Added informal generic method syntax and generic function type specs.

docs/language/informal/generic-method-syntax.md

+# Feature: Generic Method Syntax
+
+**This document** is an informal specification of the support for generic
+methods and functions which has been implemented in `dart2js` with option
+`--generic-method-syntax`, starting with commit
+[acc5f59](https://github.com/dart-lang/sdk/commit/acc5f59a99d5d8747459c935e6360ac325606cc6).
+In SDK 1.21 this feature is available by default (i.e., also without the
+option) in the virtual machine and the analyzer, as well as in `dart2js`.
+
+The **motivation for** having this **feature** is that it enables partial
+support for generic methods and functions, thus providing a bridge between
+not having generic methods and having full support for generic methods. In
+particular, code declaring and using generic methods may be type checked and
+compiled in strong mode, and the same code will now be acceptable in
+standard (non-strong) mode as well. The semantics is different in certain
+cases, but standard mode analysis will emit diagnostic messages (e.g.,
+errors) for that.
+
+In this document, the word **routine** will be used when referring to
+something which can be a method, a top level function, a local function, or
+a function literal expression.
+
+With **this feature** it is possible to compile code where generic methods
+and functions are declared, implemented, and invoked. The runtime semantics
+does not include reification of type arguments. Evaluations of the runtime
+value of a routine type parameter is a runtime error or yields `dynamic`,
+depending on the context. No type checking takes place at usages of a method
+or function type parameter in the body, and no type checking regarding
+explicitly specified or omitted type arguments takes place at call sites.
+
+In short, generic methods and functions are supported syntactically, and the
+runtime semantics prevents dynamic usages of the type argument values, but
+it allows all usages where that dynamic value is not required. For instance,
+a generic routine type parameter, `T`, cannot be used in an expression like
+`x is T`, but it can be used as a type annotation. In a context where other
+tools may perform type checking, this allows for a similar level of
+expressive power as do language designs where type arguments are erased at
+compile time.
+
+The **motivation for** this **document** is that it serves as an informal
+specification for the implementation of support for the generic method
+syntax feature in all Dart tools.
+
+## Syntax
+
+The syntactic elements which are added or modified in order to support this
+feature are as follows, based on grammar rules given in the Dart Language
+Specification (Aug 19, 2015).
+
+```
+formalParameterPart:
+  typeParameters? formalParameterList
+functionSignature:
+  metadata returnType? identifier formalParameterPart
+typeParameter:
+  metadata identifier ('extends' type)?
+functionExpression:
+  formalParameterPart functionBody
+fieldFormalParameter:
+  metadata finalConstVarOrType? 'this' '.' identifier
+  formalParameterPart?
+argumentPart:
+  typeArguments? arguments
+selector:
+  assignableSelector | argumentPart
+assignableExpression:
+  primary (argumentPart* assignableSelector)+ |
+  'super' unconditionalAssignableSelector |
+  identifier
+cascadeSection:
+  '..' (cascadeSelector argumentPart*)
+  (assignableSelector argumentPart*)*
+  (assignmentOperator expressionWithoutCascade)?
+```
+
+In a [draft specification](https://codereview.chromium.org/1177073002) of
+generic methods from June 2015, the number of grammar changes is
+significantly higher, but that form can be obtained via renaming.
+
+This extension to the grammar gives rise to an **ambiguity** where the
+same tokens may be angle brackets of a type argument list as well as
+relational operators. For instance, `foo(a<b,c>(d))`[^1] may be parsed as  
+a `postfixExpression` on the form `primary arguments` where the arguments
+are two relational expressions (`a<b` and `c>(d)`), and it may also be
+parsed such that there is a single argument which is an invocation of a
+generic function (`a<b,c>(d)`).  The ambiguity is resolved in **favor** of
+the latter.
+
+This implies that an expression like `foo(a<b,2>(d))` will be rejected
+because it is parsed such that `foo` gets one argument which must be a
+generic function invocation, but `2` cannot parse correctly as a
+`type`. This is a breaking change, because the same expression used to parse
+correctly as an invocation of `foo` with two arguments.
+
+The **reason** why the generic function invocation is favored over the
+relational expressions is that it is considered to be a rare exception that
+this ambiguity arises: It requires a balanced set of angle brackets followed
+by a left parenthesis, which is already an unusual form. On top of that, the
+style guide recommendation to use named parameters for boolean arguments
+helps making this situation even less common.
+
+If it does occur then there is an easy **workaround**: an extra set of
+parentheses (as in `foo(a<b,(2>(d)))`) will resolve the ambiguity in the
+direction of relational expressions; or we might simply be able to remove
+the parentheses around the last expression (as in `foo(a<b,2>d)`), which
+will also eliminate the ambiguity.
+
+_It should be noted that parsing techniques like recursive descent seem to
+conflict with this approach to disambiguation: Determining whether the
+remaining input starts with a balanced expression on the form `<` .. `>`
+seems to imply a need for an unbounded lookahead. However, if some type of
+"diet" parsing is used and various kinds of bracket tokens are matched up
+during the lexical analysis then it takes only a simple O(1) check in the
+parser to perform the required check._
+
+## Scope of the Mechanism
+
+With the syntax in place, it is obvious that certain potential extensions
+have **not** been **included**.
+
+For instance, constructors, setters, getters, and operators cannot be
+declared as generic: The syntax for passing actual type arguments at
+invocation sites for setters, getters, and operators is likely to be
+unwieldy and confusing, and for constructors there is a need to find
+a way to distinguish between type arguments for the new instance and
+type arguments for the constructor itself. However, there are plans
+to add support for generic constructors.
