[Fix] nvm exec/nvm run: warn when no version and no .nvmrc

Currently these commands silently fall back to the active node version when neither a version argument nor an `.nvmrc` resolves,
making them invisibly dependent on shell state and impossible to script predictably (see #3755).

Print a stderr deprecation warning in this case (suppressed by `--silent`) and continue with the active node version,
so existing callers keep working.
The follow-up change will turn this into a hard error;
pass `current` explicitly (e.g. `nvm exec current node ...`) to silence the warning and lock in the new behavior now.

Refs #3755

nvm.sh

@@ -4179,6 +4179,14 @@ nvm() {
             if [ $has_checked_nvmrc -ne 1 ]; then
               { NVM_RC_VERSION="$(NVM_SILENT="${NVM_SILENT:-0}" nvm_rc_version 3>&1 1>&4)"; } 4>&1 && has_checked_nvmrc=1
             fi
+            if [ -z "${NVM_RC_VERSION-}" ]; then
+              if [ "${NVM_SILENT:-0}" -ne 1 ]; then
+                nvm_err 'WARNING: `nvm run` was invoked without a version argument and without an .nvmrc file.'
+                nvm_err '  Falling back to the active node version; this will become an error in a future release.'
+                nvm_err '  Pass `current` explicitly (e.g. `nvm run current ...`) to silence this warning.'
+              fi
+              NVM_RC_VERSION="$(nvm_version current)" ||:
+            fi
             provided_version="${NVM_RC_VERSION}"
             IS_VERSION_FROM_NVMRC=1
             VERSION="$(nvm_version "${NVM_RC_VERSION}")" ||:
@@ -4236,19 +4244,36 @@ nvm() {
 
       local provided_version
       provided_version="$1"
+      local VERSION_SOURCE
+      VERSION_SOURCE=''
       if [ "${NVM_LTS-}" != '' ]; then
         provided_version="lts/${NVM_LTS:-*}"
         VERSION="${provided_version}"
+        VERSION_SOURCE='lts'
       elif [ -n "${provided_version}" ]; then
         VERSION="$(nvm_version "${provided_version}")" ||:
         if [ "_${VERSION}" = '_N/A' ] && ! nvm_is_valid_version "${provided_version}"; then
           { provided_version="$(NVM_SILENT="${NVM_SILENT:-0}" nvm_rc_version 3>&1 1>&4)"; } 4>&1 && has_checked_nvmrc=1
           VERSION="$(nvm_version "${provided_version}")" ||:
+          if [ -n "${provided_version}" ]; then
+            VERSION_SOURCE='nvmrc'
+          fi
         else
+          VERSION_SOURCE='arg'
           shift
         fi
       fi
 
+      if [ -z "${VERSION_SOURCE}" ]; then
+        if [ "${NVM_SILENT:-0}" -ne 1 ]; then
+          nvm_err 'WARNING: `nvm exec` was invoked without a version argument and without an .nvmrc file.'
+          nvm_err '  Falling back to the active node version; this will become an error in a future release.'
+          nvm_err '  Pass `current` explicitly (e.g. `nvm exec current ...`) to silence this warning.'
+        fi
+        provided_version='current'
+        VERSION="$(nvm_version current)" ||:
+      fi
+
       nvm_ensure_version_installed "${provided_version}"
       EXIT_CODE=$?
       if [ "${EXIT_CODE}" != "0" ]; then

test/fast/Running 'nvm exec' and 'nvm run' without a resolvable version warn and fall back

@@ -0,0 +1,63 @@
+#!/bin/sh
+
+set -ex
+
+die () { echo "$@" ; cleanup ; exit 1; }
+
+cleanup() {
+  cd "${ORIG_PWD}" 2>/dev/null || true
+  [ -n "${TMP_DIR-}" ] && rm -rf "${TMP_DIR}"
+}
+
+export NVM_DIR="$(cd ../.. && pwd)"
+
+: nvm.sh
+\. ../../nvm.sh
+
+\. ../common.sh
+
+ORIG_PWD="$(pwd)"
+
+# Run from a fresh, empty directory so no ambient .nvmrc above the test dir
+# can satisfy the lookup and mask the warning.
+TMP_DIR="$(mktemp -d)"
+cd "${TMP_DIR}" || die "could not cd to temp dir"
+
+EXEC_WARNING='WARNING: `nvm exec` was invoked without a version argument and without an .nvmrc file.'
+RUN_WARNING='WARNING: `nvm run` was invoked without a version argument and without an .nvmrc file.'
+
+# `nvm exec` with no version and no .nvmrc should warn on stderr (and fall back).
+set +ex # needed for stderr
+EXEC_STDERR="$(nvm exec </dev/null 2>&1 1>/dev/null)"
+set -ex
+case "${EXEC_STDERR}" in
+  *"${EXEC_WARNING}"*) ;;
+  *) die "'nvm exec' with no version did not warn; got >${EXEC_STDERR}<" ;;
+esac
+
+# `--silent` should suppress the warning.
+set +ex # needed for stderr
+EXEC_SILENT_STDERR="$(nvm exec --silent </dev/null 2>&1 1>/dev/null)"
+set -ex
+case "${EXEC_SILENT_STDERR}" in
+  *WARNING*) die "'nvm exec --silent' should not warn; got >${EXEC_SILENT_STDERR}<" ;;
+esac
+
+# `nvm run` with an unresolvable version and no .nvmrc should warn (and fall back).
+set +ex # needed for stderr
+RUN_STDERR="$(nvm run bogusversion </dev/null 2>&1 1>/dev/null)"
+set -ex
+case "${RUN_STDERR}" in
+  *"${RUN_WARNING}"*) ;;
+  *) die "'nvm run' with no resolvable version did not warn; got >${RUN_STDERR}<" ;;
+esac
+
+# `--silent` should suppress the warning.
+set +ex # needed for stderr
+RUN_SILENT_STDERR="$(nvm run --silent bogusversion </dev/null 2>&1 1>/dev/null)"
+set -ex
+case "${RUN_SILENT_STDERR}" in
+  *WARNING*) die "'nvm run --silent' should not warn; got >${RUN_SILENT_STDERR}<" ;;
+esac
+
+cleanup