Change order of override error checks

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -143,9 +143,10 @@ object RefChecks {
    *
    *    1.1. M must have the same or stronger access privileges as O.
    *    1.2. O must not be effectively final.
-   *    1.3. O is deferred, or M has `override` modifier.
-   *    1.4. If O is stable, then so is M.
-   *     // @M: LIFTED 1.5. Neither M nor O are a parameterized type alias
+   *    1.3. If M or O are extension methods, they must both be extension methods.
+   *         (Should be before the check for the need of `override` modifier.)
+   *    1.4. O is deferred, or M has `override` modifier.
+   *    1.5. If O is stable, then so is M.
    *    1.6. If O is a type alias, then M is an alias of O.
    *    1.7. If O is an abstract type then
    *       1.7.1 either M is an abstract type, and M's bounds are sharper than O's bounds.
@@ -156,8 +157,7 @@ object RefChecks {
    *    1.8.1  M's type is a subtype of O's type, or
    *    1.8.2  M is of type []S, O is of type ()T and S <: T, or
    *    1.8.3  M is of type ()S, O is of type []T and S <: T, or
-   *    1.9.1  If M is erased, O is erased. If O is erased, M is erased or inline.
-   *    1.9.2  If M or O are extension methods, they must both be extension methods.
+   *    1.9. If M is erased, O is erased. If O is erased, M is erased or inline.
    *    1.10.  If O is inline (and deferred, otherwise O would be final), M must be inline
    *    1.11.  If O is a Scala-2 macro, M must be a Scala-2 macro.
    *  2. Check that only abstract classes have deferred members
@@ -343,6 +343,10 @@ object RefChecks {
         overrideError("cannot be used here - classes can only override abstract types")
       else if (other.isEffectivelyFinal) // (1.2)
         overrideError(i"cannot override final member ${other.showLocated}")
+      else if (member.isAllOf(ExtensionMethod) && !other.isAllOf(ExtensionMethod)) // (1.3)
+        overrideError("is an extension method, cannot override a normal method")
+      else if (other.isAllOf(ExtensionMethod) && !member.isAllOf(ExtensionMethod)) // (1.3)
+        overrideError("is a normal method, cannot override an extension method")
       else if (!other.is(Deferred) &&
                  !other.name.is(DefaultGetterName) &&
                  !member.isAnyOverride)
@@ -381,7 +385,7 @@ object RefChecks {
         intersectionIsEmpty(member.extendedOverriddenSymbols, other.extendedOverriddenSymbols))
         overrideError("cannot override a concrete member without a third member that's overridden by both " +
           "(this rule is designed to prevent ``accidental overrides'')")
-      else if (other.isStableMember && !member.isStableMember) // (1.4)
+      else if (other.isStableMember && !member.isStableMember) // (1.5)
         overrideError("needs to be a stable, immutable value")
       else if (member.is(ModuleVal) && !other.isRealMethod && !other.isOneOf(Deferred | Lazy))
         overrideError("may not override a concrete non-lazy value")
@@ -390,14 +394,10 @@ object RefChecks {
         overrideError("may not override a non-lazy value")
       else if (other.is(Lazy) && !other.isRealMethod && !member.is(Lazy))
         overrideError("must be declared lazy to override a lazy value")
-      else if (member.is(Erased) && !other.is(Erased)) // (1.9.1)
+      else if (member.is(Erased) && !other.is(Erased)) // (1.9)
         overrideError("is erased, cannot override non-erased member")
-      else if (other.is(Erased) && !member.isOneOf(Erased | Inline)) // (1.9.1)
+      else if (other.is(Erased) && !member.isOneOf(Erased | Inline)) // (1.9)
         overrideError("is not erased, cannot override erased member")
-      else if (member.isAllOf(ExtensionMethod) && !other.isAllOf(ExtensionMethod)) // (1.9.2)
-        overrideError("is an extension method, cannot override a normal method")
-      else if (other.isAllOf(ExtensionMethod) && !member.isAllOf(ExtensionMethod)) // (1.9.2)
-        overrideError("is a normal method, cannot override an extension method")
       else if other.is(Inline) && !member.is(Inline) then // (1.10)
         overrideError("is not inline, cannot implement an inline method")
       else if (other.isScala2Macro && !member.isScala2Macro) // (1.11)

tests/neg/override-extension_normal-methods.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/override-extension_normal-methods.scala:6:6 --------------------------------------------------------
+6 |  def m[T](x: T): String  = "normal method" // error: normal method, cannot override an extension method. Also needs `override' modifier (but this error should be obfuscated).
+  |      ^
+  |      error overriding method m in trait A of type [T](t: T): String;
+  |        method m of type [T](x: T): String is a normal method, cannot override an extension method
+-- Error: tests/neg/override-extension_normal-methods.scala:14:6 -------------------------------------------------------
+14 |  def [T](t: T).m: String = "extrnsion method" // error: extension method, cannot override an normal method. Also needs `override' modifier (but this error should be obfuscated).
+   |      ^
+   |      error overriding method m in trait B of type [T](x: T): String;
+   |        method m of type [T](t: T): String is an extension method, cannot override a normal method

tests/neg/override-extension_normal-methods.scala

@@ -0,0 +1,15 @@
+trait A {
+  def [T](t: T).m: String = "extrnsion method"
+}
+
+trait AAA extends A {
+  def m[T](x: T): String  = "normal method" // error: normal method, cannot override an extension method. Also needs `override' modifier (but this error should be obfuscated).
+}
+
+trait B {
+  def m[T](x: T): String  = "normal method"
+}
+
+trait BBB extends B {
+  def [T](t: T).m: String = "extrnsion method" // error: extension method, cannot override an normal method. Also needs `override' modifier (but this error should be obfuscated).
+}