Report ERR_PatternWrongType for Union matching scenarios (#81352)

Relates to test plan: #81074

Author: AlekseyTs

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

@@ -4869,7 +4869,7 @@ private BoundExpression BindIsOperator(BinaryExpressionSyntax node, BindingDiagn
                 NamedTypeSymbol unionType = PrepareForUnionMatchingIfAppropriateAndReturnUnionType(node, ref inputType, isPatternDiagnostics);
 
                 bool hasErrors = node.Right.HasErrors;
-                var convertedExpression = BindExpressionForPattern(inputType, node.Right, ref hasErrors, isPatternDiagnostics, out var constantValueOpt, out var wasExpression, out _);
+                var convertedExpression = BindExpressionForPattern(unionType, inputType, node.Right, ref hasErrors, isPatternDiagnostics, out var constantValueOpt, out var wasExpression, out _);
                 if (wasExpression)
                 {
                     hasErrors |= constantValueOpt is null;

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

@@ -223,8 +223,9 @@ private void BuildSwitchLabels(SyntaxList<SwitchLabelSyntax> labelsSyntax, Binde
                     case SyntaxKind.CasePatternSwitchLabel:
                         // bind the pattern, to cause its pattern variables to be inferred if necessary
                         var matchLabel = (CasePatternSwitchLabelSyntax)labelSyntax;
+                        NamedTypeSymbol unionType = null;
                         _ = sectionBinder.BindPattern(
-                            matchLabel.Pattern, SwitchGoverningType, permitDesignations: true, labelSyntax.HasErrors, tempDiagnosticBag, hasUnionMatching: out _);
+                            matchLabel.Pattern, ref unionType, SwitchGoverningType, permitDesignations: true, labelSyntax.HasErrors, tempDiagnosticBag, hasUnionMatching: out _);
                         break;
 
                     default:
@@ -270,7 +271,10 @@ protected BoundExpression ConvertCaseExpression(CSharpSyntaxNode node, BoundExpr
                 caseExpression = CreateConversion(caseExpression, conversion, SwitchGoverningType, diagnostics);
             }
 
-            return ConvertPatternExpression(SwitchGoverningType, node, caseExpression, out constantValueOpt, hasErrors, diagnostics, out _);
+            // PROTOTYPE: Test that consumer actually does the right thing when a union matching is involved. Cover error scenarios as well.
+            var inputType = SwitchGoverningType;
+            NamedTypeSymbol unionType = PrepareForUnionMatchingIfAppropriateAndReturnUnionType(node, ref inputType, diagnostics);
+            return ConvertPatternExpression(unionType, inputType, node, caseExpression, out constantValueOpt, hasErrors, diagnostics, out _);
         }
 
         private static readonly object s_nullKey = new object();

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

@@ -247,8 +247,9 @@ private BoundSwitchLabel BindSwitchSectionLabel(
                     {
                         var caseLabelSyntax = (CaseSwitchLabelSyntax)node;
                         bool hasErrors = node.HasErrors;
+                        NamedTypeSymbol unionType = null;
                         BoundPattern pattern = sectionBinder.BindConstantPatternWithFallbackToTypePattern(
-                            caseLabelSyntax.Value, caseLabelSyntax.Value, SwitchGoverningType, hasErrors, diagnostics, out bool hasUnionMatching);
+                            caseLabelSyntax.Value, caseLabelSyntax.Value, ref unionType, SwitchGoverningType, hasErrors, diagnostics, out bool hasUnionMatching);
                         pattern.WasCompilerGenerated = true; // we don't have a pattern syntax here
                         reportIfConstantNamedUnderscore(pattern, caseLabelSyntax.Value);
 
@@ -279,8 +280,9 @@ private BoundSwitchLabel BindSwitchSectionLabel(
 
                         MessageID.IDS_FeaturePatternMatching.CheckFeatureAvailability(diagnostics, node.Keyword);
 
+                        NamedTypeSymbol unionType = null;
                         BoundPattern pattern = sectionBinder.BindPattern(
-                            matchLabelSyntax.Pattern, SwitchGoverningType, permitDesignations: true, node.HasErrors, diagnostics, out bool hasUnionMatching);
+                            matchLabelSyntax.Pattern, ref unionType, SwitchGoverningType, permitDesignations: true, node.HasErrors, diagnostics, out bool hasUnionMatching);
                         if (matchLabelSyntax.Pattern is ConstantPatternSyntax p)
                             reportIfConstantNamedUnderscore(pattern, p.Expression);
 

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

@@ -39,7 +39,8 @@ internal override BoundSwitchExpressionArm BindSwitchExpressionArm(SwitchExpress
             Binder armBinder = this.GetRequiredBinder(node);
             bool hasErrors = switchGoverningType.IsErrorType();
             ImmutableArray<LocalSymbol> locals = _armScopeBinder.Locals;
-            BoundPattern pattern = armBinder.BindPattern(node.Pattern, switchGoverningType, permitDesignations: true, hasErrors, diagnostics, out bool hasUnionMatching);
+            NamedTypeSymbol? unionType = null;
+            BoundPattern pattern = armBinder.BindPattern(node.Pattern, ref unionType, switchGoverningType, permitDesignations: true, hasErrors, diagnostics, out bool hasUnionMatching);
             BoundExpression? whenClause = node.WhenClause != null
                 ? armBinder.BindBooleanExpression(node.WhenClause.Condition, diagnostics)
                 : null;

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

@@ -2139,9 +2139,8 @@ internal static bool IsWellKnownTypeUnmanagedType(this TypeSymbol typeSymbol)
         internal static bool IsWellKnownTypeIsExternalInit(this TypeSymbol typeSymbol)
             => typeSymbol.IsWellKnownCompilerServicesTopLevelType("IsExternalInit");
 
-        internal static bool IsWellKnownTypeIUnion(this NamedTypeSymbol typeSymbol)
-            => typeSymbol.DeclaredAccessibility == Accessibility.Public &&
-               typeSymbol.IsInterface &&
+        internal static bool IsWellKnownTypeIUnion(this NamedTypeSymbol typeSymbol) // PROTOTYPE: Test individual conditions
+            => typeSymbol is { DeclaredAccessibility: Accessibility.Public, IsInterface: true, IsGenericType: false } &&
                typeSymbol.IsWellKnownCompilerServicesTopLevelType(WellKnownMemberNames.IUnionInterfaceName); // PROTOTYPE: Confirm namespace, the spec doesn't specify it at the moment.
 
         internal static bool IsWellKnownTypeOutAttribute(this TypeSymbol typeSymbol) => typeSymbol.IsWellKnownInteropServicesTopLevelType("OutAttribute");