[flang] Add new warnings for unused & undefined locals (#173504) Add a requested warning for completely unused local variables. The implementation runs a scan over typed expressions during the existing expression semantics pass to detect variable uses, and a routine at the end of semantics to take a pass over the symbol tables to find unused locals. The new infrastructure needed to detect variable uses, and the existing infrastructure that detects potential variable definitions, then makes it easy to detect variables that are used without any possible initialization or definition, so I did that too. The warning for unused locals is off by default -- they might indicate a misspelling (that IMPLICIT NONE would have caught), but seem otherwise generally benign. The warning for uses of completely uninitialized and undefined variables, however, is enabled by default, since that's likely to indicate a program bug that should be investigated. This patch touches a lot of files lightly. Many of these files are tests that would have produced needless warning noise; one new test was added. Fixes https://github.com/llvm/llvm-project/issues/173276. GitOrigin-RevId: 5fc77c166639afbc980c9ad19a7486af7b7604b8
Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project (https://github.com/flang-compiler/f18) with an aim to replace the previous flang project (https://github.com/flang-compiler/flang) and address its various deficiencies. F18 was subsequently accepted into the LLVM project and rechristened as Flang.
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