Add test to confirm that a bug is fixed. (#27157)

Fixes #27104

Author: Neal Gafter

src/Compilers/CSharp/Test/Symbol/SymbolDisplay/SymbolDisplayTests.cs

@@ -6580,5 +6580,36 @@ public void UnmanagedConstraint_Delegate()
                 SymbolDisplayPartKind.Space,
                 SymbolDisplayPartKind.Keyword);
         }
+
+        [Fact, WorkItem(27104, "https://github.com/dotnet/roslyn/issues/27104")]
+        public void BadDiscardInForeachLoop_01()
+        {
+            var source = @"
+class C
+{
+    void M()
+    {
+        foreach(_ in """")
+        {
+        }
+    }
+}";
+            var compilation = CreateCompilation(source);
+            compilation.VerifyDiagnostics(
+                // (6,17): error CS8186: A foreach loop must declare its iteration variables.
+                //         foreach(_ in "")
+                Diagnostic(ErrorCode.ERR_MustDeclareForeachIteration, "_").WithLocation(6, 17)
+                );
+            var tree = compilation.SyntaxTrees[0];
+            var variable = tree.GetRoot().DescendantNodes().OfType<ForEachVariableStatementSyntax>().Single().Variable;
+            var model = compilation.GetSemanticModel(tree);
+            var symbol = model.GetSymbolInfo(variable).Symbol;
+            Verify(
+                symbol.ToMinimalDisplayParts(model, variable.SpanStart),
+                "var _",
+                SymbolDisplayPartKind.ErrorTypeName, // var
+                SymbolDisplayPartKind.Space,
+                SymbolDisplayPartKind.Punctuation); // _
+        }
     }
 }