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[flang] Do not error on constant nonzero UB in substring of ZLA #174511

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Merged
eugeneepshteyn merged 2 commits into from
Jan 8, 2026
+40 −2
Merged

[flang] Do not error on constant nonzero UB in substring of ZLA #174511

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84151a1
Do not error on constant nonzero UB in substring of ZLA
tmjbios Jan 2, 2026
a674969
Updates based on feedback
tmjbios Jan 7, 2026
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5 changes: 3 additions & 2 deletions flang/lib/Semantics/expression.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1215,15 +1215,16 @@ MaybeExpr ExpressionAnalyzer::Analyze(const parser::Substring &ss) {
if (!ubValue) {
ubValue = len;
}
if (lbValue && ubValue && *lbValue > *ubValue) {
if ((len && *len == 0) ||
(lbValue && ubValue && *lbValue > *ubValue)) {
// valid, substring is empty
} else if (lbValue && *lbValue < 1 && (ubValue || !last)) {
Say("Substring must begin at 1 or later, not %jd"_err_en_US,
static_cast<std::intmax_t>(*lbValue));
return std::nullopt;
} else if (ubValue && len && *ubValue > *len &&
(lbValue || !first)) {
Say("Substring must end at %zd or earlier, not %jd"_err_en_US,
Say("Substring must end at %jd or earlier, not %jd"_err_en_US,
static_cast<std::intmax_t>(*len),
static_cast<std::intmax_t>(*ubValue));
return std::nullopt;
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37 changes: 37 additions & 0 deletions flang/test/Semantics/zero_len_char_array.f90
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Original file line number Diff line number Diff line change
@@ -0,0 +1,37 @@
! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck --allow-empty %s
! CHECK-NOT: error:
! Test that zero-length character substrings with lbound>ubounds indices
! do not produce spurious errors when the character array is zero length.
program flang_t7052
implicit none
character*(*), parameter :: param_char = ""
character*(0) :: zero_len_char

if ( param_char(init(5):init(3)) > zero_len_char(1:-2) ) then
print *,"Test failed"
endif

if ( param_char(init(5):init(3)) > zero_len_char(10:2) ) then
print *,"Test failed"
endif

if ( param_char(init(5):init(3)) > zero_len_char(init(10):2) ) then
print *,"Test failed"
endif

if ( param_char(init(5):init(3)) > zero_len_char(init(10):-2) ) then
print *,"Test failed"
endif

if ( param_char(init(5):init(3)) > zero_len_char(2:init(10)) ) then
print *,"Test failed"
endif

contains

integer function init(i)
integer, intent(in) :: i
init=i
end

end program