Give errors on implicit conversions to 'void' (#40465)

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

@@ -2069,6 +2069,12 @@ internal BoundExpression GenerateConversionForAssignment(TypeSymbol targetType,
                 return;
             }
 
+            if (targetType.IsVoidType())
+            {
+                Error(diagnostics, ErrorCode.ERR_NoImplicitConv, syntax, operand.Display, targetType);
+                return;
+            }
+
             switch (operand.Kind)
             {
                 case BoundKind.BadExpression:

src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs

@@ -1486,6 +1486,166 @@ public static void Main()
                 Diagnostic(ErrorCode.ERR_NoImplicitConv, "m").WithArguments("bool", "int").WithLocation(6, 29));
         }
 
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void ThrowExpression_ImplicitVoidConversion_Return()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        return true ? throw null : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,23): error CS0029: Cannot implicitly convert type '<throw expression>' to 'void'
+                //         return true ? throw null : M2();
+                Diagnostic(ErrorCode.ERR_NoImplicitConv, "throw null").WithArguments("<throw expression>", "void").WithLocation(6, 23));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void ThrowExpression_ImplicitVoidConversion_Assignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        object obj = true ? throw null : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,29): error CS0029: Cannot implicitly convert type '<throw expression>' to 'void'
+                //         object obj = true ? throw null : M2();
+                Diagnostic(ErrorCode.ERR_NoImplicitConv, "throw null").WithArguments("<throw expression>", "void").WithLocation(6, 29));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void IntLiteral_ImplicitVoidConversion_Assignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        object obj = true ? 0 : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,22): error CS0173: Type of conditional expression cannot be determined because there is no implicit conversion between 'int' and 'void'
+                //         object obj = true ? 0 : M2();
+                Diagnostic(ErrorCode.ERR_InvalidQM, "true ? 0 : M2()").WithArguments("int", "void").WithLocation(6, 22));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void VoidCall_ImplicitVoidConversion_Assignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        object obj = true ? M2() : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,22): error CS0029: Cannot implicitly convert type 'void' to 'object'
+                //         object obj = true ? M2() : M2();
+                Diagnostic(ErrorCode.ERR_NoImplicitConv, "true ? M2() : M2()").WithArguments("void", "object").WithLocation(6, 22));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void VoidCall_ImplicitVoidConversion_DiscardAssignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        _ = true ? M2() : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,9): error CS8209: A value of type 'void' may not be assigned.
+                //         _ = true ? M2() : M2();
+                Diagnostic(ErrorCode.ERR_VoidAssignment, "_").WithLocation(6, 9));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void VoidCall_Assignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        object obj = M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,22): error CS0029: Cannot implicitly convert type 'void' to 'object'
+                //         object obj = M2();
+                Diagnostic(ErrorCode.ERR_NoImplicitConv, "M2()").WithArguments("void", "object").WithLocation(6, 22));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void VoidCall_DiscardAssignment()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        _ = M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,9): error CS8209: A value of type 'void' may not be assigned.
+                //         _ = M2();
+                Diagnostic(ErrorCode.ERR_VoidAssignment, "_").WithLocation(6, 9));
+        }
+
+        [Fact, WorkItem(40405, "https://github.com/dotnet/roslyn/issues/40405")]
+        public void VoidCall_ImplicitVoidConversion_Return()
+        {
+            string text = @"
+class C
+{
+    void M1()
+    {
+        return true ? M2() : M2();
+    }
+
+    void M2() { }
+}
+";
+            CreateCompilation(text).VerifyDiagnostics(
+                // (6,9): error CS0127: Since 'C.M1()' returns void, a return keyword must not be followed by an object expression
+                //         return true ? M2() : M2();
+                Diagnostic(ErrorCode.ERR_RetNoObjectRequired, "return").WithArguments("C.M1()").WithLocation(6, 9));
+        }
+
         [Fact]
         public void CS0030ERR_NoExplicitConv()
         {