Remove panic, update comment

.tfdev

@@ -6,3 +6,8 @@ version_info {
 
 version_exec = false
 disable_provider_requirements = true
+
+platform {
+    os = "darwin"
+    arch = "amd64"
+}

configs/testdata/invalid-files/data-reserved-for_each.tf

@@ -1,3 +0,0 @@
-data "test" "foo" {
-  for_each = ["a"]
-}

terraform/eval_for_each.go

@@ -16,18 +16,12 @@ import (
 func evaluateResourceForEachExpression(expr hcl.Expression, ctx EvalContext) (forEach map[string]cty.Value, diags tfdiags.Diagnostics) {
 	forEachMap, known, diags := evaluateResourceForEachExpressionKnown(expr, ctx)
 	if !known {
-		// Currently this is a rather bad outcome from a UX standpoint, since we have
-		// no real mechanism to deal with this situation and all we can do is produce
-		// an error message.
-		// FIXME: In future, implement a built-in mechanism for deferring changes that
-		// can't yet be predicted, and use it to guide the user through several
-		// plan/apply steps until the desired configuration is eventually reached.
+		// Attach a diag as we do with count, with the same downsides
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagError,
 			Summary:  "Invalid forEach argument",
 			Detail:   `The "for_each" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to first apply only the resources that the for_each depends on.`,
 		})
-		panic("uh-oh")
 	}
 	return forEachMap, diags
 }