[flang] Still do scalar- constraint check.

Original-commit: flang-compiler/f18@e50b4c4b48c0281473670cdf011e335b37d526dc
Reviewed-on: https://github.com/flang-compiler/f18/pull/342

[flang] Simplify grammar for STOP/ERROR STOP.

Original-commit: flang-compiler/f18@eddaea25a2f9be240edad9127aa412fbd2923e4a
Reviewed-on: https://github.com/flang-compiler/f18/pull/342
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#341 from flang-compiler/klausler-patch-1

Update FortranForCProgrammers.md

Original-commit: flang-compiler/f18@678cff1bc70defe5bc074e840c725cb493793f68
Reviewed-on: https://github.com/flang-compiler/f18/pull/341

[flang] Update FortranForCProgrammers.md

Original-commit: flang-compiler/f18@39d81438442cc1185409240842084338c2c5d276
Reviewed-on: https://github.com/flang-compiler/f18/pull/341

[flang] Merge pull request flang-compiler/f18#338 from flang-compiler/tsk2

Name resolution for defined operators

Original-commit: flang-compiler/f18@6b3edf2b713074c33ff2bfb6f748590d247b8cbc
Reviewed-on: https://github.com/flang-compiler/f18/pull/338

[flang] Name resolution for defined operators

Instead of tracking just genericName_ while in a generic interface
block or generic statement, now we immediately create a symbol for it.
A parser::Name isn't good enough because a defined-operator or
defined-io-generic-spec doesn't have a name.

Change the parse tree to add a source field to GenericSpec. Use these
as names for symbols for defined-operator and defined-io-generic-spec
(e.g. "operator(+)" or "read(formatted)").

Change the source for defined-op-name to include the dots so that they
can be distinguished from normal symbols with the same name (e.g. you
can have both ".foo." and "foo"). These symbols have names in the symbol
table like ".foo.", not "operator(.foo.)", because references to them
have that form.

Add GenericKind enum to GenericDetails and GenericBindingDetails.
This allows us to know a symbol is "assignment(=)", for example,
without having to do a string comparison.

Add GenericSpecInfo to handle analyzing the various kinds of
generic-spec and generating symbol names and GenericKind for them.

Add reference to LanguageFeatureControl to SemanticsContext so that
they can be checked during semantics. For this change, if
LogicalAbbreviations is enabled, report an error if the user tries
to define an operator named ".T." or ".F.".

Add resolve-name-utils.cc to hold utility functions and classes that
don't have to be in the ResolveNamesVisitor class hierarchy. The goal
is to reduce the size of resolve-names.cc where possible.

Original-commit: flang-compiler/f18@3081f694e21dbcaef2554198a682c9af57f2e185
Reviewed-on: https://github.com/flang-compiler/f18/pull/338

[flang] Merge pull request flang-compiler/f18#339 from flang-compiler/pmk-spaces

Squash free-form white space in prescanner around parentheses.

Original-commit: flang-compiler/f18@11eb9c89490bfb90abac1b308781580e54f2de46
Reviewed-on: https://github.com/flang-compiler/f18/pull/339

[flang] Improve comment

Original-commit: flang-compiler/f18@140ebe060e31ffcdbcc2d069249fbd0d18c598c5
Reviewed-on: https://github.com/flang-compiler/f18/pull/339

[flang] Final tweak before review

Original-commit: flang-compiler/f18@5dda7b2f658198b21a35edc295a488ebada84244
Reviewed-on: https://github.com/flang-compiler/f18/pull/339
Tree-same-pre-rewrite: false

[flang] Simplify

Original-commit: flang-compiler/f18@42bee00a457e46e77f4ee912a3a5fcd7fb4ae44f
Reviewed-on: https://github.com/flang-compiler/f18/pull/339
Tree-same-pre-rewrite: false

[flang] Squash spaces in generic specs.

