fixes #21377; fixes @[] and {} type inference as returns in generics (#21475)

* fixes `@[]` type inference in generics

* add issue links

* fixes macros and iterators

* refactor

* add one more test

compiler/semexprs.nim

@@ -998,7 +998,7 @@ proc afterCallActions(c: PContext; n, orig: PNode, flags: TExprFlags; expectedTy
     fixAbstractType(c, result)
     analyseIfAddressTakenInCall(c, result)
     if callee.magic != mNone:
-      result = magicsAfterOverloadResolution(c, result, flags)
+      result = magicsAfterOverloadResolution(c, result, flags, expectedType)
     when false:
       if result.typ != nil and
           not (result.typ.kind == tySequence and result.typ[0].kind == tyEmpty):

compiler/seminst.nim

@@ -127,11 +127,18 @@ proc instantiateBody(c: PContext, n, params: PNode, result, orig: PSym) =
         if sfGenSym in param.flags:
           idTablePut(symMap, params[i].sym, result.typ.n[param.position+1].sym)
     freshGenSyms(c, b, result, orig, symMap)
-    
+
     if sfBorrow notin orig.flags: 
       # We do not want to generate a body for generic borrowed procs.
       # As body is a sym to the borrowed proc.
-      b = semProcBody(c, b)
+      let resultType = # todo probably refactor it into a function
+        if result.kind == skMacro:
+          sysTypeFromName(c.graph, n.info, "NimNode")
+        elif not isInlineIterator(result.typ):
+          result.typ[0]
+        else:
+          nil
+      b = semProcBody(c, b, resultType)
     result.ast[bodyPos] = hloBody(c, b)
     excl(result.flags, sfForward)
     trackProc(c, result, result.ast[bodyPos])

compiler/semmagic.nim

@@ -526,7 +526,7 @@ proc checkDefault(c: PContext, n: PNode): PNode =
     message(c.config, n.info, warnUnsafeDefault, typeToString(constructed))
 
 proc magicsAfterOverloadResolution(c: PContext, n: PNode,
-                                   flags: TExprFlags): PNode =
+                                   flags: TExprFlags; expectedType: PType = nil): PNode =
   ## This is the preferred code point to implement magics.
   ## ``c`` the current module, a symbol table to a very good approximation
   ## ``n`` the ast like it would be passed to a real macro
@@ -635,5 +635,9 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     result = n
   of mPrivateAccess:
     result = semPrivateAccess(c, n)
+  of mArrToSeq:
+    result = n
+    if result.typ != nil and expectedType != nil and result.typ.kind == tySequence and expectedType.kind == tySequence and result.typ[0].kind == tyEmpty:
+      result.typ = expectedType # type inference for empty sequence # bug #21377
   else:
     result = n

tests/types/ttopdowninference.nim

@@ -245,7 +245,7 @@ block: # bug #11777
   var s: S = {1, 2}
   doAssert 1 in s
 
-block: # regression #20807
+block: # bug #20807
   var s: seq[string]
   template fail =
     s = @[]
@@ -255,3 +255,38 @@ block: # regression #20807
   test: fail()
   doAssert not (compiles do:
     let x: seq[int] = `@`[string]([]))
+
+block: # bug #21377
+  proc b[T](v: T): seq[int] =
+    let x = 0
+    @[]
+
+  doAssert b(0) == @[]
+
+block: # bug #21377
+  proc b[T](v: T): seq[T] =
+    let x = 0
+    @[]
+
+  doAssert b(0) == @[]
+
+block: # bug #21377
+  proc b[T](v: T): set[bool] =
+    let x = 0
+    {}
+
+  doAssert b(0) == {}
+
+block: # bug #21377
+  proc b[T](v: T): array[0, int] =
+    let x = 0
+    []
+
+  doAssert b(0) == []
+
+block: # bug #21377
+  proc b[T](v: T): array[0, (string, string)] =
+    let x = 0
+    {:}
+
+  doAssert b(0) == {:}