Add support for if expressions

[cirras]

This PR implements if expressions.

Embarcadero Documentation

The language feature is documented by Embarcadero here.

They even kindly provided a table of operand type combinations with the corresponding result type.

Type 1	Type 2	Result type
AnsiChar	AnsiChar	AnsiChar
AnsiString	AnsiString, AnsiChar	AnsiString
Array	Array	Array of the same element type.
Boolean types	Boolean types	Common Boolean type or Boolean type.
Class reference	Class reference, nil	Common type, Base type, or TClass.
Dynamic array	Dynamic array	Common dynamic array type.
Enum type	Enum type	Common enum type.
File	File	Error (use pointer to File type).
Instance	Instance, nil	Common type, Base type, or TObject.
Integral types	Integral types	Common integral type, or the type that covers both ranges.
Integral types	Real types	Real type.
Interface	Interface, nil	Common type, Base type, or IInterface.
nil	AnsiChar, ShortString, AnsiString, WideChar, WideString	Pointer to WideChar
Object	Object	Object type.
Pointer to AnsiChar	AnsiChar, ShortString, AnsiString literal	Pointer to AnsiChar
Pointer to AnsiChar	Pointer to WideChar literal	Pointer to WideChar
Pointer to WideChar	AnsiChar, ShortString, AnsiString, WideChar, WideString, UnicodeString	UnicodeString
Pointer types	Pointer types	Common pointer type, or Pointer type.
Procedure	Procedure	Procedure type, if they have the same signature.
Procedure of object	Procedure of object	Procedure for object type if they have the same signature.
Real types	Real types	The real type with the greater precision.
Record	Record	Record type.
Reference to procedure Anonymous Method	Reference to procedure Anonymous Method	Reference to the procedure type, if they have the same signature.
Set	Set	Common set type, or large set type if list literals.
Short string	Short string, AnsiChar	Longer short string
Text	Text	Error (use pointer to Text type).
UnicodeString	UnicodeString, WideString, AnsiString, Short string, WideChar, AnsiChar	UnicodeString
Variant	Variant	Variant type.
WideChar	WideChar	WideChar
WideChar	AnsiChar, AnsiString	UnicodeString
WideString	WideString, AnsiString, WideChar, AnsiChar	WideString

Reality

Unfortunately, the documented operand/result type combinations are inaccurate or incomplete in various ways.
Below is what Delphi 13 actually does.

Type 1	Type 2	Result type
AnsiChar	AnsiChar	AnsiChar
AnsiString	AnsiString, AnsiChar	Left operand (code pages don't unify)
Array	Array	Identical type only; else error
Boolean types	Boolean types	Left operand, never widened
Class reference	Class reference, nil	Reference to nearest common ancestor; same class -> right operand; nil -> the reference
Dynamic array	Dynamic array, nil	Identical type only (nil -> the array); else error
Dynamic array	[...] constructor	Error
Enum type	Enum type	The enum (subranges unify to it); distinct enums error
File	File	Error (use pointer to File type).
Instance	Instance, nil	Nearest common ancestor class, never an interface; strong aliases are siblings of their base; nil -> the class
Instance	Interface	Error
Integral types	Integral types	Narrowest type covering both ranges; signed/unsigned tie -> anonymous unsigned subrange, or UInt64
Integral types	Real types	Extended; Comp/Currency where Extended is 8 bytes
Interface	Interface, nil	Nearest common ancestor; nil -> the interface
nil	AnsiChar, ShortString, AnsiString, WideChar, WideString	Error
nil	Procedure, procedure of object, reference to	The procedural type
nil	[...] constructor	Anonymous array of T
nil	Variant, any value type	Error
Object	Object	Identical type only; else error
Pointer to AnsiChar	AnsiChar, ShortString, AnsiString literal	Error
Pointer to AnsiChar	Pointer to WideChar literal	Error
Pointer to WideChar	AnsiChar, ShortString, AnsiString, WideChar, WideString, UnicodeString	UnicodeString
Pointer types	Pointer types	Left operand if either is untyped or the right is nil; nil on the left, or unrelated pointers -> Pointer
Procedure	Procedure	Left operand, if they have the same signature
Procedure of object	Procedure of object	Left operand, if they have the same signature
Mixed procedural kinds	Mixed procedural kinds	Error
Real types	Real types	Extended; Comp/Currency where Extended is 8 bytes
Record	Record	Identical type only; else error
Reference to procedure Anonymous Method	Reference to procedure Anonymous Method	The reference to type, if they have the same signature
Set	Set	The containing set (left on ties); else set of Byte or set of AnsiChar
Set	[...] constructor	The set, if the elements are compatible
[...] constructor	[...] constructor	Left operand's set reading, if all elements are ordinal; else error
ShortString	ShortString, AnsiChar	Longer ShortString
Text	Text	Error (use pointer to Text type).
UnicodeString	UnicodeString, WideString, AnsiString, Short string, WideChar, AnsiChar	Left operand
Variant	Variant	Variant (even for OleVariant mixes)
WideChar	WideChar	WideChar
WideChar	AnsiChar, AnsiString	UnicodeString
WideString	WideString, AnsiString, WideChar, AnsiChar	Left operand

Working out the actual rules required a lot of painful testing against the Delphi 13 compiler, but I think this is pretty accurate now.

It definitely isn't a "least upper bound" as commonly understood, or as documented:

• operand types can end up stuffed into ill-fitting result types
  • signed integers going into UInt64
  • large booleans going into smaller booleans (whichever the left operand happened to be)
• ties typically go to the left operand, so ordering of the expression is significant

Fun Facts

• if b then [] else DynArray crashes the 64-bit compiler with an internal error (F2084).

[fourls]

Looks good for the most part! A couple of minor comments.

[cirras]

Addressed review feedback and, in the process, discovered the resolver was handling array constructors incorrectly.
Pulling that thread a little further, it turns out the TypeInferrer was also handling them incorrectly. That was a fun incidental discovery. 😩
Patched up in a separate commit.

[fourls]

Looks really good! The array element type inference being wrong is a bit of a surprise - I guess it's a fairly niche case.

On top of the comments below, could you also rebase on master to get the Java 25 compatibility fixes?

[fourls]

Looks good! This will go a long way to unblocking Delphi 13 support.

As always, I'm sure there's a bunch of bizarre edge case behavioural quirks that we've missed and will have to slowly discover over time 😅