+
+This informal specification specifies a dynamic semantics where the values
+of **actual type arguments are not reified** at run time. A future
+extension of this mechanism may add this reification, such that dynamic
+type tests and type casts involving routine type variables will be
+supported.
+
+## Resolution and Type Checking
+
+In order to be useful, the support for generic methods and functions must be
+sufficiently complete and consistent to **avoid spurious** diagnostic
+**messages**. In particular, even though no regular type checks take place
+at usages of routine type parameters in the body where they are in scope,
+those type parameters should be resolved. If they had been ignored then any
+usage of a routine type parameter `X` would give rise to a `Cannot resolve
+type X` error message, or the usage might resolve to other declarations of
+`X` in enclosing scopes such as a class type parameter, both of which is
+unacceptable.
+
+In `dart2js` resolution, the desired behavior has been achieved by adding a
+new type parameter **scope** and putting the type parameters into that
+scope, giving each of them the bound `dynamic`. The type parameter scope is
+the current scope during resolution of the routine signature and the type
+parameter bounds, it encloses the formal parameter scope of the routine, and
+the formal parameter scope in turn encloses the body scope.
+
+This implies that every usage of a routine type parameter is treated during
+**type checking** as if it had been an alias for the type dynamic.
+
+Static checks for **invocations** of methods or functions where type
+arguments are passed are omitted entirely: The type arguments are parsed,
+but no checks are applied to certify that the given routine accepts type
+arguments, and no checks are applied for bound violations. Similarly, no
+checks are performed for invocations where no type arguments are passed,
+whether or not the given routine is statically known to accept type
+arguments.
+
+Certain usages of a routine type parameter `X` give rise to **errors**: It
+is a compile-time error if `X` is used as a type literal expression (e.g.,
+`foo(X)`), or in an expression on the form `e is X` or `e is! X`, or in a
+try/catch statement like `.. on T catch ..`.
+
+It could be argued that it should be a warning or an error if a routine type
+parameter `X` is used in an expression on the form `e as X`. The blind
+success of this test at runtime may introduce bugs into correct programs in
+situations where the type constraint is violated; in particular, this could
+cause "wrong" objects to propagate through local variables and parameters
+and even into data structures (say, when a `List<T>` is actually a
+`List<dynamic>`, because `T` is not present at runtime when the list is
+created). However, considering that these type constraint violations are
+expected to be rare, and considering that it is common to require that
+programs compile without warnings, we have chosen to omit this warning. A
+tool is still free to emit a hint, or in some other way indicate that there
+is an issue.
+
+## Dynamic semantics
+
+If a routine invocation specifies actual type arguments, e.g., `int` in the
+**invocation** `f<int>(42)`, those type arguments will not be evaluated at
+runtime, and they will not be passed to the routine in the
+invocation. Similarly, no type arguments are ever passed to a generic
+routine due to call-site inference. This corresponds to the fact that the
+type arguments have no runtime representation.
+
+When the body of a generic **routine** is **executed**, usages of the formal
+type parameters will either result in a run-time error, or they will yield
+the type dynamic, following the treatment of malformed types in
+Dart. There are the following cases:
+
+When `X` is a routine type parameter, the evaluation of `e is X`, `e is! X`,
+and `X` used as an expression proceeds as if `X` had been a malformed type,
+producing a dynamic error; the evaluation of `e as X` has the same outcome
+as the evaluation of `e`.
+
+Note that the forms containing `is` are compile-time errors, which means
+that compilers may reject the program or offer ways to compile the program
+with a different runtime semantics for these expressions. The rationale for
+`dart2js` allowing the construct and compiling it to a run time error is
+that (1) this allows more programs using generic methods to be compiled,
+and (2) an `is` expression that blindly returns `true` every time (or
+`false` every time) may silently introduce a bug into an otherwise correct
+program, so the expression must fail if it is ever evaluated.
+
+When `X` is a routine type parameter which is passed as a type argument to a
+generic class instantiation `G`, it is again treated like a malformed type,
+i.e., it is considered to denote the type dynamic.
+
+This may be surprising, so let us consider a couple of examples: When `X` is
+a routine type parameter, `42 is X` raises a dynamic error, `<int>[42] is
+List<X>` yields the value `true`, and `42 as X` yields `42`, no matter
+whether the syntax for the invocation of the routine included an actual type
+argument, and, if so, no matter which value the actual type argument would
+have had at the invocation.
+
+Object construction is similar: When `X` is a routine type parameter which
+is a passed as a type argument in a constructor invocation, the actual
+value of the type type argument will be the type dynamic, as it would have
+been with a malformed type.
+
+In **checked mode**, when `X` is a routine type parameter, no checked mode
+checks will ever fail for initialization or assignment to a local variable
+or parameter whose type annotation is `X`, and if the type annotation is a
+generic type `G` that contains `X`, checked mode checks will succeed or
+fail as if `X` had been the type dynamic. Note that this differs from the
+treatment of malformed types.
+
+## Changes
+
+2017-Jan-04: Changed 'static error' to 'compile-time error', which is the
+phrase that the language specification uses.
+
+## Notes
+
+[^1]: These expressions violate the common style in Dart with respect to
+spacing and capitalization. That is because the ambiguity implies
+conflicting requirements, and we do not want to bias the appearance in
+one of the two directions.