From 43bd0e0ceef70a657fbd932e8d9d5343c0d6f866 Mon Sep 17 00:00:00 2001
From: zerbina <100542850+zerbina@users.noreply.github.com>
Date: Sat, 17 Aug 2024 02:17:23 +0200
Subject: [PATCH] reject overloads that require conversion in `var` position
 (#1420)

## Summary

* overloads where an argument passed to a `var` parameter requires an
  implicit conversion are now rejected
* this results in clearer error messages

## Details

The `checkConstraint` template is only called with `operand`, which is
the operand before fixup (e.g., introduction of implicit conversions),
hence the `isLValue` test not failing.

`arg` (the operand after fixup) is now passed to `isLValue`, so that
implicit conversions are taken into account.
---
 compiler/sem/sigmatch.nim                      |  2 +-
 .../timplicit_conversion_var_parameter.nim     | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 tests/errmsgs/timplicit_conversion_var_parameter.nim

diff --git a/compiler/sem/sigmatch.nim b/compiler/sem/sigmatch.nim
index e14d99681bf..89e3d6d792f 100644
--- a/compiler/sem/sigmatch.nim
+++ b/compiler/sem/sigmatch.nim
@@ -2818,7 +2818,7 @@ proc matchesAux(c: PContext, n, nOrig: PNode, m: var TCandidate, marker: var Int
         if argConverter.typ.kind notin {tyVar}:
           m.error.firstMismatch.kind = kVarNeeded
           noMatch()
-      elif not isLValue(c, n):
+      elif not isLValue(c, arg):
         m.error.firstMismatch.kind = kVarNeeded
         noMatch()
 
diff --git a/tests/errmsgs/timplicit_conversion_var_parameter.nim b/tests/errmsgs/timplicit_conversion_var_parameter.nim
new file mode 100644
index 00000000000..4061dc35966
--- /dev/null
+++ b/tests/errmsgs/timplicit_conversion_var_parameter.nim
@@ -0,0 +1,18 @@
+discard """
+  action: reject
+  nimout: '''
+timplicit_conversion_var_parameter.nim(18, 2) Error: type mismatch: got <uint16>
+but expected one of:
+proc p(x: var uint32)
+  first type mismatch at position: 1
+  required type for x: var uint32
+  but expression 'arg' is immutable, not 'var'
+
+expression: p(arg)
+'''
+"""
+
+proc p(x: var uint32) = discard
+
+var arg: uint16 # requires widening conversion
+p(arg)