fix(template): add helpful hint to kubeVersion error when using defaults

On Slack a user reported that he didn't understand why `helm template`
was reporting v1.20.0. He had no Kubernetes cluster connection. This is
because we fallback on default Kube version in capabilities.go.

This change tracks whether the KubeVersion is a default fallback and
adds a helpful hint to the error message only when the default is used
(not when users explicitly set --kube-version).

Before:
```
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.20.0
```

After (with default):
```
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.20.0 (no cluster connected, using default version; specify target version with --kube-version)
```

After (with explicit --kube-version 1.27.0):
```
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.27.0
```

Testing:
```sh
$ bin/helm template test-release /tmp/test-chart
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.20.0 (no cluster connected, using default version; specify target version with --kube-version)

$ bin/helm template test-release /tmp/test-chart --kube-version 1.27.0
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.27.0

$ bin/helm template test-release /tmp/test-chart --kube-version 1.20.0
Error: chart requires kubeVersion: >= 1.28.0-0 which is incompatible with Kubernetes v1.20.0

$ bin/helm template test-release /tmp/test-chart --kube-version 1.28.0
---
apiVersion: v1
kind: ConfigMap
...
```

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

pkg/action/action.go

@@ -204,7 +204,11 @@ func (cfg *Configuration) renderResources(ch *chart.Chart, values common.Values,
 
 	if ch.Metadata.KubeVersion != "" {
 		if !chartutil.IsCompatibleRange(ch.Metadata.KubeVersion, caps.KubeVersion.String()) {
-			return hs, b, "", fmt.Errorf("chart requires kubeVersion: %s which is incompatible with Kubernetes %s", ch.Metadata.KubeVersion, caps.KubeVersion.String())
+			errMsg := fmt.Sprintf("chart requires kubeVersion: %s which is incompatible with Kubernetes %s", ch.Metadata.KubeVersion, caps.KubeVersion.String())
+			if caps.IsDefaultKubeVersion {
+				errMsg += " (no cluster connected, using default version; specify target version with --kube-version)"
+			}
+			return hs, b, "", fmt.Errorf("%s", errMsg)
 		}
 	}
 
@@ -383,15 +387,14 @@ func (cfg *Configuration) getCapabilities() (*common.Capabilities, error) {
 		}
 	}
 
-	cfg.Capabilities = &common.Capabilities{
-		APIVersions: apiVersions,
-		KubeVersion: common.KubeVersion{
+	cfg.Capabilities = common.NewCapabilitiesFromCluster(
+		common.KubeVersion{
 			Version: kubeVersion.GitVersion,
 			Major:   kubeVersion.Major,
 			Minor:   kubeVersion.Minor,
 		},
-		HelmVersion: common.DefaultCapabilities.HelmVersion,
-	}
+		apiVersions,
+	)
 	return cfg.Capabilities, nil
 }
 

pkg/action/install.go

@@ -304,7 +304,7 @@ func (i *Install) RunWithContext(ctx context.Context, ch ci.Charter, vals map[st
 		// NOTE(bacongobbler): used for `helm template`
 		i.cfg.Capabilities = common.DefaultCapabilities.Copy()
 		if i.KubeVersion != nil {
-			i.cfg.Capabilities.KubeVersion = *i.KubeVersion
+			i.cfg.Capabilities = i.cfg.Capabilities.WithKubeVersion(*i.KubeVersion)
 		}
 		i.cfg.Capabilities.APIVersions = append(i.cfg.Capabilities.APIVersions, i.APIVersions...)
 		i.cfg.KubeClient = &kubefake.PrintingKubeClient{Out: io.Discard}

pkg/chart/common/capabilities.go

@@ -47,6 +47,7 @@ var (
 		},
 		APIVersions:          DefaultVersionSet,
 		HelmVersion:          helmversion.Get(),
+		IsDefaultKubeVersion: true,
 	}
 )
 
@@ -58,6 +59,9 @@ type Capabilities struct {
 	APIVersions VersionSet
 	// HelmVersion is the build information for this helm version
 	HelmVersion helmversion.BuildInfo
+	// IsDefaultKubeVersion indicates if KubeVersion is a default fallback value
+	// (not from a real cluster or user-provided --kube-version flag)
+	IsDefaultKubeVersion bool
 }
 
 func (capabilities *Capabilities) Copy() *Capabilities {
@@ -65,6 +69,27 @@ func (capabilities *Capabilities) Copy() *Capabilities {
 		KubeVersion:          capabilities.KubeVersion,
 		APIVersions:          capabilities.APIVersions,
 		HelmVersion:          capabilities.HelmVersion,
+		IsDefaultKubeVersion: capabilities.IsDefaultKubeVersion,
+	}
+}
+
+// WithKubeVersion returns a copy of Capabilities with a custom KubeVersion.
+// Sets IsDefaultKubeVersion to false since this is an explicit override.
+func (capabilities *Capabilities) WithKubeVersion(kv KubeVersion) *Capabilities {
+	c := capabilities.Copy()
+	c.KubeVersion = kv
+	c.IsDefaultKubeVersion = false
+	return c
+}
+
+// NewCapabilitiesFromCluster creates a Capabilities from cluster discovery information.
+// IsDefaultKubeVersion is set to false since this comes from a real cluster.
+func NewCapabilitiesFromCluster(kubeVersion KubeVersion, apiVersions VersionSet) *Capabilities {
+	return &Capabilities{
+		KubeVersion:          kubeVersion,
+		APIVersions:          apiVersions,
+		HelmVersion:          DefaultCapabilities.HelmVersion,
+		IsDefaultKubeVersion: false,
 	}
 }