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Typecheck something within the context of a deriving strategy. This is of particular importance when the deriving strategy is via. For instance:

deriving via (S a) instance C (T a)

We need to typecheck S a, and moreover, we need to extend the tyvar environment with a before typechecking C (T a), since S a quantified the type variable a.

Type-check a visible type application

Used during the "kind-checking" pass in TcTyClsDecls only, and even then only for data-con declarations. See Note [Use SigTvs in kind-checking pass] in TcTyClsDecls

Bring implicitly quantified type/kind variables into scope during kind checking. Uses SigTvs, as per Note [Use SigTvs in kind-checking pass] in TcTyClsDecls.

Bring tycon tyvars into scope. This is used during the "kind-checking" pass in TcTyClsDecls. (Never in getInitialKind, never in the "type-checking"/desugaring pass.) Never emits constraints, though the thing_inside might.

Used for the type variables of a type or class decl on the second full pass (type-checking/desugaring) in TcTyClDecls. This is *not* used in the initial-kind run, nor in the "kind-checking" pass. Accordingly, everything passed to the continuation is fully zonked.

(tcTyClTyVars T [a,b] thing_inside) where T : forall k1 k2 (a:k1 -> *) (b:k1). k2 -> * calls thing_inside with arguments [k1,k2,a,b] [k1:*, k2:*, Anon (k1 -> *), Anon k1] (k2 -> *) having also extended the type environment with bindings for k1,k2,a,b

Never emits constraints.

The LHsTyVarBndrs is always user-written, and the full, generalised kind of the tycon is available in the local env.

Bring tyvars into scope, wrapping the thing_inside in an implication constraint. The implication constraint is necessary to provide SkolemInfo for the tyvars and to ensure that no unification variables made outside the scope of these tyvars (i.e. lower TcLevel) unify with the locally-scoped tyvars (i.e. higher TcLevel).

INVARIANT: The thing_inside must check only types, never terms.

Use this (not tcExtendTyVarEnv) wherever you expect a Λ or ∀ in Core. Use tcExtendTyVarEnv otherwise.

Like scopeTyVars, but allows you to specify different scoped names than the Names stored within the tyvars.

Kind-check a LHsQTyVars. If the decl under consideration has a complete, user-supplied kind signature (CUSK), generalise the result. Used in getInitialKind (for tycon kinds and other kinds) and in kind-checking (but not for tycon kinds, which are checked with tcTyClDecls). See also Note [Complete user-supplied kind signatures] in HsDecls.

This function does not do telescope checking.

Apply a type of a given kind to a list of arguments. This instantiates invisible parameters as necessary. Always consumes all the arguments, using matchExpectedFunKind as necessary. This takes an optional VarEnv Kind which maps kind variables to kinds. These kinds should be used to instantiate invisible kind variables; they come from an enclosing class for an associated type/data family.

Type-check a thing that emits only equality constraints, then solve those constraints. Fails outright if there is trouble. Use this if you're not going to get another crack at solving (because, e.g., you're checking a datatype declaration)

Instantiate a type to have at most n invisible arguments.

Whenever a type is about to be added to the environment, it's necessary to make sure that any free meta-tyvars in the type are promoted to the current TcLevel. (They might be at a higher level due to the level-bumping in tcExplicitTKBndrs, for example.) This function both zonks *and* promotes.

Make an appropriate message for an error in a function argument. Used for both expressions and types.