Original-commit: flang-compiler/f18@bd5e29f5f807abf58c9169f96ce944e600a02cae
Reviewed-on: https://github.com/flang-compiler/f18/pull/339
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#336 from flang-compiler/pmk-fixes

Fix parse of .TRUE._8

Original-commit: flang-compiler/f18@5e44e67f784dee20067c9ac6ec67080126e57908
Reviewed-on: https://github.com/flang-compiler/f18/pull/336

[flang] Fix parse of .TRUE._8

Original-commit: flang-compiler/f18@b03e7a691d21f84b53a04ed71327720f5500c924
Reviewed-on: https://github.com/flang-compiler/f18/pull/336

[flang] Merge pull request flang-compiler/f18#334 from flang-compiler/eas0

FIR sync: replace the stubs for translating terminal symbols to expressions; st…

Original-commit: flang-compiler/f18@0a93e2c393c67448f7a0494a2fef47612fd664f0
Reviewed-on: https://github.com/flang-compiler/f18/pull/334

[flang] more compiler warnings

Original-commit: flang-compiler/f18@df8c83c106ee5f8ab7c66f5ad6a87bf8571860bf
Reviewed-on: https://github.com/flang-compiler/f18/pull/334

[flang] review comments

Original-commit: flang-compiler/f18@1ba1c0268f293c3b6652d313e68a87cc118f71e3
Reviewed-on: https://github.com/flang-compiler/f18/pull/334
Tree-same-pre-rewrite: false

[flang] muzzle clang++ warnings

Original-commit: flang-compiler/f18@02decb4040e4e7c57a6404adef17215ce8f276a5
Reviewed-on: https://github.com/flang-compiler/f18/pull/334
Tree-same-pre-rewrite: false

[flang] replace the stubs for translating terminal symbols to expressions; start pushing Value type into FIR; cleanup terminator succ_blocks; remove parser element from disassociate; run clang-format; etc.

Original-commit: flang-compiler/f18@c5561646ea4f72431174f28131a49426b8a03495
Reviewed-on: https://github.com/flang-compiler/f18/pull/334
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#335 from flang-compiler/pmk-spec

Some tweaks to preprocess & parse all Fortran in SPEC CPUv6

Original-commit: flang-compiler/f18@b2828f1318e31917558c28d7ca695234b9ba9b4b
Reviewed-on: https://github.com/flang-compiler/f18/pull/335

[flang] Document extension

Original-commit: flang-compiler/f18@ca2a7dc5a8af1079a832407c23acfbbb3a12c17f
Reviewed-on: https://github.com/flang-compiler/f18/pull/335

[flang] Final tweaks, can now parse all SPEC CPUv6

Original-commit: flang-compiler/f18@015152f1ba44bf938fc581622b89130e73a5c144
Reviewed-on: https://github.com/flang-compiler/f18/pull/335
Tree-same-pre-rewrite: false

[flang] more spec work, handle classic C comments

Original-commit: flang-compiler/f18@d901ee7f500df48c81d786a2f9fcef4c65049d2f
Reviewed-on: https://github.com/flang-compiler/f18/pull/335
Tree-same-pre-rewrite: false

[flang] Accomodate missing clock_gettime(); remove f18-parse-demo dependence on lib/evaluate; allow #ifdef with no name

Original-commit: flang-compiler/f18@330fd8116febab2416afeb8c08d6216c1edd9e55
Reviewed-on: https://github.com/flang-compiler/f18/pull/335
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#330 from flang-compiler/pmk-parse-demo

f18-parse-demo

Original-commit: flang-compiler/f18@ec7b68f596ec61c523717c805885b5662e37e119
Reviewed-on: https://github.com/flang-compiler/f18/pull/330

[flang] Add reference to libFortranEvaluate for destructor

Original-commit: flang-compiler/f18@db6fb0f8cdfc9033b874662b48240ead57efcc74
Reviewed-on: https://github.com/flang-compiler/f18/pull/330

[flang] Move dump-parse-tree.h from semantics into parser.

Original-commit: flang-compiler/f18@9075adfcdf5a300471c9bd5512674ec33a9ad90d
Reviewed-on: https://github.com/flang-compiler/f18/pull/330
Tree-same-pre-rewrite: false

[flang] address review comments

Original-commit: flang-compiler/f18@be7dbdab0a0aab0b13ab020b7b2ffafd0ca63601
Reviewed-on: https://github.com/flang-compiler/f18/pull/330
Tree-same-pre-rewrite: false

[flang] f18-parse-demo

Original-commit: flang-compiler/f18@96d9aefc4f31898e104409d06b0d5d4d97007857
Reviewed-on: https://github.com/flang-compiler/f18/pull/330
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#333 from flang-compiler/pmk-include-in-continuation

Fixes and extensions to get WRF to parse

Original-commit: flang-compiler/f18@338a8578569132d9fa9749617e9b11ada5b729dd
Reviewed-on: https://github.com/flang-compiler/f18/pull/333

[flang] Get clean -fparse-only pass over WRF

Original-commit: flang-compiler/f18@ffe517abc6bdc481248308156dec6d48089e8269
Reviewed-on: https://github.com/flang-compiler/f18/pull/333

