fix nim-lang#16898 nim-lang#17621

compiler/semtypes.nim

@@ -10,7 +10,7 @@
 # this module does the semantic checking of type declarations
 # included from sem.nim
 
-import math
+import std/math
 
 const
   errStringOrIdentNodeExpected = "string or ident node expected"
@@ -1171,6 +1171,7 @@ proc liftParamType(c: PContext, procKind: TSymKind, genericParams: PNode,
   else: discard
 
 proc semParamType(c: PContext, n: PNode, constraint: var PNode): PType =
+  ## Semchecks the type of parameters.
   if n.kind == nkCurlyExpr:
     result = semTypeNode(c, n[0], nil)
     constraint = semNodeKindConstraints(n, c.config, 1)
@@ -1226,9 +1227,13 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
     if hasType:
       typ = semParamType(c, a[^2], constraint)
       # TODO: Disallow typed/untyped in procs in the compiler/stdlib
-      if kind == skProc and (typ.kind == tyTyped or typ.kind == tyUntyped):
-        if not isMagic(getCurrOwner(c)):
-          localError(c.config, a[^2].info, "'" & typ.sym.name.s & "' is only allowed in templates and macros or magic procs")
+      if kind in {skProc, skFunc}:
+        if (typ.kind == tyTyped or typ.kind == tyUntyped) and not isMagic(getCurrOwner(c)):
+          localError(c.config, a[^2].info, "'" & typ.sym.name.s & 
+                  "' is only allowed in templates and macros or magic procs")
+        elif typ.kind == tyLent:
+          localError(c.config, a[^2].info, "`lent` is only allowed in return type")
+
 
     if hasDefault:
       def = a[^1]

tests/lent/t16898.nim

@@ -0,0 +1,31 @@
+discard """
+  errormsg: "lent is only allowed in return type"
+"""
+
+# bug #16898
+type
+  Fp[N: static int, T] = object
+    big: array[N, T]
+
+type
+  QuadraticExt* = concept x
+    ## Quadratic Extension concept (like complex)
+    type BaseField = auto
+    x.c0 is BaseField
+    x.c1 is BaseField
+
+{.experimental:"views".}
+
+func prod(r: var QuadraticExt, a, b: lent QuadraticExt) =
+  discard
+
+type
+  Fp2[N: static int, T] = object
+    c0, c1: Fp[N, T]
+
+# This should be passed by reference,
+# but concepts do not respect the 24 bytes rule
+# or `byref` pragma.
+var r, a, b: Fp2[6, uint64]
+
+prod(r, a, b)

tests/lent/t17621.nim

@@ -0,0 +1,15 @@
+discard """
+  errormsg: "lent is only allowed in return type"
+"""
+
+# bug #17621
+{.experimental: "views".}
+
+type Test = ref object
+  foo: int
+
+proc modify(self: lent Test) =
+  self.foo += 1
+
+let test = Test(foo: 12)
+modify(test)