fixes #23949; cannot return lent expression from conditionals like case (#25190)

fixes #23949

It can also allow  `endsInNoReturn` in branches later

compiler/parampatterns.nim

@@ -13,7 +13,7 @@
 import ast, types, msgs, idents, renderer, wordrecg, trees,
   options
 
-import std/strutils
+import std/[strutils, assertions]
 
 # we precompile the pattern here for efficiency into some internal
 # stack based VM :-) Why? Because it's fun; I did no benchmarks to see if that
@@ -216,6 +216,11 @@ proc exprRoot*(n: PNode; allowCalls = true): PSym =
     else:
       break
 
+proc isAssignable*(owner: PSym, n: PNode): TAssignableResult
+
+proc isLentableBranch(owner: PSym, n: PNode): bool =
+  result = isAssignable(owner, n) in {arLentValue, arAddressableConst, arLentValue}
+
 proc isAssignable*(owner: PSym, n: PNode): TAssignableResult =
   ## 'owner' can be nil!
   result = arNone
@@ -308,6 +313,35 @@ proc isAssignable*(owner: PSym, n: PNode): TAssignableResult =
     # nkVarTy denotes an lvalue, but the example above is the only
     # possible code which will get us here
     result = arLValue
+  of nkIfExpr, nkIfStmt:
+    # allow 'if' expressions to be lent if all branches are lentable
+    for branch in n:
+      if branch.len == 2:
+        if not isLentableBranch(owner, branch[1]):
+          return
+      elif branch.len == 1:
+        if not isLentableBranch(owner, branch[0]):
+          return
+      else:
+        raiseAssert "Malformed `if` statement in isAssignable"
+    result = arLentValue
+  of nkCaseStmt:
+    # allow 'case' expressions to be lent if all branches are lentable
+    for i in 1 ..< n.len:
+      let branch = n[i]
+      case branch.kind
+      of nkOfBranch:
+        if not isLentableBranch(owner, branch[^1]):
+          return
+      of nkElifBranch:
+        if not isLentableBranch(owner, branch[1]):
+          return
+      of nkElse:
+        if not isLentableBranch(owner, branch[0]):
+          return
+      else:
+        raiseAssert "Malformed `case` statement in isAssignable"
+    result = arLentValue
   else:
     discard
 

tests/lent/tlents.nim

@@ -0,0 +1,25 @@
+discard """
+  targets: "c cpp"
+"""
+
+type A = object
+  field: int
+
+proc x(a: A): lent int =
+  result = case true
+    of true:
+      a.field
+    of false:
+      a.field
+
+proc y(a: A): lent int =
+  result = if true:
+      a.field
+    else:
+      a.field
+
+block:
+  var a = A(field: 1)
+  doAssert x(a) == 1
+  doAssert y(a) == 1
+