allow the use of primitive names in HCL

In order to allow for type constraints to be inserted into arbitrary
expressions using the `convert` function, we need to allow for the use
of primitive type names. Previously `string`, `number`, `bool`, and
`any` would be considered as incomplete resource references, because
there are no constraints on what providers can name resources. However
because you cannot use a standalone resource type name as a reference in
HCL, we can make an exception here for when these identifiers are not
part of a longer traversal.

internal/addrs/parse_ref.go

@@ -407,6 +407,17 @@ func parseRef(traversal hcl.Traversal) (*Reference, tfdiags.Diagnostics) {
 		}
 		remain := traversal[1:] // trim off "action"
 		return parseActionRef(rootRange, remain)
+
+	case "string", "number", "bool", "any":
+		if len(traversal) == 1 {
+			// A standalone word is a primitive type constraint.
+			return nil, diags
+		}
+
+		// There could technically be providers that implement resources by
+		// these names, so if the traversal has more parts we still fallthrough
+		// to the default resource parsing.
+		fallthrough
 	default:
 		return parseResourceRef(ManagedResourceMode, rootRange, traversal)
 	}

internal/addrs/parse_ref_test.go

@@ -1056,6 +1056,47 @@ func TestParseRef(t *testing.T) {
 			},
 			``,
 		},
+
+		// a standalone primitive name is a type constraint, and does not
+		// reference anything
+		{
+			`string`,
+			nil,
+			``,
+		},
+		{
+			`bool`,
+			nil,
+			``,
+		},
+		{
+			`number`,
+			nil,
+			``,
+		},
+		{
+			`any`,
+			nil,
+			``,
+		},
+
+		{
+			// a multi-step traversal starting with a primitive name is still
+			// treated as any other resource reference
+			`string.foo`,
+			&Reference{
+				Subject: Resource{
+					Mode: ManagedResourceMode,
+					Type: "string",
+					Name: "foo",
+				},
+				SourceRange: tfdiags.SourceRange{
+					Start: tfdiags.SourcePos{Line: 1, Column: 1, Byte: 0},
+					End:   tfdiags.SourcePos{Line: 1, Column: 11, Byte: 10},
+				},
+			},
+			``,
+		},
 	}
 
 	for _, test := range tests {