strings: type assert in the generic nextind, prevind methods (#57608)

The type assertions are valid according to the doc strings of these
functions in the case of `AbstractString`.

Should prevent some invalidation on loading user code.

Fixes #57605

Author: nsajko

base/strings/basic.jl

@@ -508,11 +508,11 @@ prevind(s::AbstractString, i::Int)                 = prevind(s, i, 1)
 
 function prevind(s::AbstractString, i::Int, n::Int)
     n < 0 && throw(ArgumentError("n cannot be negative: $n"))
-    z = ncodeunits(s) + 1
+    z = ncodeunits(s)::Int + 1
     @boundscheck 0 < i ≤ z || throw(BoundsError(s, i))
-    n == 0 && return thisind(s, i) == i ? i : string_index_err(s, i)
+    n == 0 && return thisind(s, i)::Int == i ? i : string_index_err(s, i)
     while n > 0 && 1 < i
-        @inbounds n -= isvalid(s, i -= 1)
+        @inbounds n -= isvalid(s, i -= 1)::Bool
     end
     return i - n
 end
@@ -567,11 +567,11 @@ nextind(s::AbstractString, i::Int)                 = nextind(s, i, 1)
 
 function nextind(s::AbstractString, i::Int, n::Int)
     n < 0 && throw(ArgumentError("n cannot be negative: $n"))
-    z = ncodeunits(s)
+    z = ncodeunits(s)::Int
     @boundscheck 0 ≤ i ≤ z || throw(BoundsError(s, i))
-    n == 0 && return thisind(s, i) == i ? i : string_index_err(s, i)
+    n == 0 && return thisind(s, i)::Int == i ? i : string_index_err(s, i)
     while n > 0 && i < z
-        @inbounds n -= isvalid(s, i += 1)
+        @inbounds n -= isvalid(s, i += 1)::Bool
     end
     return i + n
 end

test/strings/basic.jl

@@ -877,6 +877,11 @@ end
             end
         end
     end
+
+    @testset "return type infers to `Int`" begin
+        @test Int === Base.infer_return_type(prevind, Tuple{AbstractString, Vararg})
+        @test Int === Base.infer_return_type(nextind, Tuple{AbstractString, Vararg})
+    end
 end
 
 @testset "first and last" begin