[flang] WRF preprocessing tweaks

Original-commit: flang-compiler/f18@53f76e1c93a9df2afb7ce6689f9abee5cef6793c
Reviewed-on: https://github.com/flang-compiler/f18/pull/333
Tree-same-pre-rewrite: false

[flang] checkpoint

Original-commit: flang-compiler/f18@0c9bdc70328e3e6fb752c84c84f5d6cda0b4316d
Reviewed-on: https://github.com/flang-compiler/f18/pull/333
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#332 from flang-compiler/eas1

fix typo

Original-commit: flang-compiler/f18@4097d8a89088168380b6cf19171bdf6ffbf56056
Reviewed-on: https://github.com/flang-compiler/f18/pull/332

[flang] fix typo

Original-commit: flang-compiler/f18@fc88b92af0952476b2f592b5bfb08e4d4d120ca3
Reviewed-on: https://github.com/flang-compiler/f18/pull/332

[flang] Merge pull request flang-compiler/f18#331 from flang-compiler/tsk10

Change default for LINK_WITH_FIR to ON.

Original-commit: flang-compiler/f18@ea8944908956f73fb123b6cf4053753b98f28427
Reviewed-on: https://github.com/flang-compiler/f18/pull/331

[flang] Change default for LINK_WITH_FIR to ON.

Original-commit: flang-compiler/f18@b13f99446339666859eb00e896e8d1751423892a
Reviewed-on: https://github.com/flang-compiler/f18/pull/331

[flang] Merge pull request flang-compiler/f18#329 from flang-compiler/tsk10

Add cmake option to link with FIR and LLVM

Original-commit: flang-compiler/f18@ddb43b33ccf52e9083a90d11f00b6a85667617ed
Reviewed-on: https://github.com/flang-compiler/f18/pull/329

[flang] Add cmake option to link with FIR and LLVM

The cmake option -DLINK_WITH_FIR=ON causes the f18 driver to be linked
against FIR and LLVM. When it is set to OFF (the default), the call into
FIR is #ifdef'd out.

The source in lib/FIR is always built; this option only affects linking.

Original-commit: flang-compiler/f18@69569edd4c6abbb7a66c0d8f3595858693a06996
Reviewed-on: https://github.com/flang-compiler/f18/pull/329

[flang] Merge pull request flang-compiler/f18#328 from flang-compiler/eas0

remove unused files and clean up unused warnings

Original-commit: flang-compiler/f18@67ef747557f254d3fb3692bd7eba720a29ad69df
Reviewed-on: https://github.com/flang-compiler/f18/pull/328

[flang] Move virtual dtor to Terminator

Original-commit: flang-compiler/f18@68853d602a6a2c21f4f19b91b58fec1f88473dd6
Reviewed-on: https://github.com/flang-compiler/f18/pull/328

[flang] remove unused files
Cleanup some warnings when building with clang.

Original-commit: flang-compiler/f18@33cf6255d1431a8fa0a30fb4b883615de17bcbb7
Reviewed-on: https://github.com/flang-compiler/f18/pull/328
Tree-same-pre-rewrite: false

[flang] Silence clang warning related to ISO_Fortran_binding.h usage in C++

Original-commit: flang-compiler/f18@85b7f370b4b1e41186bc2cfb7cf55d279085b1d5

[flang] address comments

Original-commit: flang-compiler/f18@7b1fc24265b694ed365879e974810744911f12a2

[flang] remove unused CFI_ISO_FORTRAN_BINDING_FLEXIBLE_ARRAY

Original-commit: flang-compiler/f18@3cf9a625bf6b7c2a62b29074f31a564cc51e5df0
Tree-same-pre-rewrite: false

[flang] Silence clang warning from ISO_Fortran_binding.h usage in C++

Original-commit: flang-compiler/f18@88eb833b14e3f91285605dd4db0e0c021df7fa4c
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#326 from flang-compiler/BUILD_WITH_CLANG_LIBRARIES

Update CMakeList.txt to allow linking with LLVM libc++

Original-commit: flang-compiler/f18@866e188de60e5811882c4c1ea75daa2b448b7b0b
Reviewed-on: https://github.com/flang-compiler/f18/pull/326

[flang] To use LLVM's libc++ instead of GCC's libstdc++, define
BUILD_WITH_CLANG_LIBRARIES on the cmake command line to
be the path to a clang installation with bin, lib, etc.
For this to work, BUILD_WITH_CLANG must be defined too.

