ext/dynblock: Make iterator variables visible to nested block for_each

Previously we were incorrectly passing down the original forEachCtx down
to nested child blocks for recursive expansion. Instead, we must use the
iteration-specific constructed EvalContext, which then allows any nested
dynamic blocks to use the parent's iterator variable in their for_each or
labels expressions, and thus unpack nested data structures into
corresponding nested block structures:

    dynamic "parent" {
      for_each = [["a", "b"], []]
      content {
        dynamic "child" {
          for_each = parent.value
          content {}
        }
      }
    }

ext/dynblock/expand_body.go

@@ -234,7 +234,8 @@ func (b *expandBody) expandBlocks(schema *hcl.BodySchema, rawBlocks hcl.Blocks,
 }
 
 func (b *expandBody) expandChild(child hcl.Body, i *iteration) hcl.Body {
-	ret := Expand(child, b.forEachCtx)
+	chiCtx := i.EvalContext(b.forEachCtx)
+	ret := Expand(child, chiCtx)
 	ret.(*expandBody).iteration = i
 	return ret
 }

ext/dynblock/expand_body_test.go

@@ -145,6 +145,52 @@ func TestExpand(t *testing.T) {
 					},
 				}),
 			},
+			{
+				Type:        "dynamic",
+				Labels:      []string{"b"},
+				LabelRanges: []hcl.Range{hcl.Range{}},
+				Body: hcltest.MockBody(&hcl.BodyContent{
+					Attributes: hcltest.MockAttrs(map[string]hcl.Expression{
+						"for_each": hcltest.MockExprLiteral(cty.MapVal(map[string]cty.Value{
+							"foo": cty.ListVal([]cty.Value{
+								cty.StringVal("dynamic c nested 0"),
+								cty.StringVal("dynamic c nested 1"),
+							}),
+						})),
+						"iterator": hcltest.MockExprVariable("dyn_b"),
+					}),
+					Blocks: hcl.Blocks{
+						{
+							Type: "content",
+							Body: hcltest.MockBody(&hcl.BodyContent{
+								Blocks: hcl.Blocks{
+									{
+										Type:        "dynamic",
+										Labels:      []string{"c"},
+										LabelRanges: []hcl.Range{hcl.Range{}},
+										Body: hcltest.MockBody(&hcl.BodyContent{
+											Attributes: hcltest.MockAttrs(map[string]hcl.Expression{
+												"for_each": hcltest.MockExprTraversalSrc("dyn_b.value"),
+											}),
+											Blocks: hcl.Blocks{
+												{
+													Type: "content",
+													Body: hcltest.MockBody(&hcl.BodyContent{
+														Attributes: hcltest.MockAttrs(map[string]hcl.Expression{
+															"val0": hcltest.MockExprTraversalSrc("c.value"),
+															"val1": hcltest.MockExprTraversalSrc("dyn_b.key"),
+														}),
+													}),
+												},
+											},
+										}),
+									},
+								},
+							}),
+						},
+					},
+				}),
+			},
 			{
 				Type:        "a",
 				Labels:      []string{"static1"},
@@ -268,6 +314,16 @@ func TestExpand(t *testing.T) {
 					"val1": cty.StringVal("dynamic b 1"),
 				}),
 			}),
+			cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"val0": cty.StringVal("dynamic c nested 0"),
+					"val1": cty.StringVal("foo"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"val0": cty.StringVal("dynamic c nested 1"),
+					"val1": cty.StringVal("foo"),
+				}),
+			}),
 		})
 
 		if !got.RawEquals(want) {

ext/dynblock/expand_spec.go

@@ -41,11 +41,14 @@ func (b *expandBody) decodeSpec(blockS *hcl.BlockHeaderSchema, rawSpec *hcl.Bloc
 	eachVal, eachDiags := eachAttr.Expr.Value(b.forEachCtx)
 	diags = append(diags, eachDiags...)
 
-	if !eachVal.CanIterateElements() {
+	if !eachVal.CanIterateElements() && eachVal.Type() != cty.DynamicPseudoType {
+		// We skip this error for DynamicPseudoType because that means we either
+		// have a null (which is checked immediately below) or an unknown
+		// (which is handled in the expandBody Content methods).
 		diags = append(diags, &hcl.Diagnostic{
 			Severity:    hcl.DiagError,
 			Summary:     "Invalid dynamic for_each value",
-			Detail:      fmt.Sprintf("Cannot use a value of type %s in for_each. An iterable collection is required.", eachVal.Type()),
+			Detail:      fmt.Sprintf("Cannot use a %s value in for_each. An iterable collection is required.", eachVal.Type().FriendlyName()),
 			Subject:     eachAttr.Expr.Range().Ptr(),
 			Expression:  eachAttr.Expr,
 			EvalContext: b.forEachCtx,

ext/dynblock/iteration.go

@@ -30,12 +30,14 @@ func (i *iteration) Object() cty.Value {
 
 func (i *iteration) EvalContext(base *hcl.EvalContext) *hcl.EvalContext {
 	new := base.NewChild()
-	new.Variables = map[string]cty.Value{}
 
+	if i != nil {
+		new.Variables = map[string]cty.Value{}
 		for name, otherIt := range i.Inherited {
 			new.Variables[name] = otherIt.Object()
 		}
 		new.Variables[i.IteratorName] = i.Object()
+	}
 
 	return new
 }