Infer declarations are allowed in generic constraints, but do not infer generic constraint types

TypeScript Version: 4.0.2

Search Terms: generic constraint, extends infer

Code

type InferGenericConstraint<Fn> = 
    Fn extends <T extends infer Constraint>(arg: T) => any
        ? Constraint
        : never;

type GenericFunctionExample = <T extends number>(arg: T) => void;

type InferredConstraint = InferGenericConstraint<GenericFunctionExample>;

Expected behavior:

• Preferred: InferredConstraint is number.
• Alternative: infer declaration is disallowed within a generic constraint clause.

Actual behavior: InferredConstraint is never

Playground Link: https://tsplay.dev/gWoYlN

Related Issues: #1213

Start by reproducing the provided code in the linked TypeScript Playground using the reported 4.0.2 behavior and compare it with the Nightly version. The issue is done when the inferred constraint is number, or when infer declarations in generic constraint clauses are rejected consistently; no repository file or test is named.