Original-commit: flang-compiler/f18@a36d5fee421f2e90293540335c5b887412e31383
Reviewed-on: https://github.com/flang-compiler/f18/pull/326

[flang] Merge pull request flang-compiler/f18#327 from flang-compiler/pmk-nofir

Emergency temporary patch to fix build

Original-commit: flang-compiler/f18@06d71f72a5c97ac7be5cde0150cc6738772ba94a
Reviewed-on: https://github.com/flang-compiler/f18/pull/327

[flang] Temporarily disconnect FIR from f18.cc build

Original-commit: flang-compiler/f18@c610bc2744ad56adae8de09a81069a2c3df396e7
Reviewed-on: https://github.com/flang-compiler/f18/pull/327

[flang] Merge pull request flang-compiler/f18#324 from flang-compiler/eas0

Revisions for review comments.

Original-commit: flang-compiler/f18@5b066774b0c03a45dff94d2ccdf071a7690aabe5
Reviewed-on: https://github.com/flang-compiler/f18/pull/324

[flang] Revisions for review comments.

Original-commit: flang-compiler/f18@1a03f7d5bbf6b0f74a54736a906ea4a1e301f0e9
Reviewed-on: https://github.com/flang-compiler/f18/pull/324

[flang] Merge pull request flang-compiler/f18#323 from flang-compiler/eas0

Fix build breakage

Original-commit: flang-compiler/f18@46e416acc7d318d7681e17f4e10812b4ad6f626b
Reviewed-on: https://github.com/flang-compiler/f18/pull/323

[flang] Fix build breakage

Original-commit: flang-compiler/f18@72d003f89d523f84cfab6ed048897be6bcb7ed92
Reviewed-on: https://github.com/flang-compiler/f18/pull/323

[flang] Merge pull request flang-compiler/f18#319 from flang-compiler/eas0

FIR update

Original-commit: flang-compiler/f18@3f6f710889c68207573ea9eddc962270590a2669
Reviewed-on: https://github.com/flang-compiler/f18/pull/319

[flang] Address review comments; merge with latest breaking changes on master

Original-commit: flang-compiler/f18@ffde96d4866c25c3267332963363d4b6c523325d
Reviewed-on: https://github.com/flang-compiler/f18/pull/319

[flang] Run old clang-format over FIR directory

Original-commit: flang-compiler/f18@0616ac43ecea4b60bb076fb67c2a284cb94835fb
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] Repairs for new interfaces

Original-commit: flang-compiler/f18@1a080dfe6a0e87879458cac76fe809eb92437b21
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] FIR statements: various reworking of the code to decompose some
operations, get rid of unneeded nodes, restructure DO loops, etc.

Original-commit: flang-compiler/f18@0e0d67fbdae307b63f31ade0a990db8f00fed251
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] react to more review comments

Original-commit: flang-compiler/f18@b685d8a588287ac7683fd5626023c0de70fcc69b
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] cleanup merge

Original-commit: flang-compiler/f18@ea00eab799cd8e5312d45d8c97bc9a5a3c6a3ba9
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] Fixes for building with clang++ and other misc. from review

Original-commit: flang-compiler/f18@c3cdc327863036213dab26abf9e97f23a968c197
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] Move over build recipes and rename the library

Original-commit: flang-compiler/f18@51476aa1786de4b98b8189f256ba602298fd4903
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] Fortran Intermediate Representation: initial draft based on "CFG" note.

Original-commit: flang-compiler/f18@b7a759539467268c250b239f2e62c854f41ae46e
Reviewed-on: https://github.com/flang-compiler/f18/pull/319
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#322 from flang-compiler/pmk-struct-constr-disambig

Fix two ambiguous parsing cases by patching parse tree in expression semantics

Original-commit: flang-compiler/f18@d4807f136419b075adcaf38b0129ba7e79c994df
Reviewed-on: https://github.com/flang-compiler/f18/pull/322

[flang] Address review comment

Original-commit: flang-compiler/f18@8a9a41487e809e61b173b07f629a8e0251a8b921
Reviewed-on: https://github.com/flang-compiler/f18/pull/322

[flang] Fix misparsings of substrings in the parse tree, not in expressions.

