generics caching buggy and order-sensitive

[timotheecour]

• generics caching is buggy and order-sensitive
• import order affects which generic instantiation gets cached, resulting in surprising behavior when unrelated code changes
• there's no way to "recover" from a generic instantiation, see example below; note that the "first-arrived" instantiation deepDownHidden1 could be hidden really deep inside some code and it'd be hard to find where or why proc $(a: FileIndex): string = "FOO" in main.nim isn't picked up in a more complex example

Example

#t10297_types.nim:
type
  FileIndex* = distinct int32
  TLineInfo* = object
    line*: uint16
    col*: int16
    fileIndex*: FileIndex

# t10297c.nim
import t10297_types
proc deepDownHidden1() = discard $TLineInfo.default

# t10297d.nim
import t10297_types
proc `$`*(a: FileIndex): string = $a.int32
proc deepDownHidden2() = discard $TLineInfo.default

#main.nim
import t10297_types
import t10297c
import t10297d

proc `$`(a: FileIndex): string = "FOO"
echo TLineInfo.default

Current Output

# as is it shows:
(line: 0, col: 0, fileIndex: ...)
# flip order of import t10297c vs import t10297d in main:
(line: 0, col: 0, fileIndex: 0)
# comment both import t10297c and import t10297d
(line: 0, col: 0, fileIndex: FOO)

Expected Output

(line: 0, col: 0, fileIndex: FOO)

Additional Information

• devel 653de5f 1.1.1