TypeScript Language Specification v1.5

3.10.5 Contextual Signature Instantiation
During type argument inference in a function call (section 4.12.2) it is in certain circumstances necessary
to instantiate a generic call signature of an argument expression in the context of a non-generic call
signature of a parameter such that further inferences can be made. A generic call signature A is
instantiated in the context of non-generic call signature B as follows:
Using the process described in 3.10.6, inferences for A's type parameters are made from each
parameter type in B to the corresponding parameter type in A for those parameter positions that
are present in both signatures, where rest parameters correspond to an unbounded expansion of
optional parameters of the rest parameter element type.
The inferred type argument for each type parameter is the union type of the set of inferences
made for that type parameter. However, if the union type does not satisfy the constraint of the
type parameter, the inferred type argument is instead the constraint.
3.10.6 Type Inference
In certain contexts, inferences for a given set of type parameters are made from a type S, in which those
type parameters do not occur, to another type T, in which those type parameters do occur. Inferences
consist of a set of candidate type arguments collected for each of the type parameters. The inference
process recursively relates S and T to gather as many inferences as possible:
If T is one of the type parameters for which inferences are being made, S is added to the set of
inferences for that type parameter.
Otherwise, if S and T are references to the same generic type, inferences are made from each type
argument in S to each corresponding type argument in T.
Otherwise, if T is a union type:
o First, inferences are made from S to each constituent type in T that isn't simply one of the
type parameters for which inferences are being made.
o If the first step produced no inferences and exactly one constituent type in T is simply a
type parameter for which inferences are being made, inferences are made from S to that
type parameter.
Otherwise, if S is a union type, inferences are made from each constituent type in S to T.
Otherwise, if S and T are object types, then for each member M in T:
o If M is a property and S contains a property N with the same name as M, inferences are
made from the type of N to the type of M.
o If M is a call signature and a corresponding call signature N exists in S, N is instantiated
with the Any type as an argument for each type parameter (if any) and inferences are
made from parameter types in N to the corresponding parameter types in M for positions
that are present in both signatures, and from the return type of N to the return type of M.
o If M is a construct signature and a corresponding construct signature N exists in S, N is
instantiated with the Any type as an argument for each type parameter (if any) and
inferences are made from parameter types in N to the corresponding parameter types in
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