In configurable systems, conditional compilation can hide compilation errors under untested feature combinations. We investigate foundation models for detecting such errors and, in a controlled setting, restoring compilability in configurable C code. Study I evaluates GPT-OSS-20B on 5,000 synthetic snippets generated by ChatGPT-5.2 from 30 curated seeds and exhaustively compiled under all Boolean feature assignments; it also compares TypeChef and evaluates Gemini 3.6 Flash on a stratified sample. GPT-OSS-20B achieved 84.7% micro-precision and 52.1% micro-recall for affected configurations. Coverage depended on reporting style: presence conditions covered 99.4% of failing configurations, whereas explicit enumerations covered 29.5% under a prompt requesting only a minimal justifiable set. GPT-OSS-20B restored compilability for 1,930 of 2,665 faulty snippets (72.4%), while Gemini 3.6 Flash did so for 182 of 190 sampled faulty snippets (95.8%). A paired counterfactual audit found no evidence that an identified label-correlated #define property materially influenced GPT-OSS-20B's predictions. Study II evaluates Codex-GPT5.5 on 100 faulty file-level subjects from five mature configurable systems and reports target-fault-aligned problems in 94 subjects, including four of five historical bugs. Overall, foundation models can support localized detection, explanation, and triage, but should complement compiler-based and variability-aware analyses; compiler acceptance does not establish semantic correctness

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