Original-commit: flang-compiler/f18@53d0e26d922d2a7d99599a0f6b7c9a2aa0854007
Reviewed-on: https://github.com/flang-compiler/f18/pull/322
Tree-same-pre-rewrite: false

[flang] add structconst04.f90 test and fixes to pass it

Original-commit: flang-compiler/f18@d857c843f5ef2125e9675eb22f013b225a399a38
Reviewed-on: https://github.com/flang-compiler/f18/pull/322
Tree-same-pre-rewrite: false

[flang] Resolve misparse of structure constructor as function reference.

Original-commit: flang-compiler/f18@5b6b2540f6c66b3f2900b24b06d732de3436b4ea
Reviewed-on: https://github.com/flang-compiler/f18/pull/322
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#321 from flang-compiler/tsk3

Remove build dependency on clang

Original-commit: flang-compiler/f18@737998bb508819772a4f46f10c7d8d9cfaef9070
Reviewed-on: https://github.com/flang-compiler/f18/pull/321

[flang] Remove build dependency on clang

For now all we need to depend on is LLVM.
Use llvm_map_components_to_libnames to find libraries to link against.

Original-commit: flang-compiler/f18@dff1402c2a781b7fff5c691b45e6a7f2e3a6ef61
Reviewed-on: https://github.com/flang-compiler/f18/pull/321

[flang] Merge pull request flang-compiler/f18#320 from flang-compiler/pmk-expr-rearrangement

Rearrange/simplify expr analysis code

Original-commit: flang-compiler/f18@4eaccde5167370eb082ce6ef0247942a230f94f1
Reviewed-on: https://github.com/flang-compiler/f18/pull/320

[flang] Rearrange/simplify expr analysis code

Original-commit: flang-compiler/f18@aa9f3a7cca821bf8ea430c10fcae49b295ccd489
Reviewed-on: https://github.com/flang-compiler/f18/pull/320

[flang] Merge pull request flang-compiler/f18#317 from flang-compiler/tsk10

Simplify Semantics::Perform

Original-commit: flang-compiler/f18@c435fe65a4c027300365dce9a54240705db1a945
Reviewed-on: https://github.com/flang-compiler/f18/pull/317

[flang] Simplify Semantics::Perform

`Semantics::Perform` is mostly a series of calls followed by a check
for fatal errors. There is more error checking logic than real code.

To make it clearer, change each of the phases it calls to return true
on success so that `Perform` can just call them one after the other.

Original-commit: flang-compiler/f18@a218cac788fdc4018645651964e75221c6f31653
Reviewed-on: https://github.com/flang-compiler/f18/pull/317

[flang] Merge pull request flang-compiler/f18#316 from flang-compiler/pmk-expr-walk

Add Expression traversal framework, use it to reimplement IsConstantE…

Original-commit: flang-compiler/f18@90cd33adcb6c420d0791a094c456832e4d78d548
Reviewed-on: https://github.com/flang-compiler/f18/pull/316

[flang] add descender.h

Original-commit: flang-compiler/f18@cfc3082b7ae840168e891dcf25e0034a4193a3fd
Reviewed-on: https://github.com/flang-compiler/f18/pull/316

[flang] add rewriting as well as const visitation

Original-commit: flang-compiler/f18@1224eaee85c65d70724385d222ba72541dd80213
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] More rearrangement

Original-commit: flang-compiler/f18@7a675d9df6ac161d68314459b3a54575eda1d354
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] Hide implementation better

Original-commit: flang-compiler/f18@2118b463140f027c81659c647639a03a820437ad
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] back out one change

Original-commit: flang-compiler/f18@27fb43b6ed1a4874c153ffc1219dcbdca931804e
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] Address review comments

Original-commit: flang-compiler/f18@3ed6199fedc2ba0d527cc7dd46c5ab67a7f0f1c7
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] Improve detection of default Handle() callback

Original-commit: flang-compiler/f18@c09c4c9e00d15a5dd391216662665d48a908c4fd
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] some tweaks

Original-commit: flang-compiler/f18@07d196386e139b65fe18d4df32f06f994627aac3
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] Add Expression traversal framework, use it to reimplement IsConstantExpr()

Original-commit: flang-compiler/f18@f5d39159354a994cdeb5ab1d4cbfea188c14e681
Reviewed-on: https://github.com/flang-compiler/f18/pull/316
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#315 from flang-compiler/tsk10

Create framework for checking statement semantics

Original-commit: flang-compiler/f18@0fcdb68061b8d008d22e0cc0ff4ccdbb40ab8f44
Reviewed-on: https://github.com/flang-compiler/f18/pull/315

[flang] Move ExprChecker into separate pass

DoConcurrentChecker depends on expressions being fully resolved so it
can't be in the same pass as ExprChecker. The same will probably
apply to AssignmentChecker when its finished.

