Fix crash in GetDeconstructionInfo (#27605)

src/Compilers/CSharp/Portable/Compilation/MemberSemanticModel.cs

@@ -854,7 +854,11 @@ public override DeconstructionInfo GetDeconstructionInfo(AssignmentExpressionSyn
             }
 
             var boundConversion = boundDeconstruction.Right;
-            Debug.Assert(boundConversion != null || boundDeconstruction.HasAnyErrors);
+            Debug.Assert(boundConversion != null);
+            if (boundConversion is null)
+            {
+                return default;
+            }
 
             return new DeconstructionInfo(boundConversion.Conversion);
         }
@@ -869,6 +873,10 @@ public override DeconstructionInfo GetDeconstructionInfo(ForEachVariableStatemen
 
             var boundDeconstruction = boundForEach.DeconstructionOpt;
             Debug.Assert(boundDeconstruction != null || boundForEach.HasAnyErrors);
+            if (boundDeconstruction is null)
+            {
+                return default;
+            }
 
             return new DeconstructionInfo(boundDeconstruction.DeconstructionAssignment.Right.Conversion);
         }

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenDeconstructTests.cs

@@ -273,6 +273,39 @@ public void Deconstruct(out int a)
             Assert.Equal(ConversionKind.NoConversion, defaultInfo.Conversion.Value.Kind);
         }
 
+        [Fact]
+        [WorkItem(27520, "https://github.com/dotnet/roslyn/issues/27520")]
+        public void GetDeconstructionInfoOnIncompleteCode()
+        {
+            string source = @"
+class C
+{
+    static void M(string s)
+    {
+        foreach (char in s) { }
+    }
+}";
+
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (6,18): error CS1525: Invalid expression term 'char'
+                //         foreach (char in s) { }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "char").WithArguments("char").WithLocation(6, 18),
+                // (6,23): error CS0230: Type and identifier are both required in a foreach statement
+                //         foreach (char in s) { }
+                Diagnostic(ErrorCode.ERR_BadForeachDecl, "in").WithLocation(6, 23)
+                );
+
+            var tree = comp.SyntaxTrees.First();
+            var model = comp.GetSemanticModel(tree);
+            var foreachDeconstruction = (ForEachVariableStatementSyntax)tree.FindNodeOrTokenByKind(SyntaxKind.ForEachVariableStatement).AsNode();
+            Assert.Equal(@"foreach (char in s) { }", foreachDeconstruction.ToString());
+            var deconstructionInfo = model.GetDeconstructionInfo(foreachDeconstruction);
+            Assert.Equal(Conversion.NoConversion, deconstructionInfo.Conversion);
+            Assert.Null(deconstructionInfo.Method);
+            Assert.Empty(deconstructionInfo.Nested);
+        }
+
         [Fact]
         [WorkItem(15634, "https://github.com/dotnet/roslyn/issues/15634")]
         public void DeconstructMustReturnVoid()