Support static abstract members in interfaces

Author: RexJaeschke

standard/classes.md

@@ -2184,9 +2184,9 @@ Grammar notes:
 
 > *Note*: The overlapping of, and priority between, alternatives here is solely for descriptive convenience; the grammar rules could be elaborated to remove the overlap. ANTLR, and other grammar systems, adopt the same convenience and so *method_body* has the specified semantics automatically. *end note*
 
-A *method_declaration* may include a set of *attributes* ([§23](attributes.md#23-attributes)) and one of the permitted kinds of declared accessibility ([§15.3.6](classes.md#1536-access-modifiers)), the `new` ([§15.3.5](classes.md#1535-the-new-modifier)), `static` ([§15.6.3](classes.md#1563-static-and-instance-methods)), `virtual` ([§15.6.4](classes.md#1564-virtual-methods)), `override` ([§15.6.5](classes.md#1565-override-methods)), `sealed` ([§15.6.6](classes.md#1566-sealed-methods)), `abstract` ([§15.6.7](classes.md#1567-abstract-methods)), `extern` ([§15.6.8](classes.md#1568-external-methods)) and `async` ([§15.14](classes.md#1514-async-functions)). Additionally a  *method_declaration* that is contained directly by a *struct_declaration* may include the `readonly` modifier ([§16.5.12](structs.md#16512-methods)).
+A *method_declaration* may include a set of *attributes* ([§23](attributes.md#23-attributes)) and one of the permitted kinds of declared accessibility ([§15.3.6](classes.md#1536-access-modifiers)), the `new` ([§15.3.5](classes.md#1535-the-new-modifier)), `static` ([§15.6.3](classes.md#1563-static-and-instance-methods)), `virtual` ([§15.6.4](classes.md#1564-virtual-methods)), `override` ([§15.6.5](classes.md#1565-override-methods)), `sealed` ([§15.6.6](classes.md#1566-sealed-methods)), `abstract` ([§15.6.7](classes.md#1567-abstract-methods)), `extern` ([§15.6.8](classes.md#1568-external-methods)) and `async` ([§15.14](classes.md#1514-async-functions)) modifiers. Additionally a  *method_declaration* that is contained directly by a *struct_declaration* may include the `readonly` modifier ([§16.5.12](structs.md#16512-methods)).
 
-A declaration has a valid combination of modifiers if all of the following are true:
+A *method_declaration* has a valid combination of modifiers if all of the following are true. (These rules are modified slightly in the context of an interface; see [§19.4.1](standard/interfaces.md#1941-general).):
 
 - The declaration includes a valid combination of access modifiers ([§15.3.6](classes.md#1536-access-modifiers)).
 - The declaration does not include the same modifier multiple times.