Move more errors out of the parser. (#16326)

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -2958,10 +2958,6 @@ private ConversionOperatorDeclarationSyntax ParseConversionOperatorDeclaration(S
             var type = this.ParseType();
 
             var paramList = this.ParseParenthesizedParameterList();
-            if (paramList.Parameters.Count != 1)
-            {
-                paramList = this.AddErrorToFirstToken(paramList, ErrorCode.ERR_OvlUnaryOperatorExpected);
-            }
 
             BlockSyntax blockBody;
             ArrowExpressionClauseSyntax expressionBody;

src/Compilers/CSharp/Portable/Symbols/Source/SourceUserDefinedConversionSymbol.cs

@@ -48,6 +48,10 @@ internal sealed class SourceUserDefinedConversionSymbol : SourceUserDefinedOpera
                 diagnostics,
                 isExpressionBodied)
         {
+            if (syntax.ParameterList.Parameters.Count != 1)
+            {
+                diagnostics.Add(ErrorCode.ERR_OvlUnaryOperatorExpected, syntax.ParameterList.GetLocation());
+            }
         }
 
         internal new ConversionOperatorDeclarationSyntax GetSyntax()

src/Compilers/CSharp/Test/Syntax/Parsing/ParserErrorMessageTests.cs

@@ -1827,6 +1827,22 @@ public static void Main()
             ParseAndValidate(test, Diagnostic(ErrorCode.ERR_OvlUnaryOperatorExpected, "ii"));
         }
 
+        [Fact]
+        public void CS1019ERR_OvlUnaryOperatorExpected2()
+        {
+            var test = @"
+class C
+{
+    public static implicit operator int(C c1, C c2) => 0;
+}
+";
+
+            CreateCompilationWithMscorlib(test).VerifyDiagnostics(
+                // (4,40): error CS1019: Overloadable unary operator expected
+                //     public static implicit operator int(C c1, C c2) => 0;
+                Diagnostic(ErrorCode.ERR_OvlUnaryOperatorExpected, "(C c1, C c2)").WithLocation(4, 40));
+        }
+
         [WorkItem(906502, "DevDiv/Personal")]
         [Fact]
         public void CS1020ERR_OvlBinaryOperatorExpected()

src/Compilers/CSharp/Test/Syntax/Parsing/ScriptParsingTests.cs

@@ -8302,7 +8302,6 @@ public void OperatorError()
                 new ErrorDescription { Code = 1003, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1031, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1003, Line = 1, Column = 1 },
-                new ErrorDescription { Code = 1019, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1026, Line = 1, Column = 9 },
                 new ErrorDescription { Code = (int)ErrorCode.ERR_SemicolonExpected, Line = 1, Column = 9 });
         }
@@ -8315,7 +8314,6 @@ public void OperatorImplicitError()
                 new ErrorDescription { Code = 1003, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1031, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1003, Line = 1, Column = 9 },
-                new ErrorDescription { Code = 1019, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1026, Line = 1, Column = 9 },
                 new ErrorDescription { Code = (int)ErrorCode.ERR_SemicolonExpected, Line = 1, Column = 9 });
         }
@@ -8328,7 +8326,6 @@ public void OperatorExplicitError()
                 new ErrorDescription { Code = 1003, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1031, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1003, Line = 1, Column = 9 },
-                new ErrorDescription { Code = 1019, Line = 1, Column = 9 },
                 new ErrorDescription { Code = 1026, Line = 1, Column = 9 },
                 new ErrorDescription { Code = (int)ErrorCode.ERR_SemicolonExpected, Line = 1, Column = 9 });
         }