Index: flang/lib/Evaluate/fold-character.cpp
===================================================================
--- flang/lib/Evaluate/fold-character.cpp
+++ flang/lib/Evaluate/fold-character.cpp
@@ -56,7 +56,13 @@
   if (name == "achar" || name == "char") {
     using IntT = SubscriptInteger;
     return FoldElementalIntrinsic<T, IntT>(context, std::move(funcRef),
-        ScalarFunc<T, IntT>([](const Scalar<IntT> &i) {
+        ScalarFunc<T, IntT>([&](const Scalar<IntT> &i) {
+          if (i.IsNegative() || i.BGE(Scalar<IntT>{0}.IBSET(8 * KIND))) {
+            context.messages().Say(
+                "%s(I=%jd) is out of range for CHARACTER(KIND=%d)"_warn_en_US,
+                parser::ToUpperCaseLetters(name),
+                static_cast<std::intmax_t>(i.ToInt64()), KIND);
+          }
           return CharacterUtils<KIND>::CHAR(i.ToUInt64());
         }));
   } else if (name == "adjustl") {
Index: flang/test/Evaluate/errors01.f90
===================================================================
--- flang/test/Evaluate/errors01.f90
+++ flang/test/Evaluate/errors01.f90
@@ -100,6 +100,32 @@
     !CHECK: error: DIM=4 argument to SPREAD must be between 1 and 3
     integer, parameter :: bad3 = spread(matrix, 4, 1)
   end subroutine
+  subroutine s10
+    !CHECK: warning: CHAR(I=-1) is out of range for CHARACTER(KIND=1)
+    character(kind=1), parameter :: badc11 = char(-1,kind=1)
+    !CHECK: warning: ACHAR(I=-1) is out of range for CHARACTER(KIND=1)
+    character(kind=1), parameter :: bada11 = achar(-1,kind=1)
+    !CHECK: warning: CHAR(I=-1) is out of range for CHARACTER(KIND=2)
+    character(kind=2), parameter :: badc21 = char(-1,kind=2)
+    !CHECK: warning: ACHAR(I=-1) is out of range for CHARACTER(KIND=2)
+    character(kind=2), parameter :: bada21 = achar(-1,kind=2)
+    !CHECK: warning: CHAR(I=-1) is out of range for CHARACTER(KIND=4)
+    character(kind=4), parameter :: badc41 = char(-1,kind=4)
+    !CHECK: warning: ACHAR(I=-1) is out of range for CHARACTER(KIND=4)
+    character(kind=4), parameter :: bada41 = achar(-1,kind=4)
+    !CHECK: warning: CHAR(I=256) is out of range for CHARACTER(KIND=1)
+    character(kind=1), parameter :: badc12 = char(256,kind=1)
+    !CHECK: warning: ACHAR(I=256) is out of range for CHARACTER(KIND=1)
+    character(kind=1), parameter :: bada12 = achar(256,kind=1)
+    !CHECK: warning: CHAR(I=65536) is out of range for CHARACTER(KIND=2)
+    character(kind=2), parameter :: badc22 = char(65536,kind=2)
+    !CHECK: warning: ACHAR(I=65536) is out of range for CHARACTER(KIND=2)
+    character(kind=2), parameter :: bada22 = achar(65536,kind=2)
+    !CHECK: warning: CHAR(I=4294967296) is out of range for CHARACTER(KIND=4)
+    character(kind=4), parameter :: badc42 = char(4294967296_8,kind=4)
+    !CHECK: warning: ACHAR(I=4294967296) is out of range for CHARACTER(KIND=4)
+    character(kind=4), parameter :: bada42 = achar(4294967296_8,kind=4)
+  end subroutine
   subroutine warnings
     real, parameter :: ok1 = scale(0.0, 99999) ! 0.0
     real, parameter :: ok2 = scale(1.0, -99999) ! 0.0