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An assignment to a whole polymorphic allocatable changes its dynamic type to the type of the right-hand side expression. But when the assignment is under control of a WHERE statement, or a FORALL / DO CONCURRENT with a mask expression, there is no interpretation of the assignment, as the type of a variable must be the same for all of its elements. There is no restriction in the standard against this usage, and no other Fortran compiler complains about it. But it is not possible to implement it in general, and the behavior produced by other compilers is not reasonable, much less worthy of emulating. It's best to simply disallow it with an error message. Fixes https://github.com/llvm/llvm-project/issues/133669, or more accurately, resolves it.
Flang
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.
Please note that flang is not ready yet for production usage.
Getting Started
Read more about flang in the docs directory. Start with the compiler overview.
To better understand Fortran as a language and the specific grammar accepted by flang, read Fortran For C Programmers and flang's specifications of the Fortran grammar and the OpenMP grammar.
Treatment of language extensions is covered in this document.
To understand the compilers handling of intrinsics, see the discussion of intrinsics.
To understand how a flang program communicates with libraries at runtime, see the discussion of runtime descriptors.
If you're interested in contributing to the compiler, read the style guide and also review how flang uses modern C++ features.
If you are interested in writing new documentation, follow LLVM's Markdown style guide.
Consult the Getting Started with Flang for information on building and running flang.