Checks that don't depend on expressions can go in the first pass
with ExprChecker.

Original-commit: flang-compiler/f18@c0785ec06f58098d262cf2f791a98e74bdcef11b
Reviewed-on: https://github.com/flang-compiler/f18/pull/315

[flang] Add test for AssignmentChecker

Add a test to verify that AssignmentChecker is actually running.

Original-commit: flang-compiler/f18@2c91604e9bff06eb8d63d79abda916802966602f
Reviewed-on: https://github.com/flang-compiler/f18/pull/315
Tree-same-pre-rewrite: false

[flang] Create framework for checking statement semantics

Add `SemanticsVisitor` as the visitor class to perform statement
semantics checks. Its template parameters are "checker" classes
that perform the checks. They have `Enter` and `Leave` functions
that are called for the corresponding parse tree nodes (`Enter`
before the children, `Leave` after). Unlike `Pre` and `Post` in
visitors they cannot prevent the parse tree walker from visiting
child nodes.

Existing checks have been incorporated into this framework:
- `ExprChecker` replaces `AnalyzeExpressions()`
- `AssignmentChecker` replaces `AnalyzeAssignments()`
- `DoConcurrentChecker` replaces `CheckDoConcurrentConstraints()`

Adding a new checker requires:
- defining the checker class:
  - with BaseChecker as virtual base class
  - constructible from `SemanticsContext`
  - with Enter/Leave functions for nodes of interest
- add the checker class to the template parameters of `StatementSemantics`

Because these checkers and also `ResolveNamesVisitor` require tracking
the current statement source location, that has been moved into
`SemanticsContext`. `ResolveNamesVisitor` and `SemanticsVisitor`
update the location when `Statement` nodes are encountered, making it
available for error messages.

`AnalyzeKindSelector()` now has access to the current statement through
the context and so no longer needs to have it passed in.

Test `assign01.f90` was added to verify that `AssignmentChecker` is
actually doing something.

Original-commit: flang-compiler/f18@3a222c36731029fabf026e5301dc60f0587595be
Reviewed-on: https://github.com/flang-compiler/f18/pull/315
Tree-same-pre-rewrite: false

[flang] Merge pull request flang-compiler/f18#311 from flang-compiler/pmk-struct-checks

More structure constructor semantics checking

Original-commit: flang-compiler/f18@5561c9012c07c21cd043b0b0b8b2a03d6e42bebe
Reviewed-on: https://github.com/flang-compiler/f18/pull/311

[flang] Tweak API per review

Original-commit: flang-compiler/f18@c0ab787cd1eb82a13fca9a473973b25e737da77a
Reviewed-on: https://github.com/flang-compiler/f18/pull/311

[flang] Use value()/has_value() on Indirection class templates instead of operator*/operator->

Original-commit: flang-compiler/f18@a97f377ae6051d877df82036983cc6209a2e0d95
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Dodge bogus G++ 8.1.0 build warning

Original-commit: flang-compiler/f18@582c6d1eca5bc29fddadae4d25e418da4ace2c11
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Improve error messages

Original-commit: flang-compiler/f18@40461345124b95de1f35828885e95d8bf0a235f1
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Add lib/semantics/tools.{h,cc}

Original-commit: flang-compiler/f18@c9823da20746313005ee017413f8207a720a10b0
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] C1594 constraint checking on pointer components in struct constructors

Original-commit: flang-compiler/f18@386cd8a9b43f4e65c3e42febbae991fa87d277a9
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Length conversions in array constructors, and fix their formatting

Original-commit: flang-compiler/f18@88cdb49f4892a20f2628f11373a0f1c9c32a6c1e
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Character length conversion on structure constructor component assignments

Original-commit: flang-compiler/f18@051c1dd923a3b321d1a19768414caf93732a3426
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Add SetLength operator to make character length changes explicit

Original-commit: flang-compiler/f18@f2d751d8a6c1bf31f69031bbdab0a823999ebf5c
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] snapshot of work in progress

Original-commit: flang-compiler/f18@56e83d4dd64dd7d30b2ce4bbf124f2c39bc14b58
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false

[flang] Represent NULL()

Original-commit: flang-compiler/f18@2c3368fb5f6178bb997936bb506deecc2d8b7d03
Reviewed-on: https://github.com/flang-compiler/f18/pull/311
Tree-same-pre-rewrite: false