hclsyntax: Detect and reject invalid nested splat result

hclsyntax/expression.go

@@ -1938,6 +1938,22 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			diags = append(diags, tyDiags...)
 			return cty.ListValEmpty(ty.ElementType()).WithMarks(marks), diags
 		}
+		// Unfortunately it's possible for a nested splat on scalar values to
+		// generate non-homogenously-typed vals, and we discovered this bad
+		// interaction after the two conflicting behaviors were both
+		// well-established so it isn't clear how to change them without
+		// breaking existing code. Therefore we just make that an error for
+		// now, to avoid crashing trying to constuct an impossible list.
+		if !cty.CanListVal(vals) {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid nested splat expressions",
+				Detail:   "The second level of splat expression produced elements of different types, so it isn't possible to construct a valid list to represent the top-level result.\n\nConsider using a for expression instead, to produce a tuple-typed result which can therefore have non-homogenous element types.",
+				Subject:  e.Each.Range().Ptr(),
+				Context:  e.Range().Ptr(), // encourage a diagnostic renderer to also include the "source" part of the expression in its code snippet
+			})
+			return cty.DynamicVal, diags
+		}
 		return cty.ListVal(vals).WithMarks(marks), diags
 	default:
 		return cty.TupleVal(vals).WithMarks(marks), diags

hclsyntax/expression_test.go

@@ -1387,6 +1387,54 @@ upper(
 			cty.DynamicVal,
 			1, // splat cannot be applied to null sequence
 		},
+		{
+			`listofobj[*].scalar[*]`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"listofobj": cty.ListVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"scalar": cty.StringVal("foo"),
+						}),
+						cty.ObjectVal(map[string]cty.Value{
+							"scalar": cty.StringVal("bar"),
+						}),
+					}),
+				},
+			},
+			cty.ListVal([]cty.Value{
+				// The second-level splat promotes the scalars to single-element tuples.
+				cty.TupleVal([]cty.Value{cty.StringVal("foo")}),
+				cty.TupleVal([]cty.Value{cty.StringVal("bar")}),
+			}),
+			0,
+		},
+		{
+			// This is a particularly tricky case where two splat rules interact in
+			// a sub-optimal way:
+			// 1. The top-level splat is applied to a list and so it wants to return a list.
+			// 2. The nested splat is applied to a scalar, and so it wants to return different tuple types depending on the nullness.
+			// Rule 2 breaks rule 1, because we can't make a list with elements of different types.
+			// For now we're treating this as an error because we didn't learn of this bad
+			// interaction until long after both of these rules were in separate wide use,
+			// and so it isn't clear how to make this work without potentially breaking other
+			// behavior. Perhaps this can become valid in future if we find a viable way to
+			// do it.
+			`listofobj[*].scalar[*]`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"listofobj": cty.ListVal([]cty.Value{
+						cty.ObjectVal(map[string]cty.Value{
+							"scalar": cty.NullVal(cty.String),
+						}),
+						cty.ObjectVal(map[string]cty.Value{
+							"scalar": cty.StringVal("bar"),
+						}),
+					}),
+				},
+			},
+			cty.DynamicVal,
+			1, // nested splat produces non-homogenously-typed results in this case, so cannot produce a valid list
+		},
 		{
 			`["hello", "goodbye"].*`,
 			nil,