[flang] Implement conditional expressions lowering (F2023) (#186490)
## Implement Lowering for Fortran 2023 Conditional Expressions (R1002)
***This PR contains the lowering steps only for ease of review. DO NOT
MERGE until #186489 is merged.***
Implements Fortran 2023 conditional expressions with syntax: `result =
(condition ? value1 : condition2 ? value2 : ... : elseValue)`
Issue: #176999
Discourse:
https://discourse.llvm.org/t/rfc-adding-conditional-expressions-in-flang-f2023/89869/1
-- note that some of the details provided in the RFC post are no longer
accurate
### Implementation Details
**Lowering to HLFIR:**
- Lazy evaluation via nested if-then-else control flow
- Only the selected branch is evaluated
- Temporary allocation with proper cleanup
- Special handling for:
- CHARACTER types with deferred length
- Arrays (shape determined by selected branch per F2023 10.1.4(7))
- Derived types
**LIT Testing:**
- Lowering tests: HLFIR code generation verification
- Note: Executable tests will be added to the llvm-test-suite repo
(https://github.com/llvm/llvm-test-suite/pull/369)
**Limitations**
- Conditional arguments are not yet supported. This work is planned
- #180592
- Polymorphic types (CLASS) not yet supported in lowering
- Both limitations will emit clear error message if encountered
### Examples
```
! Simple conditional
x = (flag ? 10 : 20)
! Chained
result = (x > 0 ? 1 : x < 0 ? -1 : 0)
! Examples from F2023
( ABS (RESIDUAL)<=TOLERANCE ? ’ok’ : ’did not converge’ )
( I>0 .AND. I<=SIZE (A) ? A (I) : PRESENT (VAL) ? VAL : 0.0 )
```
AI Usage Disclosure: AI tools (Claude Sonnet 4.5) were used to assist
with implementation of this feature and test code generation. I have
reviewed, modified, and tested all AI-generated code.
llvm-project
b25ecac3 - [flang] Implement conditional expressions lowering (F2023) (#186490)
