssignmentOperation (OperationKind.SimpleAssignment, Type: System.Object, IsInvalid, IsImplicit) (Syntax: '(o1 ?? o2)  ...  (s1 ?? s2)')
+                  Left:
                     IFlowCaptureReferenceOperation: 2 (OperationKind.FlowCaptureReference, Type: System.Object, IsInvalid, IsImplicit) (Syntax: 'o1 ?? o2')
-                  Right: 
-                    IFlowCaptureReferenceOperation: 5 (OperationKind.FlowCaptureReference, Type: System.String, IsImplicit) (Syntax: 's1 ?? s2')
+                  Right:
+                    IConversionOperation (TryCast: False, Unchecked) (OperationKind.Conversion, Type: System.Object, IsImplicit) (Syntax: 's1 ?? s2')
+                      Conversion: CommonConversion (Exists: True, IsIdentity: False, IsNumeric: False, IsReference: True, IsUserDefined: False) (MethodSymbol: null)
+                        (ImplicitReference)
+                      Operand:
+                        IFlowCaptureReferenceOperation: 5 (OperationKind.FlowCaptureReference, Type: System.String, IsImplicit) (Syntax: 's1 ?? s2')
 
             Next (Regular) Block[B10]
                 Leaving: {R5} {R1}