Are vowels long before "gn"?

Question

Allen and Greenough, §10d, provide a general rule:

A vowel before ns, nf, gn, is long: as in cōnstāns, īnferō, māgnus [emphasis modified]

This seems to agree with Priscian:

'gnus' quoque vel 'gna' vel 'gnum' terminantia longam habent vocalem paenultimam, ut 'rēgnum', 'stāgnum', 'benīgnus', 'malīgnus', 'abiēgnus', 'privīgnus', 'Pelīgnus'. (Keil, II.82)

However, Priscian does not mention magnus, and Lewis and Short disagree with A&G and say it has a short vowel. Interestingly, Bennet includes ns and nf in his parallel rule, but does not mention gn.

So which is it? Are vowels before gn generally long or not?

Answer 1

First of all, it's important to note that syllables containing a vowel + gn combination are long (or, less confusingly, "heavy"), regardless of the length of the vowel itself. As Bennet says:

A syllable is long,—
a) if it contains a long vowel; [...]
c) if it contains a short vowel followed by x, z, or any two consonants (5.B.1)

It does not follow, however, that the vowel preceding gn is necessarily long. For that, we'll need to turn to other sources.

W. Sydney Allen addresses the matter in Vox Latina, 71ff:

It is commonly, and mistakenly, believed that vowels in Latin are regularly long before the consonant-group gn.

Allen contends that:

• The "rule" rests on a single passage in Priscian
• The passage in Priscian is an interpolation, not original
• Historical evidence frequently contradicts the rule
• Only a few words—rēgnum, stāgnum, sēgnis, and abiēgnus—are likely to have a long vowel preceding gn

First, Allen addresses Priscian, and argues that the passage quoted above, in Keil II.82, is an interpolation first based on context:

The interpolatory nature of the 'gnus' passage is strongly suggested by its irrelevant interruption of the discussion of proper names, more particularly those with adjectives in -īnus; and by introduction of nouns into a chapter which is exclusively concerned with adjectives.

The interpolator, Allen continues, misinterpreted Priscian's discussion of Anagnīnus, where the vowel following gn, not preceding it, is long. Furthermore, the examples of the rule are not convincing: in several we would expect a long vowel for historical/phonological reasons, while many common words, for which we have evidence of a short vowel, are not mentioned. Allen summarizes:

It seems most probable, therefore, that what the interpolator states as a general rule is not so, and that he was at a loss to cite much more by way of example.

Turning to inscriptions, Allen says "we nowhere find even so common a word as magnus marked with a long a." There is some evidence for a long i in a few of the interpolator's examples, but Allen suggests that this could be explained by phonetic influences on the short i before [ŋ] that made its quality (not length) approach that of the long ī. This explanation is corroborated by long ī appearing occassionally in front of other inscriptional instances of [ŋ], like ng, nc and nqu.

Thus, Allen concludes:

We may safely say, then, that the vowel is long in rēgnum, stāgnum, sēgnis, abiēgnus, but probably NOT before gn in any other instances.

Answer 2

There seems to be a certain amount of evidence that suggests the possibility of a phonetic process of vowel lengthening, or perhaps raising* before -gn-. However, evidence from Romance languages indicates that any lengthening or raising sound change did not run to completion (or was reversed) in this context: Italian has [e] in degno and segnare. For this reason, most recent sources that I've read only mark a vowel before -gn- with a macron if it is etymologically long.

Relevant inscriptional evidence is mentioned in Allen 1989 (Vox Latina, pp. 72-73; cited in Nathaniel's answer) and Leumann 1977 (Lateinische Laui- und Formenlehre, §127, p. 113). Actually, the most complete description I've found so far is in Lindsay 1894 (The Latin Language: An Historical Account of Latin Sounds, Stems and Flexions, p 138), which mentions:

• prIvIgno (Corpus Inscriptionum Latinarum vi. 3541)
• dIgne (Corpus Inscriptionum Latinarum vi. 6314)
• sIgnum (C.I.L. vi. 10234), sIgnificabo (vi. 16664)

Leumann mentions inscriptions seignum "D 44" and "sIgnum 353": I'm not sure what the source for these is (the table of abbreviations says D = Diehl).

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*A separate historical sound change that raised etymological *ĕ to i before -gn- was clearly complete already in Classical Latin. This is usually interpreted as a change of *ĕŋ to ĭŋ. What would be arguable would be a further change of ĭŋ to [iːŋ] or [iŋ] (as opposed to [ɪŋ], with more or less the same value as for ĭ in other contexts).