Introduce SingleValueStmtExpr, which allows the
embedding of a statement in an expression context.
This then allows us to parse and type-check `if`
and `switch` statements as expressions, gated
behind the `IfSwitchExpression` experimental
feature for now. In the future,
SingleValueStmtExpr could also be used for e.g
`do` expressions.
For now, only single expression branches are
supported for producing a value from an
`if`/`switch` expression, and each branch is
type-checked independently. A multi-statement
branch may only appear if it ends with a `throw`,
and it may not `break`, `continue`, or `return`.
The placement of `if`/`switch` expressions is also
currently limited by a syntactic use diagnostic.
Currently they're only allowed in bindings,
assignments, throws, and returns. But this could
be lifted in the future if desired.
For a single expression closure, just use the
expression as the body in the case where we're
coercing to Void, as the return is already
implied. This avoids crashing in
`ClosureExpr::getSingleExpressionBody` with a
double braced body.
Surprisingly it seems nothing is currently calling
`ClosureExpr::getSingleExpressionBody` after
type-checking, so no test case, but later commits
in this patch will exercise this case.
All of the type variables referenced by a type had to be
handled by `inferVariables` otherwise it would to possible
to bring non-representative variable into scope which in
turn later would get simplied into a variable that doesn't
exist in the active scope and solver would crash.
Resolves: rdar://83418797
`transformedBody` is available only if the flag is set, so there
is no reason to double-check it before application, the presence
of the transformed body is evidence enough.
Invalid syntax could introduce type variables into sequence which
have to be brought into scope otherwise solver is going to crash.
Resolves: rdar://100753270
Replace `AnyFunctionRef` as "context" for syntactic element with
a custom `SyntacticElementContext` to support type-checking of
other constructs in the future.
Split out brace element handling for a couple of
walkers into a new function. The diff for this is
best viewed without whitespace changes. This
should be NFC.
